Monthly Archives: September 2022

Sound Class 8 Science NCERT Textbook Questions

Question 1.
Choose the correct answer.
Sound can travel through
(a) gases only
(b) solids only
(c) liquids only
(d) solids, liquids, and gases
Answer:
(d) solids, liquids, and gases.

Question 2.
Voice of which of the following is likely to have a minimum frequency?
(a) Baby girl
(b) Baby boy
(c) A man
(d) A woman
Answer:
(c) A man

Question 3.
In the following statements, tick ‘T’ against those which are true and ‘F’ against those which are false.

1. Sound cannot travel in a vacuum.
2. The number of oscillations per second of a vibrating object is called its time period.
3. If the amplitude of vibration is large, the sound is feeble.
4. For human ears, the audible range is 20 Hz to 20,000 Hz.
5. The lower the frequency of vibration, the higher is the pitch.
6. Unwanted or unpleasant sound is termed as music.
7. Noise pollution may cause partial hearing impairment.

Answer:

1. True
2. False
3. False
4. True
5. False
6. False
7. True

Question 4.
Fill in the blanks with suitable words.

1. Time taken by an object to complete one oscillation is called _______
2. Loudness is determined by the ________ of vibration.
3. The unit of frequency is ________
4. Unwanted sound is called _______
5. The shrillness of a sound is determined by the ______ of vibration.

Answer:

1. Time period
2. Amplitude
3. Hertz (Hz)
4. Noise
5. Frequency

Question 5.
A pendulum oscillates 40 times in 4 seconds. Find its time period and frequency.
Answer:
No. of oscillation = 40
Total time is taken = 4 seconds

Question 6.
The sound from a mosquito is produced when it vibrates its wings at an average rate of 500 vibrations per second. What is the time period of the vibration?
Answer:
Number of vibrations per second = 500

Question 7.
Identify the part which vibrates to produce sound in the following instruments.

1. Dholak
2. Sitar
3. Flute

Answer:

1. Stretched membrane
2. String of sitar
3. Air column

Question 8.
What is the difference between noise and music? Can music become noise sometimes?
Answer:
The type of sound which are unpleasant to listen is known as noise whereas music is a pleasant sound, which produces a sensation.
Yes, music can become noise when it’s too loud.

Question 9.
List the sources of noise pollution in your surroundings.
Answer:
Following are the major sources of noise pollution:

• Sound of vehicles
• Sound of kitchen appliances
• Sound of bursting crackers
• Sound of loudspeakers, TV, transistors

Question 10.
Explain in what way noise pollution is harmful to humans.
Answer:
Noise pollution causes:
(a) Lack of sleep
(b) Anxiety
(c) Hypertension
and these are harmful to health.

Question 11.
Your parents are going to buy a house. They have been offered one on the roadside and another three lanes away from the roadside. Which house would you suggest your parents should buy? Explain your answer.
Answer:
I would suggest my parents buy a house three lanes away from the roadside because house on the roadside would be much noisy in both days and night due to running vehicles. Whereas, a house three lanes away would be comparatively quieter as the intensity of noise decreases with the distance between the source and the listener.

Sound Class 8 Science NCERT Intext Activities Solved

Activity 1
Take a metal plate (or a shallow pan). Hang it at a convenient place in such a way that it does not touch any wall. Now strike it with a stick (Fig. 13.1). Touch the plate or pan gently with your finger. Do you feel the vibrations? Again strike the plate with the stick and hold it tightly with your hands immediately after striking. Do you still hear the sound? Touch the plate after it stops producing sound. Can you feel vibrations now?

Solution:
When we touch the pan gently with our finger after striking we feel the vibration. When we hold the pan tightly after striking it, we do not hear the sound. When the pan stops producing sound it also stops vibrating. Thus, we can conclude that vibrating body produces sound.

Activity 2
Jake a rubber band. Put it around the, longer side of a pencil box (Fig. 13.2). Insert two pencils between the box and the stretched rubber. Now, pluck the rubber band somewhere in the middle. Do you hear any sound? Does the band vibrate?

Solution:
Yes, we hear the sound on plucking the rubber band. Also, we find that the band is vibrating. Thus, all vibrating bodies produce sound.

Activity 3
Take a metal dish. Pour water in it. Strike it at its edge with a spoon (Fig. 13.3). Do you hear a sound? Again strike the dish and then touch it. Can you feel the dish vibrating? Strike the dish again. Look at the surface of water. Do you see any waves there? Now hold the dish. What change do you observe on the surface of water? Can you explain the change? Is there a hint to connect sound with the vibrations of a body?

Solution:
On striking the metal dish we hear sound and on touching it we feel the dish vibrating. Striking the dish with water we see circular wave are produced. Thus vibrating object produces sound.

Activity 4
Take a hollow coconut shell and make a musical instrument ektara. You can also make it with the help of an earthen pot (Fig. 13.4). Play this instrument and identify its vibrating part.

Solution:
We observed that the vibrating part of the musical instrument ektara is stretched string.

Activity 5
Take 6-8 bowls or tumblers. Fill them with water upto different levels, increasing gradually from one end to the other. Now take a pencil and strike the bowls gently. Strike all of them in succession. You will hear pleasant sounds. This is your Jaltarang (Fig. 13.5).

Solution:
We can hear a pleasant sound. This is due to different levels of water in the bowls.
Thus, we find that shorter the length of the vibrating air column, higher is the pitch of the sound produced.

Activity 6
Take two rubber strips of the same size. Place these two pieces one above the other and stretch them tight. Now blow air through the gap between them [Fig. 13.6(a)]. As the air blows through the stretched rubber strips, a sound is produced. You can also take a piece of paper with a narrow slit and hold it between your fingers as shown in [Fig. 13.6(b)]. Now blow through the slit and listen to the sound.

Solution:
This activity shows that vocal cords also produce sound in a similar manner when they vibrate.

Activity 7
Take a metal or glass tumbler. Make sure that it is dry. Place a cell phone in it. Ask your friend to give a ring on this cell phone from another cell phone. Listen to the ring carefully.
Now, surround the rim of the tumbler with your hands (Fig. 13.7). Put your mouth on the opening between your hands. Indicate to your friend to give a ring again. Listen to the ring while sucking air from the tumbler. Does the sound become fainter as you suck air?
Remove the tumbler from your mouth. Does the sound become loud again?

Solution:
We observed that sound becomes fainter than earlier when we try to suck air. But when we remove tumbler from our mouth the sound again becomes loud. Thus, sound needs a medium to travel.

Activity 8
Take a bucket or a bathtub. Fill it with clean water.
Take a small bell in one hand. Shake this bell inside the water to produce sound. Make sure that the bell does not touch the body of the bucket or the tub. Place your ear gently on the water surface (Fig. 13.8). Can you hear the sound of the bell? Does it indicate that sound can travel through liquids?

Solution:
We can hear the sound of the bell which indicates that sound can travel through liquids.

Activity 9
Take a metre scale ora long metal rod and hold its one end to your ear. Ask your friend to gently scratch or tap at the other end of the scale (Fig. 13.9).
Can you hear the sound of the scratching? Ask your friends around you if they were able to hear the same sound?

Solution:
Yes, we find that we can hear the sound of the scratch. But, the people standing around us cannot hear the same sound or we can say that it is limping not audible to them.

Activity 10

Take a plastic or tin can. Cut its ends. Stretch a piece of rubber balloon across one end of the can and fasten it with a rubber band. Put four or five grains of dry cereal on the stretched rubber. Now ask your friend to speak”Hurrey, Hurrey”from the open end (Fig. 13.10). Observe what happens to the grain. Why do the grain jump up and down?

Solution:
The grain jump up and down because of the vibration caused underneath the stretched rubber. Thus when sound waves fall on the eardrum, it starts vibrating back and forth rapidly.

NCERT Solutions for Class 8 Science Chapter 13 – 1 Mark Questions and Answers

Question 1.
Choose the correct answer. Sound can travel through

• Gases only
• Solids only
• Liquids only
• Solids, liquids and gases.

Answer:
Solids, liquids and gases.

Question 2.
Voice of which of the following is likely to have minimum frequency ? [NCERT]

• Baby girl
• Baby boy
• A man
• A woman

Answer:
Aman

Question 3.
Identify the part which vibrates to produce sound in the following instruments. [NCERT]

• Dholak
• Sitar
• Flute

Answer:

• Dholak – stretched membrane
• Sitar – stretched string
• Flute – air column

Question 4.
In the following statements, tick ‘T’ against those which are true and ‘F’ against those which are false. [NCERT]

1. Sound cannot travel in vacuum. (T/F)
2. The number of oscillations per second of a vibrating object is called its time period. (T/F)
3. If the amplitude of vibration is large, sound is feeble. (T/F)
4. For human ears, the audible range is 20 Hz to 20,000 Hz. (T/F)
5. The lower the frequency of vibration, the higher is the pitch. (T/F)
6. Unwanted or unpleasant sound is termed as music. (T/F)
7. Noise pollution may cause partial hearing impairment. (T/F)

Answer:

1. T
2. F
3. F
4. T
5. F
6. F
7. T

Question 5.
Fill in the blanks with suitable words. [NCERT]

1. Time taken by an object to complete one oscillation is called …………
2. Loudness is determined by the ………….. of vibration.
3. The unit of frequency is ……………
4. Unwanted sound is called …………….
5. Shrillness of a sound is determined by the …………….. of vibration.

Answer:

1. time period
2. amplitude
3. Hertz
4. noise
5. frequency

Question 6.
Define vibration.
Answer:
Vibration is the to and fro or back and forth motion of an object.

Question 7.
Which part of the human body is responsible for producing sound ? [NCT 2011]
Answer:
In humans, the sound is produced by the voice box or larynx

Question 8.
What is the length of vocal cords in men ?
Answer:
The vocal cords in men are about 20 mm long.

Question 9.
Can sound travel in vacuum ?
Answer:
No, sound cannot travel in vacuum.

Question 10.
What is meant by oscillatory motion ?
Answer:
The to and fro motion of an object is known as oscillatory motion.

NCERT Solutions for Class 8 Science Chapter 13 – 2 Mark Questions and Answers

Question 1.
The sound from a mosquito is produced when it vibrates its wings at an average rate of 500 vibrations per second. What is the time period of the vibration ? [NCERT]
Answer:
Time taken for 500 vibrations = 1 second
Time taken for 1 vibration = 1/500 second.
∴ Time period = 1/500 second.

Question 2.
How do plants help in reducing noise pollution ?
Answer:
Plants absorb sound and so help us in minimizing noise pollution.

Question 3.
How can we control the sources of noise pollution ?
Answer:
We can control noise pollution by designing and installing silencing devices in machines.

Question 4.
How can a hearing impaired child communicate ?
Answer:
A hearing impaired child can communicate effectively by using sign language.

Question 5.
If the amplitude increases 3 times, by how much will the loudness increase ?
Answer:
If the amplitude increases three times, the loudness will increase by a factor of 9.

Question 6.
The frequency of a given sound is 1.5 kHz. How many vibrations is it completing in one second ?
Answer:
Frequency = No.of vibrations/time
∴ No. of vibrations = Frequency x time = 1.5 x 1000 x 1 = 1500 vibrations

Question 7.
Which characteristic of a vibrating body determines

1. loudness
2. pitch of the sound produced by it ?

Answer:

1. Amplitude.
2. Frequency.

Question 8.
Why do we not hear echoes in our ordinary surroundings ?
Answer:
We do not hear echoes in our ordinary surroundings because the distance to hear echo should be more than 17 m.

Question 9.
We cannot hear the sound of the exploding meteors in the sky, though we can see them. Why ?
Answer:
Sound cannot travel through vacuum. In space there is vacuum. Light can travel through vacuum, so we can see the exploding meteor but cannot hear the explosion.

Question 10.
We can hear the supersonic jet planes flying. How ?
Answer:
The supersonic jet planes fly in the air. Since sound can travel through air, we can hear then flying.

Question 11.
What are vocal cords ? What is their function ? [NCT 2011]
Answer:
The larynx has a pair of membranes known as vocal cords stretched across their length. The vocal cords vibrate and produce sound.

Question 12.
When does a thud become music ?
Answer:
When thuds are repeated at’regular intervals, it becomes music, e.g., beating of drums or wood.

Question 13.
How do birds and insects produce sound ?
Answer:
Birds chirp with the help syrinx in their wind pipe. Insects produce sound by flapping their wings.

Question 14.
What is the function of eusfachian tube in human ear ?
Answer:
The vibrations of the spoken words reach our ears through eustachian tubes.

Question 15.

1. In our body which part of the ear receives sound waves ?
2. What may happen if the eardrum is absent from our ear ?

Answer:

1. Pinna helps in receiving sound waves.
2. If the eardrum is absent we would not be able to hear.

NCERT Solutions for Class 8 Science Chapter 13 – 3 Mark Questions and Answers

Question 1.
List sources of noise pollution in your surroundings. [NCERT]
Answer:
The major sources of noise pollution are sounds of vehicles, explosions, machines, loudspeakers.

Question 2.
What are the effects of noise pollution ?
Answer:
Due to noise pollution a person may suffer from lack of sleep, hypertension and anxiety. If a person is exposed to noise continuously he may get temporary or permanent deafness.

Question 3.
How can the noise pollution be controlled in residential area ?
Answer:

1. The noisy operations must be conducted away from residential areas.
2. Noise producing industries should be set away from such areas.
3. Use of automobile horns be minimized.
4. TV and music systems should be run at lower volumes.

Question 4.
A pendulum oscillates 40 times in 4 seconds. Find its time period and frequency. [NCT 2011, NCERT]
Answer:
40 vibrations in 4 seconds.
10 vibrations in 1 second
Frequency =10 vibrations/sec. or 10 Hz.
∴ Time period = 1/10 sec.

Question 5.
Your parents are going to buy a house. They have been offered one on the roadside and another three lanes away from the roadside. Which house would you suggest your parents should buy ? Explain your answer. [NCERT]
Answer:
I would advise my parents to buy the house three lanes away from the roadside because there the noise from automobiles would be much less.

Question 6.
What happens when we pluck the strings of a sitar ?
Answer:
When we pluck the strings of a sitar, the whole instrument vibrates and the sound is heard.

Question 7.
Why is the voice of men, women and children different ?
Answer:
The voice of men, women and children are different because the length of vocal cords are different. The length of vocal cords is longest in men and shortest in children.

Question 8.
How are we able to hear sound ?
Answer:
The eardrum is like a stretched rubber sheet. Sound vibrations make the eardrum vibrate. The eardrum sends vibrations to the inner ear. From there, the signal goes to the brain and we are able to hear.

Question 9.
What sources in the home may lead to noise ?
Answer:
Television and transistor at high volumes, some kitchen appliances, desert coolers, air conditioners all contribute to noise pollution.

Question 10.
What is the-difference between noise and music ? Can music become noise sometimes,?
Answer:
Unpleasant sounds are called noise.
Music is a sound which produces a pleasing sensation.
If the music is too loud, it becomes noise.

Question 11.
Draw a labelled diagram showing the structure of the human ear.
Answer:

Question 12.
What is the function of:

1. External ear.
2. Internal ear.

Answer:

1. The external ear helps us in receiving the sound waves and directing them to the eardrum.
2. The internal ear has cochlea which is filled with a fluid and having tiny hair cells inside. The hairy cells change the sound vibrations into nerve impulse which travels to the brain.
   The internal ear also helps us in balancing the body.

Question 13.
Give some suggestions by which we can keep our ears healthy.
Answer:

1. Never insert any pointed object into the ear. Tt can damage the eardrum and make a person deaf.
2. Never shout loudly in someone’s ear.
3. Never hit anyone hard on their ear.

Question 14.
Can you hear the sound on the moon ? Explain.
Answer:
We cannot hear the sound on the moon because sound requires a material medium to travel. On the moon there is no atmosphere and sound cannot travel in vacuum.

Question 15.
What are ultrasounds ? How are they useful to us?
Answer:
Sound having frequency higher than 20kHz is known as ultrasound, is used for

• detecting finer faults in metal sheets.
• scanning and imaging the body for stones, tumour and foetus.

NCERT Solutions for Class 8 Science Chapter 13 – 5 Mark Questions and Answers

Question 1.
Sketch larynx and explain its function in your own words. [NCERT]
Answer:
We produce sound in the larynx of our throats. The larynx has two vocal cords, which are folds of tissue with a slit like opening between them. When we speak, air passes through the opening and the vocal cords vibrate to produce sound.

Question 2.
Lightning and thunder take place in the sky at the same time and at the same distance from us. Lightning is seen earlier and thunder is heard later. Can you explain why ? [NCERT]
Answer:
The speed of light is more than the speed of sound. Therefore, even though thunder and lightning take place simultaneously, we see the lightning earlier.

Question 3.

1. What is SONAR?
2. What is the basic principle of its working ?
3. Explain its use.

Answer:

1. SONAR refers to Sound Navigation and Ranging.
2. The principle of reflection of sound is used in SONAR.
3. SONAR is used to measure the depth of the ocean. Ultrasonic waves are sent from the ship down into the sea. They are received back after reflection from the sea bed. The depth is calculated by noting the time period.

Question 4.
What is the use of ultrasound in medicine and industry ?
Answer:
Use of ultrasound in medicine :

• For scanning and imaging the body for stones, tumour and foetus.
• For relieving pain in muscles and joints.

Use of ultrasound in industry :

• For detecting finer faults in metal sheets.
• In dish washing machines where water and detergent are vibrate with ultrasonic vibrators.
• For homogenising milk in milk plants.

Question 5.
What is a sonogram ? Why is it preferred to X-rays ?
Answer:
Sonogram is image of the internal organs. Ultrasound can pass through the human body and are reflected back. The reflections are recorded by computer and images are generated on the screen.
Sonogram is not harmful and is therefore used for studying the foetus or stone or tumor in the organs. On the other hand, X-rays can be harmful if humans are exposed for longer time.

Question 6.

1. Name a property of sound which is
   (i) similar to the property of light.
   (ii) different from that of light.
2. Why do some people have hearing impairment ? How do they communicate with others ?

Answer:

1. (i) The property of sound similar to light is that in both reflection takes place.
   (ii) Sound can travel around comers but light cannot.
2. Some people suffer from hearing impairment because their ear drum is damaged or absent. This can be from birth or may occur later on. Such people communicate with “sign language”. They can also use “hearing aids”.

NCERT Solutions for Class 8 Science Chapter 13 MCQs

Question 1.
The maximum displacement of a vibrating body on either side of its mean position, is known as its
(a) Frequency
(b) Loudness
(c) Amplitude
(d) Pitch
Answer:
(c)

Question 2.
The frequency of a given sound is 1.5 kHz. The vibrating body is
(a) completing 1,500 vibrations in one second.
(b) taking 1,500 seconds to complete one vibration.
(c) taking 1.5 seconds to complete one vibration.
(d) completing 1.5 vibrations in one second
Answer:
(a)

Question 3.
A given sound is inaudible to the human ear, if
(a) its amplitude is too small.
(b) its frequency is below 20 Hz.
(c) its frequency is above 20 kHz.
(d) it has any of the three characteristics listed above.
Answer:
(d)

Question 4.
Sound can propagate
(a) through vacuum as well as gases
(b) only through gases and liquids
(c) only through gases and solids
(d) any of the three states of the matter.
Answer:
(d)

Question 5.
When lightning and thunder take place, they
(a) occur together and are also observed together.
(b) occur one after the other but are observed together.
(c) occur together but the thunder is observed a little after the lightning.
(d) occur together but the thunder is observed a little before the lightning
Answer:
(c)

Question 6.
Soundshaving frequency more than 20 Hz are called
(a) Infrasonic
(b) Supersonic
(c) Ultrasonic
(d) None of these
Answer:
(c)

Question 7.
Hertz is the unit of
(a) Amplitude
(b) Frequency
(c) Pitch
(d) Wavelength
Answer:
(b)

Question 8.
Loudness of sound is expressed in
(a) Hertz
(b) Decibel
(c) Seconds
(d) None of these
Answer:
(b)

Friction Class 8 Science NCERT Textbook Questions

Question 1.
Fill in the blanks.
(a) Friction opposes the ______ between the surfaces in contact with each other.
(b) Friction depends on the ______ of surfaces.
(c) Friction produces ______
(d) The sprinkling of powder on the carrom board _______ friction.
(e) Sliding friction is _______ than the static friction.
Solution:
(a) relative motion
(b) smoothness (or irregularities or nature)
(c) heat
(d) reduces
(e) less

Question 2.
Four children were asked to arrange forces due to rolling, static and sliding frictions in decreasing order. Their arrangements are given below. Choose the correct arrangement.
(a) rolling, static, sliding
(b) rolling, sliding, static
(c) static, sliding, rolling
(d) sliding, static, rolling
Solution:
(c) static, sliding, rolling.

Question 3.
Alida runs her toy car on a dry marble floor, wet marble floor, newspaper and towel spread on the floor. The force of friction acting on the car on different surfaces in increasing order will be
(a) wet marble floor, dry marble floor, newspaper and towel.
(b) newspaper, towel, dry marble floor, wet marble floor.
(c) towel, newspaper, dry marble floor, wet marble floor.
(d) wet marble floor, dry marble floor, towel, newspaper.
Answer:
(a) wet marble floor, dry marble floor, newspaper and towel.

Question 4.
Suppose your writing desk is tilted a little. A book kept on it starts sliding down. Show the direction of frictional force acting on it.
Answer:
Frictional force will act upward, i.e., the direction opposite to that of sliding book.

Question 5.
You spill a bucket of soapy water on a marble floor accidentally. Would it make it easier or more difficult for you to walk on the floor? Why?
Answer:
The layer of soap makes the floor smooth due to which the friction is reduced. This makes the floor slippery and the foot cannot make a proper grip on the floor. Therefore it is difficult to walk on a soapy floor. We may slip on the floor.

Question 6.
Explain why sportsmen use shoes with spikes.
Answer:
Sportsmen use shoes with spikes to increase the friction between shoes and the surface. So the shoes with spikes do not slip while the sportsmen run and play.

Question 7.
Iqbal has to push a lighter box and Seema has to push a similar heavier box on the same floor. Who will have to apply a larger force and why?
Answer:
A heavy object produces more friction as it is pressed hard against the opposite surface. So Seema will have to apply a larger force.

Question 8.
Explain why sliding friction is less than static friction.
Answer:
The sliding friction is less than static friction because the sliding object get less time to interlock into the contact points on the floor. So it is somewhat easier to move an object already in motion than to get it started.

Question 9.
Give examples to show that friction is both a friend and a foe.
Answer:
Some points are given below which show that friction is both a friend and a foe:

1. Friction as a friend:
   • It allows us to grip and catch any object.
   • It helps us to walk comfortably on the floor.
   • It helps to minimise the speed or to stop any moving object.
   • It helps us to write.
2. Friction as a foe:
   • It causes wear and tears in objects.
   • It causes damage to the parts of machines and tools which further require money to get them repaired.
   • It reduces the speed of moving objects, so more force is required.
   • It produces hurdles in moving any object freely.

Question 10.
Explain why objects moving in fluids must have special shapes.
Answer:
The objects moving in fluids must have a special shape to overcome the fluid friction acting on them. Efforts are therefore made to minimise the friction, so objects are given special shape having pointed fronts with little broader middle portion which gets tapered at the back called streamlined.

Friction Class 8 Science NCERT Intext Activities Solved

Activity 1
Gently push a book on a table [Fig. 12.2(a)]. You observe that it stops after moving for some distance. Repeat this activity pushing the book from the opposite direction [Fig. 12.2 (b)]. Does the book stop this time, too? Can you think of an explanation? Can we say that a force must be acting on the book opposing its motion? This force is called the force of friction.

Friction opposes relative motion between the surfaces of the book and the table
Solution:
Yes, the book stops this time too.
After sliding for some time, the book stops. As the surface of the table provides the frictional force to oppose the sliding • of the book.
Yes, we can say that a force must be acting on the book opposing its motion.

Activity 2

Tie a string around a brick. Pull the brick by a spring balance (Fig. 12.3). You need to apply some force. Note down the reading on the spring balance when the brick just begins to move. It gives you a measure of the force of friction between the surface of the brick and the floor.

Now wrap a piece of polythene around the brick and repeat the activity. Do you observe any difference in the readings of the spring balance in the above two cases? What might be the reason for this difference? Repeat this activity by wrapping a piece of jute bag around the brick. What do you observe?
Solution:
It is observed that the reading of the spring balance in the case when brick is wrapped with a piece of poly thene is less than the other cases.
The reason for this difference could be that in the first case, since the brick surface in contact with the ground is rough, the surface gets greater force of friction. But in second case, when the brick is wrapped with polythene, the surface gets smoother. Hence, less frictional force is required to move it.
By wrapping a piece of jute bag around the brick, it is observed that the reading of spring balance is more than the second
case, when the brick was wrapped with polythene.

Activity 3
Make an inclined plane on a smooth floor, or on a table. You may use a wooden board supported by bricks, or books. [Fig. 12.4 (a)]. Put a mark with a pen at any point A on the inclined plane. Now let a pencil cell move down from this point. How far does it move on the table before coming to rest? Note down the distance. Now spread a piece of cloth over the table. Make sure that there are no wrinkles in the cloth. Try the activity again [Fig. 12.4 (b)].

The pencil cell covers different distances on different surfaces Repeat this activity by spreading a thin layer of sand over the table. Maintain the same slope throughout the activity.
Solution:
The distance covered by the pencil cell is different every time because of the different amounts of friction provided by different surfaces. The distance covered with a thin layer of sand is least, as this surface provides the maximum friction.

Activity 4
Take a few pencils which are cylindrical in shape. Place them parallel to each other on a table. Place a thick book over it (Fig. 12.5). Now push the book. You observe the pencils rolling as the book moves. Do you feel it easier to move the book in this way than to slide it? Do you think that resistance to the motion of the book has been reduced? Have you seen heavy machinery being moved by placing logs under it?

Solution:
It is observed that the book moves efficiently when placed on cylindrical pencils or rollers. It can be concluded that rolling friction is smaller than the sliding friction.

NCERT Solutions for Class 8 Science Chapter 12 – 1 Mark Questions and Answers

Question 1.
Define friction.
Answer:
Friction is the force which resists the motion of a body, while moving on a surface.

Question 2.
What is meant by sliding friction ?
Answer:
When one surface is sliding over the other, sliding friction comes into play.

Question 3.
When does static friction come into play ?
Answer:
Static friction comes into play to counter balance the applied force on the body.

Question 4.
What are lubricants ?
Answer:
The substances which reduce friction are called lubricants.

Question 5.
What is rolling friction ?
Answer:
When one body rolls over the surface of another body, the resistance to its motion is called the rolling friction.

Question 6.
What are fluids ?
Answer:
Fluids is the common name given to gases and liquids.

Question 7.
Do fluids exert friction ?
Answer:
Yes, fluids also exert friction.

Question 8.
What is meant by air resistance ?
Answer:
There is friction between a moving object and the air through which it moves, known as air resistance.

Question 9.
What is meant by drag ?
Answer:
The frictional force exerted by fluids is known as drag.

Question 10.
Is friction a contact force or non-contact force ?
Answer:
Contact force.

Question 11.
Why is it difficult to walk on ice ?
Answer:
It is difficult to walk on ice because the friction between feet and ice is less,

Question 12.
When a boat is moving in the river, does the water offer force of friction to its movement ?
Answer:
Yes, the water opposes the movement of the boat.

NCERT Solutions for Class 8 Science Chapter 12 – 2 Mark Questions and Answers

Question 1.
When your writing desk is tilted a little, a book kept on it starts sliding down. Mention the direction of the frictional force acting on it.
Answer:
The frictional force acts in the upward direction.

Question 2.
You spill a bucket of soapy water on a marble floor accidently. Would it make easier or more difficult for you to walk on the floor ? Why ? [NCERT]
Answer:
It would be more difficult to walk on the floor with soapy water because when we walk on a soapy floor, there is little friction between the floor and the feet, so it becomes difficult to walk.

Question 3.
Put a tick mark on the correct choice :
When one object moves over the surface of other object.

• surface of only lower object exerts frictional force on the upper object.
• surface of only upper object exerts frictional force on the lower object.
• surfaces of both the objects exert force on each other.
• surfaces of both the objects exert force in a direction opposite to each other.

Answer:
Surfaces of both the objects exert force in a direction opposite to each other.

Question 4.
Four children have got similar toy cars. They run the car on the dry marble floor, wet marble floor, newspaper and towel simultaneously. The force of friction between the car and different surfaces in increasing order will be :

• Wet marble floor, dry marble floor, newspaper and towel.
• Newspaper, towel, dry marble floor, wet marble floor.
• Towel, newspaper, dry marble floor, wet marble floor.
• Dry marble floor, wet marble floor, towel, newspaper.

Answer:
Wet marble floor, dry marble floor, newspaper and towel.

Question 5.
Explain why sportsmen use shoes with spikes. [NCERT]
Answer:
Sportsman use shoes with spikes to increase the friction. In this way, the sportsman can run faster.

Question 6.
Iqbal has to push a lighter box and Seema has to push a similar heavier box on the Who will have to apply a larger force and why ? [NCERT]
Answer:
Seema will have to apply more force because the weight of the box increase and the friction increases. For a heavier box the ridges of the surfaces get interlocked more.

Question 7.
Explain why sliding friction is less than static friction. [NCERT]
Answer:
Sliding friction is less than static friction because it comes into play once the body has started sliding. To make the body slide, more force is required to overcome the irregularities in the surface of the body.

Question 8.
Explain why objects moving in fluids must have special shapes. [NCERT]
Answer:
Fluids also exert a frictional force on the body moving through them. Therefore, the shape of the body is streamlined to over the frictional force of fluids and to move smoothly through them.

Question 9.
Why does the force of friction exist ?
Answer:
Friction is caused by the irregularities on the two surfaces in contact.

Question 10.
Can we have surfaces with zero friction ?
Answer:
No, the friction can be less in some surfaces, but it can never be zero.

NCERT Solutions for Class 8 Science Chapter 12 – 3 Mark Questions and Answers

Question 1.
Give examples to show that friction is both a friend and a foe. [NCERT]
Answer:

1. Friction is a friend in the following ways — it helps us to walk, to write on the blackboard and to apply the brakes of a car.
2. Friction is a foe in the following ways — it causes wear and tear of machinery and shoes, it produces heat in the machinery.

Question 2.
Why does friction increase if the two surfaces are pressed ?
Answer:
Since the friction is due to the interlocking of irregularities in the two surfaces which slide with respect to each other, it increases when the two surfaces are pressed harder.

Question 3.
Why it is easier to move a box which is already moving than one which is at rest ?
Answer:
When the box starts sliding, the contact points on its surfaces, do not get enough time to lock, into the contact points on the floor. So, the sliding friction is slightly less than static friction.

Question 4.
Why does a car stop when brakes are applied ?
Answer:
In cars, the friction is increased by using brake pads in the brake system. The brake pads do not touch the wheels but when we press the brake level, these pads arrest the motion of the run due to friction and the wheels stop moving.

Question 5.
Give three factors on which the frictional force in fluids depends.
Answer:
The frictional force in fluids depends on :

• The speed of object with respect to the fluid
• The shape of the object.
• Nature of the fluid.

Question 6.
A labourer has to push a heavy trunk. What should be done to reduce his effort in pushing it and why ?
Answer:
The labourer should put some cylindrical logs of wood under the trunk. In this way the area of contact between the two surfaces is reduced. Rolling friction is less than sliding friction.

Question 7.
Give three harmful effects of friction.
Answer:

• Friction generates heat, for e.g., between the parts of machinery.
• Friction causes wear and tear of the rubbing surfaces e.g., soles of shoes wear out.
• Friction reduces the speed of the body and increases the consumption of energy.

Question 8.
Why is friction essential ? Give three examples.
Answer:

• Friction is necessary between the soles of our feet and ground so that we can walk without slipping.
• Friction helps us in writing on paper.
• Friction makes it possible to light a match stick, by rubbing it along the rough surface of the match box.

Question 9.

1. How do trees help reduce friction ?
2. Why do meteors bum on entering the atmosphere ?

Answer:

1. Trees slow down strong winds and save us from wind storms.
2. The speed of the meteor is very high. Therefore, there is a large amount of friction between the air and meteor. This generates a lot of heat and the meteor starts burning.

Question 10.
Gaurav went to the market on his bicycle. He found that he had to apply a lot of force while pedalling. Why ? What can he do ?
Answer:
Gaurav had to apply more force because there was more friction between the parts of the bicycle. He should apply oil to the parts to reduce the friction.

NCERT Solutions for Class 8 Science Chapter 12 – 5 Mark Questions and Answers

Question 1.
Fill in the blanks : [NCERT]

1. Friction opposes the ………….. between the surfaces in contact with each other.
2. Friction depends on the ………….. of surfaces.
3. Friction produces …………..
4. Sprinkling of powder on the carom board ……….. friction.
5. Sliding friction is …………… than the static friction.

Answer:

1. Relative motion
2. Nature
3. Heat
4. Reduces
5. Surface area

Question 2.
Give five examples to show that friction is increased deliberately.
Answer:

1. Atheletes wear spiked shoes to help them run fast.
2. Mountaineers wear spiked shoes while walking on snow.
3. The tyres of vehicles are grooved.
4. The roads are made rough.
5. The blackboard is made rough.

Question 3.
Why are heat resistant tiles laid along the surface of spaceships ?
Answer:
The spaceships are sent into the space from the rockets with a very high speed. This creates a very high frictional force between the air and spaceship. As result, the spaceship gets heated to red hot. The heat resistant tiles save the spaceship from burning.

Question 4.
What is meant by lubrication ? Why do we lubricate the moving parts of our vehicles ? Name some lubricants and where they are used.
Answer:
Lubrication means oiling the machines from time to time. Lubrication of the moving parts is done to reduce friction between them. We can use oil or grease in machines. Graphite can be used in machines which reach a very high temperature.

Question 5.
What is meant by “streamlined shape” ? Where and why is this shape used ? To whom has nature provided streamlined body ?
Answer:
Streamlined shape means they are narrow in front and broader at the back. The bodies of aeroplanes, missiles and rockets are streamlined to reduce friction with air. Ships and boats have a streamlined shape to reduce friction with air. Nature has given birds and fish streamlined bodies.

NCERT Solutions for Class 8 Science Chapter 12 MCQs

Question 1.
Suppose your writing desk is tilted a little. A book kept on it, starts sliding down. The figure, showing the correct direction of frictional-force acting on it, is

Answer:
(d)

Question 2.
Friction, that exists between two surfaces in contact, when there is no relative motion between them, is called
(a) sliding friction
(b) static friction
(c) viscous drag
(d) rolling friction
Answer:
(b)

Question 3.
Four students were asked to arrange forces due to rolling, static and sliding frictions in an increasing order. Their arrangements are listed below. Choose the correct arrangement. [NCERT]
(a) rolling, static, sliding
(b) rolling, sliding, static
(c) static, sliding, rolling
(d) static, rolling, sliding
Answer:
(b)

Question 4.
The energy required to overcome friction is mainly converted into
(a) sound energy
(b) heat energy
(c) light energy
(d) chemical energy
Answer:
(b)

Question 5.
Out of the following, the better lubricant to be used in the moving parts of a machine,
(a) water
(b) air
(c) chalk powder
(d) turpentine oil
Answer:
(d)

Question 6.
Force of friction is more in
(a) marble tiles
(b) wooden floor
(c) playground
(d) glass table
Answer:
(c)

Question 7.
Once a body starts moving on table, the friction which comes into play is
(a) static friction
(b) sliding friction
(c) limiting friction
(d) none of these
Answer:
(b)

Question 8.
The force of friction that comes into play when one body rolls over another surface, is
(a) sliding friction
(b) limiting friction
(c) rolling friction
(d) static friction
Answer:
(c)

Question 9.
These days we use suitcases with wheels because
(a) they look smart
(b) they are easy to carry
(c) they make less noise a
(d) none of these
Answer:
(b)

Question 10.
Tyres have cut grooves in them
(a) to increase friction
(b) to decrease friction .
(c) to make them look attractive
(d) to save rubber
Answer:
(a)

Force and Pressure Class 8 Science NCERT Textbook Questions

Question 1.
Give two examples each of the situations in which you push or pull to change the state of motion of objects.
Answer:
(i) Push: We close drawer by pushing.
We move a wooden box by pushing.
(ii) Pull: We draw water from a well by pulling the rope.
A horse pulls a cart.

Question 2.
Give two examples of situations in which applied force causes a change in the shape of an object.
Answer:
When we apply force on a rubber band to stretch it and on clay to change its shape.

Question 3.
Fill in the blanks in the following statements.
(a) To draw water from a well we have to ______ at the rope.
(b) A charged body ______ an uncharged body towards it.
(c) To move a loaded trolley we have to ______ it.
(d) The north pole of a magnet _______ the north pole of another magnet.
Answer:
(a) pull
(b) attracts
(c) push
(d) repels

Question 4.
An archer stretches her bow while taking aim at the target. She then releases the arrow, which begins to move towards the target. Based on this information fill up the gaps in the following statements using the following terms:
muscular, contact, non-contact, gravity, friction, shape, attraction
(a) To stretch the bow, the archer applies a force that causes a change in its ______
(b) The force applied by the archer to stretch the bow is an example of ______ force.
(c) The type of force responsible for a change in the state of motion of the arrow is an example of a ______ force.
(d) While the arrow moves towards its target, the forces acting on it are due to _______ and that due to _____ of air.
Answer:
(a) shape
(b) muscular
(c) contact
(d) gravity, friction

Question 5.
In the following situations identify the agent exerting the force and the object on which it acts. State the effect of the force in each case.
(a) Squeezing a piece of lemon between the fingers to extract its juice.
(b) Taking out paste from a toothpaste tube.
(c) A load suspended from a spring while its other end is on a hook fixed to a wall.
(d) An athlete making a high jump to clear the bar at a certain height.
Answer:
(a) Agents are fingers, object is lemon, effect of force changes the shape of lemon.
(b) Agents are fingers of the person squeezing the tube, object is toothpaste tube and effect of the force can be observed as the paste coming out of the tube (change in shape).
(c) Agent is the load suspended, object is the spring and effort can be seen in the form of elongation of spring on suspension of load (change in shape).
(d) Agent is muscles of athlete, object is athlete himself and effect of the force changes the state of motion of the athlete.

Question 6.
A blacksmith hammers a hot piece of iron while making a tool. How does the force due to hammering affect the piece of iron?
Answer:
The force due to hammering causes the change in the shape of the iron and iron can be moulded in the shape of the required tool.

Question 7.
An inflated balloon was pressed against a wall after it has been rubbed with a piece of synthetic cloth. It was found that the balloon sticks to the wall. What force might be responsible for the attraction between the balloon and the wall?
Answer:
Electrostatic force.

Question 8.
Name the forces acting on a plastic bucket containing water held above ground level in your hand. Discuss why the forces acting on the bucket do not bring a change in its state of motion.
Answer:
Forces acting on bucket are as follows:
(i) Muscular force of arms acting upward.
(ii) Force of gravity acting downward.
Both the forces do not bring any change in the state of motion because both of them are acting in equal and opposite directions and thus they cancel each other’s effect.

Question 9.
A rocket has been fired upwards to launch a satellite in its orbit. Name the two forces acting on the rocket immediately after leaving the launching pad.
Answer:
The forces that act when a rocket leaves launching pad are as follows:
(i) Gravitational force of the earth (downward)
(ii) Frictional force of air (in opposite direction)

Question 10.
When we press the bulb of a dropper with its nozzle kept in water, air in the dropper is seen to escape in the form of bubbles. Once we release the pressure on the bulb, water gets filled in the dropper. The rise of water in the dropper is due to
(a) pressure of water
(b) gravity of the earth
(c) shape of rubber bulb
(d) atmospheric pressure
Answer:
(d) atmospheric pressure

Force and Pressure Class 8 Science NCERT Intext Activities Solved

Activity 1
Table 11.1 gives some examples of familiar situations involving the motion of objects. You can add more such situations or replace those given here. Try to identify action involved in each case as a push and/or a pull and record your observations. One example has been given to help you.
Identifying Actions as Push or Pull

Activity 2

Choose a heavy object like a table or a box, which you can move only by pushing hard. Try to push it all by yourself. Can you move it? Now ask one of your friends to help you in pushing it in the same direction [Fig. 11.1(a)]. Is it easier to move it now? Can you explain why? Now push the same object, but ask your friend to push it from the op¬posite side [Fig. 11.1 (b)]. Does the object move? If it does, note the direction in which it moves. Can you guess which one of you is applying a larger force?

Solution:
(i) The box moves with difficulty when we push alone but moves easily if helped by our friend. Thus, force applied on an object in the same direction add to one another.
(ii) When the object is pushed from the opposite side, it does not move. Thus, if two forces acting in the opposite direction on an object the net force acting on it is the difference between the two forces.

Activity 3
Jake a rubber ball and place it on a level surface such as a tabletop or a concrete floor. Now, gently push the ball along the level surface (Fig. 11.2). Does the ball begin to move? Push the ball again while it is still moving. Is there any change in its speed? Does it increase or decrease?
Next, place your palm in front of the moving ball. Remove your palm as soon as the moving ball touches it. Does your palm apply a force on the ball? What happens to the speed of the ball now? Does it increase or decrease? What would happen if you let your palm hold the moving ball?

Solution:
When we push the ball along the level surface, the ball begin to move. When we push the ball again while it is still moving, increases its speed.
When we place our palm in front of the moving ball, decreases its speed. Hence, force can change the state of motion.

Activity 4

Take a ball and place it on a level surface as you did in Activity 11.3. Make the ball move by giving it a push. Now place the ruler from your geometry box in its path as shown in Fig. 11.3. In doing so, you would apply a force on the moving ball. Does the ball continue to move in the same direction after it strikes the ruler? Repeat the activity and try to obstruct the moving ball by placing the ruler such that it makes different angles to its path. In each case note your observations about the direction of motion of the ball after it strikes the ruler.
Solution:
Ball changes direction after it strikes the ruler. Thus, a force can change direction of motion of a moving object.

(a) A ball set in motion by pushing it along a level surface and
(b) the direction of motion of the ball after it strikes the ruler placed in its path

Activity 5
Some situations have been given in Column 1 of Table 11.2 in which objects are not free to move. Column 2 of the Table suggests the manner in which a force can be applied to each object while Column 3 shows a diagram of the action. Try to observe the effect of the force in as many situations as possible. You can also add similar situations using available material from your environment. Note your observations in Columns 4 and 5 of the table.
Studying the Effect of Force on Objects

Solution:
From the above actions, we can conclude that force can change the shape of an object.

Activity 6

Take a pair of bar magnets. Place the longer side of one of the magnets over three round shaped pencils or wooden rollers as shown in Fig. 11.4. Now bring one end of the other magnet near the end of the magnet placed on the rollers. Make sure that the two magnets do not touch each other. Observe what happens. Next, bring the other end of the magnet near the same end of the magnet placed on the rollers (Fig. 11.4). Note what happens to the magnet placed on the rollers every time another magnet is brought near it.

Solution:
When unlike poles of the magnet are brought near the other magnet, they attract each other and moves in the direction of another magnet. When like poles of a magnet are brought near to the other magnet, they repel each other and moves in the direction away from the other magnet.
Thus, the force exerted by a magnet on another magnet is a type of non-contact force.

Activity 7
Take a plastic straw and cut it into nearly two equal pieces. Suspend one of the pieces from the edge of a table with the help of a piece of thread (Fig. 11.5). Now hold the other piece of straw in your hand and rub its free end with a sheet of paper. Bring the rubbed end of the straw near the suspended straw. Make sure that the two pieces do not touch each other. What do you observe?
Next, rub the free end of the suspended piece of straw with a sheet of paper. Again, bring the piece of straw that was rubbed earlier with paper near the free end of the suspended straw. What do you observe now?

Solution:
A straw rubbed with paper attracts another straw but repels it if it has also been rubbed with a sheet of paper.The force exerted by a charged body on another charged or uncharged body is known as electrostatic force.

Activity 8
Take a transparent glass tube or a plastic pipe. The length of the pipe/tube should be about 15 cm and its diameter should be 5-7.5 cm. Also take a piece of thin sheet of a good quality rubber, say, a rubber balloon. Stretch the rubber sheet tightly over one end of the pipe. Hold the pipe at the middle, keeping it in a vertical position (Fig. 11.6). Ask one of your friends to pour some water in the pipe. Does the rubber sheet bulge out? Note also the height of the water column in the pipe. Pour some more water. Observe again the bulge in the rubber sheet and the height of the water column in the pipe. Repeat this process a few more times. Can you see any relation between the amount of the bulge in the rubber sheet and the height of the water column in the pipe?

Solution:
The amount of bulge in the rubber sheet increases with the height of the water column in the pipe. Thus, the pressure exerted by water at the bottom of the container depends upon the height of its column.

Activity 9
Take a plastic bottle. You can take a discarded water or soft drink bottle. Fix a cylindrical glass tube, a few cm long near its bottom as shown in Fig. 11.7. You can do so by slightly heating one end of the glass tube and then quickly inserting it near the bottom of the bottle. Make sure that the water does not leak from the joint. If there is any leakage, seal it with molten wax. Cover the mouth of the glass tube with a thin rubber sheet as you did in Activity 11.8. Now fill the bottle upto half with water. What do you observe? Why does the rubber sheet fixed to the glass tube bulge this time? Pour some more water in the bottle. Is there any change in the bulge’of the rubber sheet?

Solution:
Yes, the rubber tube bulge out and the bulging increases with the amount of water in the bottle. Thus, liquid exert pressure on the walls of the container.

Activity 10
Take an empty plastic bottle or a cylindrical container. You can take a used tin of talcum powder or a plastic bottle. Drill four holes all around near the bottom of the bottle. Make sure that the holes are at the same height from the bottom (Fig. 11.8). Now fill the bottle with water. What do you observe?
Do the different streams of water coming out of the holes fall at the same distance from the bottle? What does this indicate?

Solution:
We observed that different streams of a waterfall at the same distance from the bottle. Thus, it indicates that liquids exert equal pressure at the same depth.

Activity 11
Take a good quality rubber sucker. It looks like a small rubber cup (Fig. 11.9). Press it hard on a smooth plane surface. Does it stick to the surface? Now try to pull it off the surface. Can you do it?

Solution:
when we press the sucker, most of the air between its cup and the surface escapes out. The sucker sticks to the surface because the pressure of the atmosphere acts on it. To pull the sucker off the surface, the applied force should be large enough to overcome the atmospheric pressure. Thus, this activity gives us an idea about the magnitude of atmospheric pressure.

NCERT Solutions for Class 8 Science Chapter 11 – 1 Mark Questions and Answers

Question 1.
When we press the bulb of a dropper with its nozzle kept in water, air in the dropper seems to escape in the form of bubbles. Once we release the pressure on the bulb, water gets filled in the dropper. The rise of water in the dropper is due to [NCERT]

• pressure of water
• gravity of the earth.
• shape of rubber bulb.
• atmospheric pressure.

Answer:
atmospheric pressure

Question 2.
An inflated balloon was pressed against a wall after it has been rubbed with a piece of synthetic cloth. It was found that the balloon sticks to the wall. What force might be responsible for the attraction between the balloon and the wall ? [NCERT]
Answer:
Electrostatic force.

Question 3.
Define force.
Answer:
Force is a push or pull on an object.

Question 4.
How many objects should be present for a force to come into play ?
Answer:
There should be atleast two objects for a force to come into play.

Question 5.
Two friends A and B are applying a force of 2 newton and 4 newton on a box in the same direction. What will be the total force applied by them ?
Answer:
The total force will be 6 newton, i.e., the sum of their individual forces.

Question 6.
In a tug of war, side A applies 10 newton force and side B applies 8 newton force. Which side will the rope move ?
Answer:
The rope will move towards side A as more force is applied by side A.

Question 7.
What happens to the speed of a body when a force is applied ?
Answer:
The speed of a body can be increased or decreased by applying force.

Question 8.
Can we change the direction of the moving object by applying a force ?
Answer:
Yes, we can change the direction of the moving object by applying a force.

Question 9.
What is meant by change in state of motion of the object ?
Answer:
Any change in the speed or direction of motion or both means a change in state of motion of the object.

Question 10.
Is it possible that a force changes the direction of motion but not the speed of an object ?
Answer:
Yes, it is possible when a body is moving on a circular path.

Question 11.
Give an example to show that force can change the shape of an object.
Answer:
Pressing a rubber ball with the hand changes its shape

Question 12.
What is meant by muscular force ?
Answer:
The force resulting due to the action of muscles is known” as the muscular force.

Question 13.
What is meant by contact force ?
Answer:
A force which is applied only when it is in contact with an object is called a contact force.

Question 14.
Does the force of friction also act on the objects moving in the air ?
Answer:
Yes, air also offers friction to objects moving in air.

Question 15.
Is it essential for the agent applying a force on an object to be in contact ?
Answer:
No, the force can also act from a distance. It is known as non-contact force.

Question 16.
Give one example of a force which can act from a distance.
Answer:
Magnetic force, i.e., the force exerted by a magnet on another magnet or a piece of iron.

Question 17.
What is meant by force of gravitation ?
Answer:
The force of attraction exerted by the earth on all objects is called the force of gravitation.

Question 18.
Is the force of gravity a contact force or non-contact force ?
Answer:
Force of gravity is a non-contact force.

Question 19.
Which force is responsible for the weight of objects ?
Answer:
The force of gravity is responsible for the weight of objects.

Question 20.
Does the force of gravitation exist between two objects on the earth ?
Answer:
Yes, the force of gravitation exists between two objects on the earth but it is very weak.

NCERT Solutions for Class 8 Science Chapter 11 – 2 Mark Questions and Answers

Question 1.
Why is it comfortable to lift a school bag with broad straps than thin straps ? [NCT 2008]
Answer:
Pressure is inversely proportional to area. Since broader straps have greater area, therefore, the pressure decreases.

Question 2.
Why do mountaineers suffer from nose bleeding at high altitudes ?
Answer:
The atmospheric pressure decreases with high altitude. Since the pressure of the blood inside the body is high, the nose starts bleeding.

Question 3.
Why is easier to hammer a sharp nail into wood than a blunt one ?
Answer:
Pressure = force / area.
Therefore, when we hammer a sharp nail, force acts on a smaller area, and it exerts more pressure on the nail.

Question 4.
How would pressure change if

1. area is doubled keeping force constant
2. force is doubled keeping area constant ?

Answer:

1. If area is doubled keeping the force constant, then pressure becomes half.
2. If force is doubled keeping area constant, then pressure becomes double.

Question 5.
Why are caterpillar tracks used in battle tanks instead of tyres ?
Answer:
Caterpillar tracks are used in battle tanks instead of tyres to increase the area of contact. As a result they can even cross sinking grounds as the pressure exerted on the ground is less.

Question 6.
Give two examples each of situations in which you push or pull to change the state of motion of objects. [NCERT]
Answer:
Push — moving a loaded cart, batsman hitting a ball.
Pull — opening a drawer, drawing a bucket of water from a well.

Question 7.
Give two examples of situations in which applied force causes a change in the shape of an object. [NCERT]
Answer:

• Pressing a lump of dough with hand.
• Pressing an inflated balloon.

Question 8.
A blacksmith hammers a hot piece of iron while making a tool. How does the force due to hammering affect the piece of iron ? [NCERT]
Answer:
The piece of iron becomes flattened due to the force of hammering.

Question 9.
A camel is able to move fast on sand. Why ?
Answer:
A camel is able to move fast on sand because it has flat broad feet. This increases the area of contact and therefore, the pressure exerted by the camel on the sand reduces. Hence, the camel’s feet sink very little in the sand.

Question 10.
When does a force come into play ?
Answer:
An object’s interaction with another object results in a force between the two objects.

Question 11.
If the force is applied opposite to the motion, what will happen to the speed of the object ?
Answer:
When the force is applied opposite to the motion of the object, then either the speed decreases or the direction changes.

Question 12.
State the two factors which describe the state of motion of an object.
Answer:
The state of motion is described by its speed and direction of motion.

Question 13.
A stone is tied to a thread and moved in a circular path. Is any force required to do this ? Is there any change in speed of the stone ?
Answer:
Yes, a force is required to keep the stone moving along a circular path with a constant speed, i.e., the speed does not change.

Question 14.
How do the mud particles fly off the wheels of a vehicle moving on the wet road ?
Answer:
The direction of the mud particles change at every point as the wheels of the vehicle move.

Question 15.
Can muscular force be applied on an object without being directly in contact with it ?
Answer:
No, muscular force can be applied only when it is in contact with an object.

Question 16.
Why does a boat come to rest when we stop rowing it ?
Answer:
The boat comes to rest when we stop rowing it due to the force of friction acting between the surface of water and the boat.

NCERT Solutions for Class 8 Science Chapter 11 – 3 Mark Questions and Answers

Question 1.
Define Pressure. Write the relation between pressure force and area. Name the instrument used to measure atmospheric pressure. [NCT 2009]
Answer:
Pressure is force per unit area.
Pressure = Force/Area
Barometer is use to measure atmospheric pressure.

Question 2.
Why is it difficult to cut vegetables with a blunt knife ?
Answer:
Pressure is inversely proportional to area. The area of the blunt knife is more and therefore, the effect of the force is less. Therefore, more force has to be applied.

Question 3.
Trucks intended to carry heavy loads have eight tyres instead of four tyres. Why ?
Answer:
Trucks intended to carry heavy loads have eight tyres, so as to increase the area of contact with the road. Since pressure is inversely proportional to area, less pressure is applied on the road.

Question 4.
Give reasons for the following :

1. The skiers use flat and broad skis
2. Deep sea divers wear special suits.

Answer:

1. The skiers use flat and broad skis to ski on the snow. The larger surface of skis reduces pressure on snow and helps them to slide instead of sinking.
2. Deep sea divers wear special suits, because the pressure of water increases with depth. The increased pressure may hurt the body of divers.

Question 5.
How does the medicine enter a dropper ?
Answer:
When the dropper is pressed, the air inside the dropper is driven out. The pressure inside the dropper decreases and the medicine rushes inside the dropper.

Question 6.
You are given rubber sucker with a hook. Can you use it for hanging articles ? Explain how ? Give the principle involved.
Answer:
We can use it for hanging articles. On pressing the sucker, the air between the cup and the surface escapes out. The pressure inside is reduced, but the atmospheric pressure is more. Therefore, the sucker remains stuck.

Question 7.
A boy throws a ball upwards, but it comes down after sometime. Why ? Which force is acting on the ball ? What type of force is it ?
Answer:
The ball comes down to the force of gravity exerted by the earth. Force of gravity is a non-contact force and it acts on a body even though the two are not in contact with each other.

Question 8.

1. Name the two contact forces.
2. Why do we observe sparks coming out from a synthetic shirt when we take it off our body ?
3. Which force pulls the iron nails towards a magnet ?

Answer:

1. Muscular force and force of friction.
2. The sparks come out from a synthetic shirt due to the electrostatic charge on it.
3. Magnetic force.

NCERT Solutions for Class 8 Science Chapter 11 – 5 Mark Questions and Answers

Question 1.
Fill in the blanks in the following statements : [NCERT]

1. To draw water from a well we have to ………….. at the rope.
2. A charged body ……….. an uncharged body towards it.
3. To move a loaded trolley we have to ……………. it.
4.  The north pole of a magnet …………… the north pole of another magnet.

Answer:

1. pull.
2. attracts
3. push
4. repels

Question 2.
An archer stretches her bow while taking aim at the target. She then releases the arrow, which begins to move towards the target. Based on this information fill up the gaps in the following tatements using the following terms.
muscular, contact, non-contact, gravity, friction, shape, attraction. [NCERT]

1. To stretch the bow, the archer applies a force that causes a change in its ………….
2. The force applied by the archer to stretch the bow is an example of ………….. force.
3. The type of force responsible for a change in the state of motion of the arrow is an example of a ……………. force.
4. While the arrow moves towards its target, the forces acting on it are due to and ……………… of air.

Answer:

1. shape
2. muscular
3. contact
4. gravity, friction

Question 3.
In the following situations identify the agent exerting the force and the object on which it acts. State the effect of the force in each case. [NCERT]
(a) Squeezing a piece of lemon between the fingers to extract its juice.
(b) Taking out paste from a toothpaste tube.
(c) A load suspended from a spring while its other end is on a hook fixed to a wall.
(d) An athlete making a high jump to clear the bar at a certain height.
Answer:

 Question 4.
Name the forces acting on a plastic bucket containing* water held above ground level in your hand, Discuss why the forces acting on the bucket do not bring a change in its state of motion. [NCERT]
Answer:
The forces acting on the bucket is its own weight acting downwards and the muscular force of the hand acting upwards. Since both the forces are equal and acting in opposite directions, they balance each other. So, they donot bring any change in the state of motion of bucket. The hand feels tired due to the weight of bucket.

Question 5.

1. While constructing dams, the base is made more wide. Why ?
2. Why does blood ooze out when there is a slight cut on your body ?

Answer:

1. The pressure of liquids increases as the depth increases. Therefore, the base is made wider to withstand the high pressure of water.
2. Blood pressure is little greater than atmospheric pressure. Due to this, the blood oozes out when we have a slight cut on our body.

Question 6.

1. Define atmospheric pressure.
2. What happens to the atmospheric pressure on mountains ?
3. What makes a balloon stretch if it is filled with air ?

Answer:

1. Atmospheric pressure is the pressure exerted by air.
2. As we go on the mountains, the atmospheric pressure decreases.
3.  The air exerts pressure on the balloon and therefore, it stretches.

Question 7.

1. Define one atmosphere.
2. Where is the pressure greater, 10 m below the surface of sea or 20 m below ?
3. Where is pressure greatest and the least inside a bottle filled with water.

Answer:

1. The pressure which can support 76 cm ornercury in a mercury barometer, is called one atmosphere.
2. 20 m below the surface of sea the pressure is greater.
3. The pressure is greatest at the bottom and least on the surface of water filled in a bottle.

Question 8.

1. What is weight ?
2. What is the unit of weight ? .
3. Name the device used for measuring the weight of an object.
4. Can weight be taken as a measure of force ?

Answer:

1. Weight of an object is the force of gravity acting on the object.
2. Unit of weight is newton or kilogram weight.
3. Spring balance is used for measuring the weight of an object.
4. Yes, the weight can be taken as a measure of force.

Question 9.
Name the type of force in the following cases.

1. Raindrops falling on the earth.
2. Holding a book on your hand.
3. Running a comb through your dry hair.
4. A bar magnet suspended freely.
5. Bullocks ploughing the field.

Answer:

1. Force of gravity.
2. Force of gravitation (weight).
3. Electrostatic force.
4. Magnetic force.
5. Muscular force.

Question 10.
Using a manometer how can you show that the liquid pressure remains same at the same depth?
Answer:
Take a large glass jar full of water. Slowly lower the manometer into the water. Keep the funnel at the same depth, but pointing in different directions. You will notice that the difference in the liquid level remains the same. This proves that the pressure in a liquid remains the same in all directions at the same depth.

Question 11.
What happens to the atmospheric pressure if,

1. the temperature is high.
2. the humidity in air increases.
3. metrological office; predicts fair weather
4. there is a storm. .
5. the weather is dry.

Answer:

1. Atmospheric pressure decreases.
2. Atmospheric pressure decreases.
3. Atmospheric pressure increases.
4. Atmospheric pressure decreases.
5. Atmospheric pressure increases.

NCERT Solutions for Class 8 Science Chapter 11 MCQs

Question 1.
A batsman hits a cricket ball which then rolls on the level ground. After covering a short
distance the ball comes to rest. The ball stops due to
(a) magnetic force
(b) frictional force
(c) gravitational force
(d) muscular force
Answer:
(b)

Question 2.
When two forces applied on an object are equal and opposite, then these forces
(a) may move the object.
(b) may stop the object.
(c) may move the object and also cause a change in its shape.
(d) do not move the object but may cause a change in its shape.
Answer:
(d)

Question 3.
When two unbalanced forces act on a body, in opposite directions, the net force is equal to
(a) the sum of the individual unbalanced forces.
(b) zero.
(c) difference between the two unbalanced forces and is in the direction of the larger force.
(d) difference between the two unbalanced forces and is in the direction of smaller force.
Answer:
(c)

Question 4.
Nails have pointed ends. This results in
(a) a decrease in the force exerted on them.
(b) a decrease in the effect of the force exerted on them.
(c) an increase in the force exerted on them.
(d) an increase in the effect of the force exerted on them.
Answer:
(c)

Question 5.
Which of the following is an example of contact force ?
(a) Magnetic force
(b) Muscular force
(c) Electric force
(d) Gravitational force
Answer:
(b)

Question 6.
Fruits falling from trees is an example of
(a) gravitational force
(b) muscular force
(c) frictional force
(d) electric force
Answer:
(a)

Question 7.
The unit of measuring pressure is
(a) newton
(b) newton/metre
(c) metre2
(d) metre2/newton
Answer:
(b)

Question 8.
In liquids, the pressure
(a) increases with depth
(b) decreases with depth
(c) remains same at all depths
(d) sometimes increases sometimes decreases
Answer:
(a)

Question 9.
During dry weather, rubbing a plastic scale with dry hair, attracts small pieces of paper. This is due to
(a) gravitational force
(b) electrostatic force
(c) frictional force
(d) muscular force
Answer:
(b)

Reaching the Age of Adolescence Class 8 Science NCERT Textbook Questions

Question 1.
What is the term used for secretions of endocrine glands responsible for changes taking place in the body?
Answer:
Hormones

Question 2.
Define adolescence.
Answer:
The period of life, when the body undergoes changes, leading to reproductive maturity, is called adolescence. The period of adolescence is normally 11 years to 19 years.

Question 3.
What is menstruation? Explain.
Answer:
In females, the ova or eggs begin to mature with the onset of puberty (10 to 12 years of age) one egg get matured and is released by one of the ovaries once in about 28 to 30 days. During this period, the wall of the uterus becomes thick so as to receive the fertilized egg. In case it is fertilized, it begins to develop. This results in pregnancy. If the fertilization fails to take place, the ovum or the released egg and the thickened lining of the uterus along with its blood vessels are shed off. This causes bleeding in women. This is called menstruation. It occurs once in 28 to 30 days.

Question 4.
List changes in the body that take place at puberty.
Answer:
The changes take place differently in the body of boys and girls at puberty. These change may be listed as below:

• There is sudden increase in the height of both boys and girls. However, both reach their maximum height at the age of 17 to 18 years.
• The voice of boys become hoarse and that of girls become shrill.
• Shoulders become broader and muscles grow more prominently in boys. In girls, the regions below the waist becomes wider.
• Hair start growing in different parts of boys and girls.
• In most of the teenagers, due to increased secretion of sweat and sebaceous gland, get acne and pimples on their face.
• Onset of puberty brings changes in secondary sexual characters in both male and female.
• Boys and girls become capable of reproduction.
• In girls, menstruation starts.
• Different types of hormones start to release in initiating a reproductive function.

Question 5.
Prepare a table having two columns depicting names of endocrine glands and hormones secreted by them.
Answer:

Question 6.
What are sex hormones? Why are they named so? State their function.
Answer:
The hormones which help and control the formation of secondary sexual characters are called sex hormones. They are named so because they control the sexual activities and are secreted by males and females separately.
Functions of sex hormones are given below separately:
Male sex hormone: It is also called testosterone. It is secreted by testes and causes changes in secondary sexual character of boys like growth of facial hair like moustache, beard, etc. It also stimulates spermatogenesis.
Female sex hormone: It is also called estrogen. It is secreted by ovaries and controls the secondary sexual characters in females, appearance of mammary glands, etc. It also maintains pregnancy.

Question 7.
Choose the correct option.
(a) Adolescents should be careful about what they eat, because:
(i) proper diet develops their brains.
(ii) proper diet is needed for the rapid growth taking place in their body.
(iii) adolescents feel hungry all the time.
(iv) taste buds are well developed in teenagers.

(b) Reproductive age in women starts when their:
(i) menstruation starts.
(ii) breasts start developing.
(iii) body weight increases.
(iv) height increases.

(c) The right meal for adolescents consists of:
(i) chips, noodles, coke.
(ii) chapati, dal, vegetables.
(iii) rice, noodles and burger.
(iv) vegetable cutlets, chips and lemon drink.
Answer:
(a) (ii)
(b) (i)
(c) (ii)

Question 8.
Write notes on:
(a) Adam’s apple
(b) Secondary sexual characters.
(c) Sex determination in the unborn baby.
Answer:
(a) Adam’s apple: During puberty, a change in the voice of boys and girls takes place due to increase in the size of the voice box or larynx. In boys, the voice box tends to protrude out on the upper part of the neck, below the chin and is often referred to as Adam’s apple.

(b) Secondary sexual characters: Those characters which are visible more apparently and help in distinguishing a male from a female are called secondary sexual characters. For example, in girls, breasts and in boys, facial hair, i.e., moustache and beard.

(c) Sex determination in the unborn baby: The sex of a child, i.e., whether it is a male or a female is determined at the time of fertilisation when a male gametes fuse with a female gamete. All human beings have 23 pairs of chromosomes in the nuclei of their cells. Two chromosomes out of these are sex chromosomes. A female has two X chromosomes, while a male has one X and one Y chromosome. The gametes (egg and sperm) have only one set of chromosomes. The unfertilised egg always has one X chromosome.

But sperms are of two kinds—One having X chromosome, and the other having Y chromosome. When a sperm containing X chromosome fertilises the egg, the zygote would have two X chromosomes and develop into a female child. If the sperm contributes a Y chromosome to the egg or ovum at fertilisation, the zygote would develop into a male child. It is thus also clear that the sex chromosomes of the father determine the sex of an unborn baby.

Question 9.
Word game: Use the clues to work out the words.
Across:
3. Protruding voice box in boys
4. Glands without ducts
7. Endocrine gland attached to brain
8. Secretion of endocrine glands
9. Pancreatic hormone
10. Female hormone
Down:
1. Male hormone
2. Secretes thyroxine
3. Another term for teenage
5. Hormone reaches here through blood stream
6. Voice box
7. Term for changes at adolescence

Answer:

Question 10.
The table below shows the data on likely heights of boys and girls as they grow in age. Draw graphs showing height and age for both boys and girls on the same graph paper. What conclusions can be drawn from these graphs?

Answer:
From the above graph, we conclude that initially boys grow faster than girls from 0-8 years, and girls grow faster than boys from 8-16 years. But both reach their maximum height at the age of 20. Here boys show a greater increase in height than girls.

Reaching the Age of Adolescence Class 8 Science NCERT Intext Activities Solved

Activity 1
The following chart gives the average rate of growth in height of boys and girls with age. The figures in columns 2 and 3, give the percentage of the height a person has reached at the age given in column 1. For example, by the age 11, a boy has reached 81 % of this probable full height, while a girl has reached 88% of her full height. These figures are only representative and there may be individual variations.
Use the Table for your friends and work out how tall they are likely to be. Find out who is likely to be the tallest and who might be the shortest in your class.

Activity 2

Calculation for full height (cm) = Presentheight(cm)percentageoffullheightatthisage×100 (as given in the chart)
Example: A boy is 9 years old and 120 cm tall. At the end of the growth period he is likely to be 12075×100 = 160 cm tall
Solution:
Students are suggested to find out the tallest and the shortest boy in their class, using the table shown above. Using the formula you can calculate your present height.

Use the data given in Activity 10.1 to draw a graph. Take age on the X-axis and per cent growth in height on the Y-axis. Highlight the point representing your age on the graph. Find out the percentage of height you have already reached. Calculate the height you might eventually reach. Tally your graph with the one given here (Fig. 10.2).

Solution:
You will notice that initially, girls grow faster than boys but by about 18 years of age, both reach their maximum height. The rate of growth in height varies in different individuals. Some may grow suddenly at puberty and then slow down, while others may grow gradually.

Activity 3

Collect information from magazines or from doctors and prepare a note on the importance of consuming iodised salt. You can also look for this information on the internet.
Solution:
Iodine is necessary for the thyroid gland to produce thyroxine hormone. It regulates the metabolism of carbohydrates, fats and proteins, so as to produce the best balance for the growth. Deficiency of iodine in our diet leads to reduction in the formation of thyroxine hormone causing goitre disease.

Activity 4

Make a group with your friends. Write down the items of food in your breakfast, lunch and dinner you had on the previous day. Identify the items responsible for proper growth. Also identify the junk food that you consumed the previous day.
Solution:
Do it yourself.

Activity 5
Get ideas from the pictures given in Fig. 10.3. Prepare charts or posters and paste them in the class so that you are aware of the diet for adolescents. You may use your creative ideas and present it like an advertisement. You may even organise a competition on this topic.

Solution:
You have already studied about food and its components and you are aware of the importance of various ingredients of food like vitamins, proteins, carbohydrates, fats, minerals, water, etc. So you can do this activity yourself.

Activity 6
Collect data on the number of children in your class who exercise regularly and who do not exercise regularly. Did you notice any difference in their fitness and health? Prepare a report on the benefits of regular exercise.
Solution:
Doing exercises daily keeps one fit and healthy. It is a popular saying that a sound mind resides in a sound body’, i.e., if you are healthy or physically fit, you are mentally sound also.
You can collect data on the number of children in your class who do exercise regularly and who do not. You will notice the difference clearly in their fitness and health. Also, prepare a note on the benefits of regular exercise.

NCERT Solutions for Class 8 Science Chapter 10 – 1 Mark Questions and Answers

Question 1.
What is the other name for duct gland ?
Answer:
Exocrine glands.

Question 2.
Name two duct glands.
Answer:
Tear glands and sweat glands.

Question 3.
What are ductless glands ?
Answer:
Ductless glands are those glands, from which the hormones are directly poured into the blood.

Question 4.
Name any two ductless glands.
Answer:
Pituitary gland and thyroid gland.

Question 5.
Define hormones.
Answer:
Hormones are secretions from endocrine glands.

Question 6.
Name a gland whose size gets reduced as the child grows older.
Answer:
Thymus gland.

Question 7.
Name the hormone secreted by

• adrenal gland
• pancreas.

Answer:

• Adrenalin
• Insulin.

Question 8.
Which gland secretes the hormone thyroxine ?
Answer:
Thyroid gland.

Question 9.
Which gland is known as the master gland ?
Answer:
Pituitary gland.

Question 10.
Name the female sex hormone and where is it produced ?
Answer:
Estrogen is produced in the ovary.

Question 11.
Name the male sex hormone and where is it produced ?
Answer:
Testosterone is produced in the testes.

Question 12.
What is the main function of the sex hormones ?
Answer:
The main function of sex hormones is to control sexual characteristics.

Question 13.
Name the sex glands in male and female.
Answer:
The sex glands are testes in male and ovaries in female.

Question 14.
Apart from estrogen, name the other female hormone secreted by ovaries.
Answer:
Progesterone.

Question 15.
Which gland is responsible for the fatness in the body ?
Answer:
Thyroid gland.

Question 16.
Which hormone causes sweating under fear ?
Answer:
Adrenalin.

Question 17.
What do you understand by puberty ?
Answer:
The age at which girls and boys attain reproductive maturity is known as puberty.

Question 18.
Which is the age of adolescence ?
Answer:
The period of development from childhood to womanhood or manhood is the period of adolescence.

Question 19.
Where are hormones made in the human body ?
Answer:
Endocrine or ductless glands.

Question 20.
What is menarche ?
Answer:
The first menstrual flow is known as menarche.

NCERT Solutions for Class 8 Science Chapter 10 – 2 Mark Questions and Answers

Question 1.
Give the right word for : [NCT 2005]

1. A hormone that helps in reproduction.
2. A gland in the neck, which when swollen causes goitre.
3. A hormone which helps to balance the suger in the body.
4. An element without which thyroxine hormone is not formed. .

Answer:

1. Gonadotropin
2. Thyroid
3. Insulin
4. Iodine

Question 2.
Distinguish between exocrine and endocrine glands.
Answer:
Distinguish between exocrine and endocrine glands :

Question 3.
Name the glands

1. which secrete juices
2. which secrete hormones.

Give one example in each case.

Answer:

1. Exocrine glands, e.g., liver secretes bile.
2. Endocrine glands, e.g., adrenal gland.

Question 4.
Why should adolescents eat a balanced diet ?
Answer:
A balanced diet during adolescence helps in proper development of reproductive organs and a beautiful body.

Question 5.
What is “female foeticide” ?
Answer:
Killing of the female foetus in the mother’s womb is known as female foeticide.

Question 6.
Which is the main change in boys at puberty ?
Answer:
The most obvious change in body is the sudden increase in height. The person looks tall because the bones of the legs and arms suddenly become long.

Question 7.
What do you think is the main difference in the boys and girls of your class ?
Answer:
In girls, there is more growth in the body parts below the waist. The boys have wider chest and shoulders.

Question 8.
What is the difference in the voice of boys and girls during adolescence ?
Answer:
The voice of boys becomes hoarse and deep but the voice of girls become shrill but sweet during adolescence.

Question 9.
What is the difference in the reproductive activity of male and female humans ?
Answer:
In male humans the reproductive activity is from puberty to 60 or 70 years.
In female humans the reproductive activity is from puberty to 45 to 50 years.

Question 10.
What do you understand by X and Y chromosomes ?
Answer:
X chromosomes stand for female characters and Y chromosomes for male characters.

Question 11.

1. What will be the sex of the baby developing from XX zygote ?
2. What will be the sex of the baby developing from XY zygote ?

Answer:

1. Female baby.
2. Male baby.

NCERT Solutions for Class 8 Science Chapter 10 – 3 Mark Questions and Answers

Question 1.
Name the hormones secreted by pancreas. Give function of each secretion.
Answer:
Glucagon and insulin are secreted by pancreas.

• Glucagon which helps to break down glycogen in liver to raise blood glucose level.
• Insulin which helps to reduce blood glucose level and maintains the sugar level.

Question 2.
A mother is angry at her child for misbehaving. The child is also afraid of scolding. Which hormones would be released in both their bodies ? What will be the effect of the hormone ?
Answer:
When the mother is angry and the child is afraid more adrenalin would be released from the adrenal gland. Due to this there will be heavy sweating and lowering of body temperature.

Question 3.
Name the endocrine gland and the hormone which

1. maintain calcium in the blood.
2. regulate calcium in the body.
3. regulate menstrual cycle.

Answer:
Gland               Hormone
Thyroid             Calcitonin
Parathyroid     Parathyroid Hormone
Ovaries            Progesterone.

Question 4.
Give one similarity and one difference in the secondary sex characters developed in boys and girl.
Answer:
Similarity : In both boys and girls hair start to grow in arm pits and the pubic region above the thighs.
Difference : In boys, moustaches and beard appear due to growth of facial hair.
In girls, the breast develop and menstrual cycle starts.

Question 5.
What is the function of Follicle Stimulating Hormone (FSH) from pituitary ?
Answer:
The function of FSH is

• to make the testes release testosterone hormone to initiate the formation of sperms.
• to make the ova mature in the ovaries and prepare the uterus to receive the zygote.

Question 6.

1. Why is menstruation also known as ‘period’ ?
2. What is menarche ?
3. What is menopause ?

Answer:

1. Since menstruation occurs once in about 28 to 30 days it is known as‘period’or menstrual cycle.
2. The first menstrual cycle is known as menarche.
3. The stage in the life of a woman when the menstruation stops.

Question 7.
Why should adolescents avoid sexual contact ?
Answer:
Sexual contact by adolescents should be avoided because :

• to avoid unwanted pregnancies.
• to avoid getting HIV (AIDS) infection or any other sexually transmitted disease.
• the responsibilities of parenthood should only be taken when you are settled in life.

Question 8.
What circumstances led to female foeticide being illegal ?
Answer:
In our country female foeticide is illegal because the female population is decreasing. In Indian society, girls are considered a burden. Lot of expenditure is incurred as dowry on girls marriage. Also due to ill treatment by family members, many social groups kill their daughters.

Question 9.
How are identical twins formed ? Why are identical twins always of the same sex ?
Answer:
Identical twins come from the same ovum. After fertilisation the zygote splits into two, each developing into an independent baby. Therefore, these twins are identical and are of the same sex.

Question 10.
One of the paternal twins may be a girl and the other may be a boy. Why ?
Answer:
Paternal twins are not of the same sex. They are a result of double fertilisation — fertilisation of two ova from two sperms.

Question 11.
Show diagrammatically how the sex of the child is determined ?
Answer:

Question 12.
What is menstruation ? Explain.
Answer:
Before a zygote is received in the uterus, the uterus wall becomes thick with a lining of soft and spongy tissue full of blood vessels. In case fertilisation does not take place, the ovum along with the lining and blood vessels along the uterus wall shed off. This results in bleeding in women which continues for 2 to 3 days. This is called menstruation.

Question 13.
Calculate the full height of Esha 8 years old and 110 cm tall at the end of the growth period. (% of full height at this age = 77%).
Answer:
Full height in cm = (Present height (cm)*100) / % of full height at this age
Full height in cm = (110*100) / 77
Full height = = 142 6/2 cm.

NCERT Solutions for Class 8 Science Chapter 10 – 5 Mark Questions and Answers

Question 1.
Give the similar changes taking place in both boys and girls during adolescence.
Answer:
The following changes take place both in boys and girls :

• Increase in height.
• Better development of muscles.
• Change in body shape.
• More of sweating.
• More secretion from sweat glands.

Question 2.
How should the adolescents take care of their health ?
Answer:
During adolescence one should take care of personal health and hygiene.

• Physical exercise should be taken.
• Personal hygiene should be maintained by taking bath daily.
• Proper nutrition should be taken by eating balanced diet.
• Drugs and smoking should be avoided.
• Sexual contact of any kind should be avoided.

Question 3.
Give some myths about sex life prevailing in our society ?
Answer:

1. Girls are not as intelligent as boys.
2. Loss of semen is the loss of energy.
3. A girl should not handle food duration menstruation.
4. The mother is responsible for the sex of her child.
5. Just by touching a boy, the girl will get pregnant.

Question 4.
Explain how menstruation takes place.
Answer:
Before a zygote is received in the uterus, the uterus wall becomes thick with a lining of soft and spongy tissue full of blood vessels. This is to receive the zygote. If the fertilisation does not take place, the ovum along with lining and the blood vessels along the uterus wall shed off. This results in bleeding in women which continues for 2 or 3 days. This is known as menstruation.

Question 5.
Name the six endocrine glands. Where is each gland located ?
Answer:
Name of gland           Position in the body

• Pituitary gland                beneath the brain.
• Thyroid gland                 on the windpipe.
• Parathyroid gland          on the larynx.
• Adrenal gland                 one on each kidney.
• Pancreas                          near the intestines.
• Sex gland                         testes in males and ovaries in females.

Question 6.
Why is population control important ?
Answer:
Controlling the growth of population is important because it is difficult to provide food, shelter and other necessities of life for a large population. If the population is high the poverty cannot be removed from the country. To improve the economic standard of a country it is important to control the population.

Question 7.
How can the population be controlled ?
Answer:
Population can be controlled by controlling the number of pregnancies. Many methods have been developed to avoid the sperm cells from meeting the ovum. A large number of family welfare centres have been set-up to educate the people on family planning. They also educate the people on various methods of birth control and safe sex.

Question 8.
What are the reasons for increase in population ?
Answer:

1. Ignorance — many people do not know the various methods available.
2. Traditional beliefs — many people think children are the gift of God.
3. Economic reasons — poor people think children are helping hands to increase the income of the family.
4. Mortality rate — the number of children dying among poor people is more, so they tend to have more children.

Question 9.
List changes in the body that take place at puberty.
Answer:

1. Increased height.
2. Better development of muscles.
3. Change in body shape.
4. Change in voice.
5. More sweating from sweat glands.
6. More secretions from oil glands.
7. Development of secondary sex characters.
8. Attainment of mental, intellectual and emotional stability.

Question 10.
Prepare a table having two columns depicting names of endocrine glands and hormones secreted by them.
Answer:
Endocrine                      Gland Hormone

• Pituitary Gland                    Growth hormone
• Thyroid Gland                    Thyroxine and calcitonin
• Parathyroid Gland             Parathyroid hormone
• Adrenal Gland                    Adrenalin
• Pancreas                             Glucagon and insulin
• Sex Glands                         Sex hormones – testosterone, oestrogen and progesterone.

Question 11.
Write notes on :

1. Adam’s apple
2. Secondary sexual characters
3. Sex determination in the unborn baby.

Answer:

1. In boys the voice changes due to better growth of larynx, which protrudes on the throat forming Adam’s apple. Thus, the voice becomes deep and hoarse.
2. Secondary sex characters develop-
   • In both, boys and girls, hair starts growing in arm pits and the pubic region.
   • In boys moustache and beard appear. In girls, oestrogen makes the breasts develop with milk secreting glands inside.
3. The sex of the baby is determined by the XX and XY chromosomes in a zygote. X chromosome stands for female and Y chromosome for male character. When X and X chromosomes form zygote (XX) the baby is female. When X and Y chromosomes form zygote (XY) the baby is male.

Question 12.
“Say ‘No’ to drugs”. Comment.
Answer:
During adolescence children may feel insecure at times. Then they may feel like taking drugs. They should not take them’until prescribed by a doctor. The child can be addicted to drugs which may harm his health – both physical and sexual.

Question 13.
Give some ways in which AIDS can be transmitted.
Answer:
AIDS can be transmitted by-

• Sexual contact with another HIV infected person.
• Using syringes already used by infected person.
• Through the milk of the infected mother to the infant.

NCERT Solutions for Class 8 Science Chapter 10 MCQs

Question 1.
The spurt in physical growth is evident in girls between [NCERT]
(a) 11-13 years
(b) 13-15 years
(c) 15-17 years
(d) 17-19 years
Answer:
(b)

Question 2.
An adolescent covers individuals from
(a) 5-10 years
(b) 12-18 years
(c) 10-15 years
(d) 10-20 years
Answer:
(a)

Question 3.
Adam’s apple is observed in
(a) boys
(b) girls
(c) both boys and girls
(d) none of these
Answer:
(a)

Question 4.
Testosterone in boys is produce from secretion of
(a) adrenal gland
(b) pituitary gland
(c) thyroid gland
(d) parathyroid gland
Answer:
(b)

Question 5.
In amphibians, metamorphosis is done by
(a) thyroid gland
(b) adrenal gland
(c) pituitary gland
(d) pineal gland
Answer:
(a)

Question 6.
Once a girl starts menstruating, her body needs more
(a) iron
(b) calcium
(c) Vitamin C
(d) Vitamin A
Answer:
(a)

Question 7.
If an adolescent does not eat sufficient fresh fruits and vegetables, he will be deficient in
(a) vitamin A
(b) vitamin D
(c) vitamin C
(d) vitamin B-complex
Answer:
(c)

Question 8.
One ovum is released in females once in every
(a) 28 days
(b) 38 days
(c) 18 days
(d) 08 days
Answer:
(a)

Question 9.
Pituitary gland secretes which hormone from ovaries in girls
(a) testosterone
(b) estrogen
(c) insulin
(d) thyroxine
Answer:
(b)

Question 10.
When we are under stress or angry, the hormone which helps us to adjust, is
(a) insulin
(b) adrenaline
(c) thyroxine
(d) none of these
Answer:
(b)

Reproduction in Animals Class 8 Science NCERT Textbook Questions

Question 1.
Explain the importance of reproduction in organisms.
Answer:
Reproduction is a vital phenomenon on this planet earth which is essential for existence and continuity of life and species on it, generation after generation.

Question 2.
Describe the process of fertilisation in human beings.
Answer:
In the process of fertilisation, sperms come in contact with an ova (egg). One of the sperms may fuse with the egg. The nuclei of the sperm and the egg fuse to form a single nucleus resulting in the formation of a fertilised egg called zygote. In human being, the fertilisation takes place inside the female body, known as internal fertilisation.

Question 3.
Choose the most appropriate answer.
(a) Internal fertilisation occurs
(i) in female body.
(ii) outside female body.
(iii) in male body.
(iv) outside male body.
(b) A tadpole develops into an adult frog by the process of:
(i) fertilisation
(ii) metamorphosis
(iii) embedding
(iv) budding
(c) The number of nuclei present in a zygote is:
(i) none
(ii) one
(iii) two
(iv) four
Answer:
(a) (i)
(b) (ii)
(c) (ii)

Question 4.
Indicate whether the following statements are True (T) or False (F).

1. Oviparous animals give birth to young ones.
2. Each sperm is a single cell.
3. External fertilisation takes place in frog.
4. A new human individual develops from a cell called gamete.
5. Egg laid after fertilisation is made up of a single cell.
6. Amoeba reproduces by budding.
7. Fertilisation is necessary even in asexual reproduction.
8. Binary fission is a method of asexual reproduction.
9. A zygote is formed as a result of fertilisation.
10. An embryo is made up of a single cell.

Answer:

1. False
2. True
3. True
4. False
5. True
6. False
7. False
8. True
9. True
10. False

Question 5.
Give two differences between a zygote and a foetus.
Answer:

Question 6.
Define asexual reproduction. Describe two methods of asexual reproduction in animals.
Answer:
The mode of reproduction in which only a single parent is involved is called asexual reproduction. In this type of reproduction, sex cells (gametes) are not produced and no fusion of gametes takes place for the reproduction of zygote or offsprings. Asexual reproduction takes place in Amoeba, Hydra, yeast, starfish, sponges, etc.
There are mainly two methods of asexual reproduction:
(i) Binary fission: In binary fission, a single parent cell is divided into two equal individual cells as in Amoeba. It divides into two by division of their bodies, each of them gets one nucleus and develops into separate individual. The figure given below shows how binary fission occurs in Amoeba.

(ii) Budding: In budding, the organism develops a bulge called bud which further develops into an adult organism and separates itself from the parent body to lead an independent life. This type of reproduction is shown in Hydra. The following figure shows budding in Hydra.

Question 7.
In which female reproductive organ does the embryo get embedded?
Answer:
Uterus

Question 8.
What is metamorphosis? Give examples.
Answer:
The transformation of larva into an adult involving sudden and abrupt changes in the body of an ani¬mal during the life cycle of an invertebrate or amphibian is called metamorphosis. Example, frog and butterfly.

Question 9.
Differentiate between internal fertilisation and external fertilisation.
Answer:

Question 10.
Complete the crossword puzzle using the hints given below.
Across
1. The process of the fusion of the gametes.
6. The type of fertilisation in a hen.
7. The term used for bulges observed on the sides of the body of Hydra.
8. Eggs are produced here.
Down
2. Sperms are produced in these male reproductive organs
3. Another term for the fertilised egg.
4. These animals lay eggs.
5. A type of fission in Amoeba.

Answer:

Reproduction in Animals Class 8 Science NCERT Intext Activities Solved

Activity 1

Visit some ponds or slow-flowing streams during spring or rainy season. Look out for clusters of frog’s eggs floating in water. Write down the colour and size of the eggs.
Solution:
The colour of the eggs is dull white and the size is less than a centimetre.

Activity 2
Try to collect eggs of the following organisms—frog, lizard, butterfly or moth, hen and crow or any other bird. Were you able to collect eggs of all of them? Make drawings of the eggs that you have collected.
Solution:
The picture of the eggs of some organisms are shown below (Fig. 9.9). You can make drawings of the eggs of other animals if you have collected them.

Activity 3
Get permanent slides of Hydra. Observe them using hand lens or a microscope. Look out for any bulges from the parent body. Count the number of bulges that you see in different slides. Also, note the size of the bulges. Draw the diagram of Hydra, as you see it. Compare it with Fig. 9.10.

Solution:
We observe that Hydra use regenerative cells for developing new individuals in the process of budding. Also buds develop in Hydra as outgrowth due to repeated cell division at one specific site. Those buds develop into small size individuals and when they are fully matured, they are separated from their parent’s body and become new young ones. This entire process is called budding which is an asexual mode of reproduction in which only a single parent is involved.

NCERT Solutions for Class 8 Science Chapter 9 – 1 Mark Questions and Answers

Question 1.
Choose the most appropriate answer [NCERT]

1. Internal fertilisation occurs :
   • in female body.
   • outside female body
   • in male body
   • outside male body
2.  A tadpole develops into an adult by the process of [NCERT]
   • fertilisation
   • metamorphosis
   • embedding
   • budding
3. The number of nuclei present in a zygote is :
   • none
   • one
   • two
   • four

Answer:

1. in female body.
2. Metamorphosis.
3. one

Question 2.
In which female reproductive organ does the embryo get embedded ? [NCERT]
Answer:
Embedding of the embryo takes place in the wall of the uterus.

Question 3.
Name the male reproductive organs.
Answer:
The male reproductive organs are a pair of testis, sperm ducts and a penis.

Question 4.
Name the organ producing the male gametes.
Answer:
Testis produce the male gametes

Question 5.
What are sperms ?
Answer:
The male gametes produced by the testis are known as sperms.

Question 6.
Give the structure of sperm.
Answer:
A sperm has a head, a middle piece and a tail.

Question 7.
Draw a labelled diagram of sperm.
Answer:

Question 8.
Is the sperm a single cell ?
Answer:
Yes, the sperm is a single cell with all the cell components.

Question 9.
Name the female reproductive organs.
Answer:
The female reproductive organs are a pair of ovaries, oviducts and the uterus.

Question 10.
Name the organ in the female body where development of the baby takes place.
Answer:
Uterus.

Question 11.
Is the egg single cell ?
Answer:
Yes, the egg is also a single cell.

Question 12.
What type of fertilisation takes place in humans and dogs ?
Answer:
Internal fertilisation.

Question 13.
What type of fertilisation takes place in fish ?
Answer:
External fertilisation.

Question 14.
What is an embryo ?
Answer:
The zygote divides repeatedly to form an embryo.

Question 15.
What is a foetus ?
Answer:
The stage of the embryo in which all the body parts are identifiable is called a foetus.

Question 16.
When does the mother giye birth to the young one ?
Answer:
When the development of the foetus is complete, the mother gives birth to the young one.

Question 17.
How much time does the embryo of hen take to develop into a chick ?
Answer:
About three weeks.

Question 18.
What name is given to animals which give birth to young ones ?
Answer:
Viviparous animals.

Question 19.
What name is given to animals which lay their eggs ?
Answer:
Oviparous animals.

Question 20.
Do you think metamorphosis takes place in humans ?
Answer:
No, in humans the body parts similar to the adults are present from the time of the birth.

NCERT Solutions for Class 8 Science Chapter 9 – 2 Mark Questions and Answers

Question 1.
All animals finally die, yet the species survives. Give reasons for this.
Answer:
During the life span, all aniamls reproduce to produce offsprings. So, even after they die, the species survives.

Question 2.
What is metamorphosis ? Give examples.
Answer:
The drastic change which takes place during the development of an animal is called metamorphosis. Silkworm and frog undergo metamorphosis.

Question 3.
Differentiate between internal fertilisation and external fertilisation. [NCERT]
Answer:

 Question 4.
Explain the importance of reproduction in organisms. [NCERT]
Answer:
Reproduction is essential for the survival of organisms. It ensures the continuation of similar kinds of individuals, generation after generation.

Question 5.
Differentiate between sexual and asexual reproduction.
Answer:

 Question 6.
How many sperms are produced by the testis ? What is the function of tail in sperms ?
Answer:
Millions of sperms are produced by the testis. The sperms move in oviduct with the help of tail.

Question 7.
What is the function of ovaries ? In human beings, how many eggs are produced every month ?
Answer:
Ovaries produce female gametes called ova. In human beings, a single mature egg is released every month.

Question 8.
If millions of sperms are transferred from the male to the female body, how many fuse with the egg?
Answer:
Only one sperm fuses with the egg.

Question 9.
Draw a diagram to show fertilisation in humans.
Answer:

Question 10.
Differentiate between viviparous and oviparous animals.
Answer:

 Question 11.
How does the size of the egg varies in different animals ?
Answer:
The egg is very small in humans, much larger in hens and the largest in ostrich.

Question 12.
The different stages of the life cycle of mosquito and silkmoth are given below. Arrange them in proper sequence.

1. Mosquito – Pupa —–> Eggs ———> Larva ——-> Adult.
2. Silk moth – Larvae ——-> Zygote ——> Cocoon ——> Adult ——–> Caterpillar.

Answer:

1. Eggs —–> Larva ——> Pupa ——–> Adult
2. Zygote ——> Larvae ——> Caterpillar ——> Cocoon ——-> Adult.

Question 13.
What are hermaphrodite animals and plants ? Give examples.
Answer:
Organisms bearing both male and female sex organs on the same body are known as hermaphrodite animals. Example, pea plant, earthworms.

Question 14.

1. When was Dolly bom and for how long did she live ?
2. Did Dolly have babies ?
3. Answer:
4. Dolly was bom on 5th July 1996 and died on 14th February 2003.
5. Dolly led a normal life and produced babies of her own through sexual means.

NCERT Solutions for Class 8 Science Chapter 9 – 3 Mark Questions and Answers

Question 1.
What is fertilisation ? Differentiate between external and internal fertilisation. Give two examples of each. [NCT 2006]
Answer:
Fusion of male sperms and female ova is called fertilisation.
Differences:

Question 2.
Describe the process of fertilisation in human beings. [NCERT]
Answer:
During fertilisation, the nuclei of the sperm and the egg fuse to form a single nucleus. This results in the formation of the fertilised egg or zygote. During the process of fertilisation, the new individual inherits some characteristics from the mother and some from the father.

Question 3.
Draw a labelled diagram to show the male reproductive system in human beings.
Answer:

Question 4.
Draw a labelled diagram to show the female reproductive system in human beings.
Answer:

Question 5.
Explain how external fertilisation takes place in frogs.
Answer:
During rainy season, frogs and toads move to ponds. When the male and female come together in water, the female lays hundreds of eggs. A layer of jelly holds the eggs together. The male deposits the sperms over them and fertilisation takes place.

Question 6.
Why animals which undergo external fertilisation lay hundreds of eggs ?
Answer:
These animals lay hundreds of eggs but all the eggs do not fertilise and develop into new individuals. The eggs get exposed to water movement, wind and rainfall. Other animals also feed on eggs. Thus, production of large number of eggs ensure fertilisation of at least few of them.

Question 7.
How does the embryo develop into the foetus ?
Answer:
The embryo develops in the uterus. It gradually develops the body parts such as hands, legs, head, eyes, ears, etc. This stage of the embryo in which all the body parts are identifiable is called a foetus.

Question 8.
Is it possible to find out whether the foetus is healthy or not ?
Answer:
Yes, it is possible to find out whether the foetus is healthy or not by using an ultrasound machine.

Question 9.
Can the technique of ultrasound of foetus be misused ?
Answer:
Yes, sometimes the technique of ultrasound can be misused. It can be used to identify the sex of the foetus which may lead to foeticide.

Question 10.
Show the diagrammatic representation how Dolly was bom ? Name the process also.
Answer:

Question 11.
Give the names of the following processes

1. Product of fertilisation.
2. Baby forming from body cell.
3. Changes taking place during the development of an animal.

Answer:

1. Zygote
2. Cloning.
3. Metamorphosis.

Question 12.
What is the function of

1. Scrotal sac
2. Vas deferens
3. Penis in human beings ?

Answer:

1. Scrotal sac is a sac in which the testis are placed.
2. Vas deferens or sperm duct carry the sperms from the testis to the uretra.
3. The penis is the organ that transfers sperms into the vagina of the female body.

NCERT Solutions for Class 8 Science Chapter 9 – 5 Mark Questions and Answers

Question 1.
Indicate whether the following statements are True (T) or False (F). [NCERT]

1. Oviparous animals give birth to young ones. ( )
2. Each sperm is a single cell. ( )
3. External fertilisation takes place in frog. ( )
4. A new human individual develops from a cell called gamete. ( )
5. Egg laid after fertilisation is made up of a single cell. ( )
6. Amoeba reproduces by budding. ( )
7. Fertilisation is necessary even in asexual reproduction. ( )
8. Binary fission is a method of asexual reproduction. ( )
9. A zygote is formed as a result of fertilisation. ( )
10. An embryo is made up of a single cell. ( )

Answer:

1. F
2. T
3. T
4. F
5. T
6. F
7. F
8. T
9. T
10. F

Question 2.
Define asexual reproduction. Describe two methods of asexual reproduction in animals. [NCERT]
Answer:
Asexual reproduction is the reproduction in which only a single parent is involved. The two methods of asexual reproduction are

• Budding in which new individuals develop from the buds, e.g., in hydra and yeast.
• Binary fission in which an animal reproduces by dividing into two individuals, e.g., in amoeba.

Question 3.

1. What is reproduction ?
2. Why is reproduction essential ?
3. What are the two modes of reproduction in animals ?
4. What is cloning ?

Answer:

1. Reproduction : Reproduction means producing new individuals of the same kind.
2. Reproduction is essential as it ensures the continuation of similar kinds of individuals, generation after generation.
3. The two modes of reproduction are — sexual and asexual reproduction.
4. Cloning is the production of an identical cell, any other living part or a complete organism.

Question 4.
Why does the child look like the parents ?
Answer:
The process of fertilisation is the meeting of an egg cell from the mother and a sperm cell from the father. So, the new individual inherits some characteristics from the mother and some from the father.

Question 5.
Draw diagram to show the formation of embryo from zygote.
Answer:

Question 6.
Explain the process of development of fertilised egg.
Answer:
In hens, soon after fertilisation, the zygote divides repeatedly and travels down oviduct. As it travels down, many protective layers are formed around it. The hard shell in a hen’s egg is one such layer. The hen finally lays the eggs. After about three weeks, the eggs are hatched.

Question 7.

1. What is meant by in vitro fertilisation ?
2. How is it done ?
3. What name is given to babies bom through this technique ?
4. What may be the possible reasons for adopting this technique ?

Answer:

1. The fertilisation which takes place outside the body of mother is known as in vitro fertilisation.
2. The freshly released egg from the mother is taken and kept with the sperms from the father in a test-tube with proper nutritional base at suitable temperature. Fertilisation takes place and after the zygote is about a week old it is transfered to the mothers uterus.
3. Babies bom under this technique are known as Test-tube babies.
4. Possible reasons for adopting this technique are – less number of sperms or blocked fallopian tubes.

Question 8.
Explain how cloning was done to produce Dolly.
Answer:
In cloning, egg from sheep ‘A’ with taken and its nucleus was replaced with a nucleus from body cell of sheep ‘B’. The cell thus produced was implanted in the womb of sheep A where it developes into a normal baby sheep. The baby sheep thus bom was found to be exactly similar to-sheep ‘B’ whose nucleus was taken. Thus, Dolly was bom on 5th July 1996.

Question 9.
Explain binary fission in Amoeba with the help of diagram. [NCT 2011]
Answer:
Amoeba is a unicellular organism, the nucleus of the amoeba gradually divides into two nuclei. Then the cytoplasm also divides and each part gets one nucleus. This is known as binary fission and it asexual reproduction.

Question 10.
The diagrams is given showing reproduction in an organism. Observe the diagrams and answer the following questions.

1. Name the organisms.
2. Name the type of reproduction.
3. Label the diagram.
4. Explain the reproduction process.
   

Answer:

1. The organism is hydra.
2. Asexual reproduction i.e., budding.
3. A and B – bud.
   C – young hydra.
4. A small out growth called bud arises on the surface of the parent hydra. The bud develops mouth, tentacles and grows into a tiny hydra. The tiny hydra separates from the parent hydra and develops into an adult.

NCERT Solutions for Class 8 Science Chapter 9 MCQs

Question 1.
Multiple fission is observed in
(a) Amoeba
(b) Bacteria
(c) Hydra
(d) Plasmodium
Answer:
(d)

Question 2.
Yeast reproduces by
(a) sexual reproduction
(b) asexual reproduction
(c) parthenogenesis
(d) none of these.
Answer:
(b)

Question 3.
External fertilisation and external development takes place in
(a) hen
(b) frog
(c) elephant
(d) human beings
Answer:
(b)

Question 4.
Testes are found in
(a) males only
(b) females only
(c) both males and females
(d) none of these
Answer:
(a)

Question 5.
When the embryo can be identified with body parts, it is known as
(a) zygote
(b) foetus
(c) infant
(d) egg
Answer:
(b)

Question 6.
Metamorphosis can be observed in
(a) tadpole
(b) earthworm
(c) hen
(d) Hydra
Answer:
(a)

Question 7.
Internal fertilisation and internal development takes place in
(a) hen
(b) frogs
(c) earthworm
(d) elephants
Answer:
(d)

Question 8.
Ovaries are present in
(a) male’s
(b) females
(c) both males and females
(d) none of these
Answer:
(b)

Question 9.
Breaking of the egg shell and the chick coming out is known as
(a) hatching
(b) incubation
(c) fertilisation
(d) metamorphosis
Answer:
(a)

Cell Structure and Functions Class 8 Science NCERT Textbook Questions

Question 1.
Indicate whether the following statements are True (T) or False (F).
(a) Unicellular organisms have a one-celled body.
(b) Muscle cells are branched.
(c) The basic living unit of an organism is an organ.
(d) Amoeba has an irregular shape.
Answer:
(a) True
(b) True
(c) False
(d) True

Question 2.
Make a sketch of the human nerve cell. What function do nerve cells perform?
Answer:
Function of nerve cells: The function of the nerve cell is to receive and transfer messages, it helps to control and coordinate the working of different parts of the body.

Question 3.
Write short notes on the following.
(a) Cytoplasm
(b) Nucleus of a cell
Answer:
(a) Cytoplasm: The jelly-like substance found between the nucleus and the cell membrane is called cytoplasm. It is made up of basic elements like C, H, O, N. Various other components or organelles, like mitochondria, Golgi bodies, ribosomes, etc., of cells are present in the cytoplasm.

(b) Nucleus of a cell: Nucleus of a cell is an important component of the living cell. It is located at the centre of the cell. It is separated from the cytoplasm by a membrane called nuclear membrane. It contains genetic material.

Question 4.
Which part of the cell contains organelles?
Answer:
Cytoplasm

Question 5.
Make sketches of animal and plant cells. State three differences between them.
Answer:

Question 6.
State the difference between eukaryotes and prokaryotes.
Solution:

Question 7.
Where are chromosomes found in a cell? State their function.
Answer:
Chromosomes are present in the nucleus. The functions of chromosomes is to carry genes on them and to transfer the character from parents to the next generation.

Question 8.
‘Cells are the basic structural units of living organisms.’ Explain.
Answer:
Different cells combine to form tissues and tissues combine to form organs. Similarly, organs combine to form body. Thus they are termed as the basic structural unit of every living organism.

Question 9.
Explain why chloroplast are found only in plant cells?
Answer:
Chloroplasts are plastids required for the food making process, called photosynthesis, and thus they are only present in plant cells.

Question 10.
Complete the crossword with the help of clues given below.
Across
1. This is necessary for photosynthesis.
3. Term for component present in the cytoplasm.
6. The living substance in the cell.
8. Units of inheritance present on the chromosomes.
Down
1. Green plastids.
2. Formed by collection of tissues.
4. It separates the contents of the cell from the surrounding medium.
5. Empty structure in the cytoplasm.
7. A group of cells.
Solution:

Cell Structure and Functions Class 8 Science NCERT Intext Activities Solved

Activity 1
The teacher may show a permanent slide of Amoeba and Paramecium under a microscope. Alternatively, the teacher can collect pond water and show these organisms by preparing the slides.
Solution:
Do it yourself.

Activity 2

Boil a hen’s egg. Remove the shell. What do you observe? A white material surrounds the yellow part. The white material is albumin which solidifies on boiling. The yellow part is yolk. It is part of the single cell. You can observe this single cell without any magnifying device.
Solution:
Do it yourself.

Activity 3
In order to observe the basic components of the cell, take an onion bulb. Remove the dry pink coverings (peels). You can easily separate these from the fleshy white layers of the bulb with the help of forceps or even with your hand. You can also break the onion bulb and separate out thin layers. Place a small piece of the thin onion peel in a drop of water on a glass slide. The thin layer can be cut into smaller pieces with the help of a blade or forceps. Add a drop of methylene blue solution to the layer and place a coverslip on it. While placing the coverslip ensure that there are no air bubbles under the coverslip. Observe the slide under the microscope. Draw and label.
Solution:
The boundary of the onion cell is covered by a thick covering called the cell wall. The central dense round body in the centre is called the nucleus. The jelly-like substance between the nucleus and the cell membrane is called cytoptasm.

Activity 4

Take a clean tooth pick, or a matchstick with the tip broken. Scrape inside of your cheek without hurting it. Place it in a drop of water on a glass slide. Add a drop of iodine and place a coverslip over it. Alternatively, add 1 -2 drops of methylene blue solution. Observe it under the microscope. You may notice several cells in the scraped material (Fig. 8.2). You can identify the cell membrane, the cytoplasm and nucleus. A cell wall is absent in animal cells.
Solution:
Do it yourself.

NCERT Solutions for Class 8 Science Chapter 8 – 1 Mark Questions and Answers

Question 1.
………… is the outermost layer of an animal cell. [KVS 2008; MSE (Chandigarh) 2006]
Answer:
Plasma membrane/Cell membrane

Question 2.
What is the name given to the green plastids ? [MSE (Chandigarh) 2007]
Answer:
The green plastids are called chloroplasts.

Question 3.
Name two organelles present in the plant cell but not in the animal cell. [KVS 2005]
Answer:
Cell wall and chloroplast are found in plant cell but not in animal cell.

Question 4.
Which part of the cell contains organelles ? [NCERT]
Answer:
Cytoplasm contains the organelles.

Question 5.
Why cells could not be observed before 17th century ?
Answer:
Cells could not be observed before 17th century because microscope was not available for viewing the cells.

Question 6.
Why Hooke had to take thin slices of cork ?
Answer:
He made thin slices of cork because the cork was solid and its details could not be seen.

Question 7.
Where did Hooke demonstrate cork slice ?
Answer:
Hooke demonstrated cork slice in Royal society of London.

Question 8.
Single celled organisms are also called unicellular organisms (True/False)
Answer:
True.

Question 9.
Name the cells having branched structure.
Answer:
Nerve cell.

Question 10.
Which cell is observable with unaided eye ?
Answer:
Ostrich egg.

Question 11.
Name the outermost layer of animal cell.
Answer:
Cell membrane or plasma membrane.

Question 12.
Mention the layer outside the plasma membrane of a plant cell.
Answer:
Cell wall.

Question 13.
Which four basic elements constitute 90% of protoplasm ?
Answer:
90% of protoplasm is made up of carbon, hydrogen, nitrogen and oxygen.

Question 14.
The term cell was coined by ………….
Answer:
Robert Hooke.

Question 15.
Cell wall is present in …………. only.
Answer:
Plant cell.

Question 16.
Which organism has smallest cell ?
Answer:
Bacterium mycoplasmas has the smallest cell.

Question 17.
How do you differentiate protoplasm from cytoplasm ?
Answer:
Cytoplasm is the jelly-like substance occupying most of the space inside the cell. Protoplasm includes the cell membrane, the cytoplasm and the nucleus.

Question 18.
Draw a typical cell. Label important organelles.
Answer:

Question 19.
What name is given to living things having more than one cell ?
Answer:
Multicellular organisms.

Question 20.
Can unicellular organisms be seen with the naked eye ?
Answer:
Unicellular organisms can only be viewed with the help of a microscope.

NCERT Solutions for Class 8 Science Chapter 8 – 2 Mark Questions and Answers

Question 1.
What is a cell ? Name the longest cell in human body. Draw its diagram also. [NCT 2007]
Answer:
All organisms are made of basic units known as Cell. Nerve Cell is the longest cell in hyman body.

Question 2.
Why are mitochondria known as the “power house of the cell” ? [DAV2005]
Answer:
Mitochondria is known as the power house of the cell because they perform the function of respiration and provide the cell with energy.

Question 3.
Are the cells of an elephant larger than cells of a rat ?
Answer:
No, the size of the cell has no relation with the size of the body of the animal or plant.

Question 4.
What are the “building blocks of life” ? Why are they so called ?
Answer:
Cells are building blocks of life because all living things are made up of one or more cells.

Question 5.
What is the difference between tissue and organ ?
Answer:
Group of cells of the same type make up the different tissues of the organisms, e.g., muscle tissue.
Several different types of tissues together form an organ, e.g., a stomach.

Question 6.
Differentiate between an organ and a system.
Answer:
Several different types of tissues that work together to perform one or more life activities is known as an organ.
An organ system is a group of organs that work together to carry on life activities.

Question 7.
Name an organ system in the human body and the major organs that make up that system.
Answer:
Organ system — Digestive system.
It is made up of organs such as intestines, liver, stomach, pancreas, gall bladder.

Question 8.
What features are possessed by both plant cells and animal cells ?
Answer:
All plant and animal cells have three parts — cell membrane, cytoplasm and nucleus.

Question 9.
Name the organelle known as “suicide bags” ? Why is it called so ?
Answer:
Lysosomes are known as suicide bags. They contain enzymes which help in breaking down or destroying the various materials.

Question 10.
Give the functions of cell wall.
Answer:

• It provides rigidity to the cell wall.
• It provides protection against plant viruses and pathogens.

Question 11.
Draw diagrams to show the difference between plant cell and animal cell. [NCT 2010]
Answer:

NCERT Solutions for Class 8 Science Chapter 8 – 3 Mark Questions and Answers

Question 1.
Name any three elements which form major part of protoplasm. [MSE (Chandigarh) 2006]
Answer:
Protoplasm is made up of compounds of carbon, hydrogen, nitrogen and oxygen.

Question 2.

1. Why are plant cells more rigid in shape than animal cells ? [DAV2006]
2. Name the largest and the smallest cells in the living world.
3. Tomatoes are red and leaves are green. Why ?

Answer:

1. Plant cells are more rigid in shape than animal cells due to the presence of cell wall.
2. Largest – Ostrich’s egg.
   Smallest – PPLO (Pleuro Pneumonia Like Organisms).
3. Tomatoes are red because of chromoplasts in their cells.
   Leaves are green because of chloroplasts in their cells.

Question 3.
Distinguish between prokaryotes and eukaryotes.
Answer:
Differences:

Question 4.
Make a sketch of the human nerve cell. What function do nerve cells perform ?
Answer:
Nerve cell — Nerve cells receive messages through dendron and transfer it through axon.

Question 5.
If you boil a hen’s egg, what changes do you observe ?
Answer:
When a hen’s egg is boiled, a white material surrounds the yellow part. White material is albumin which solidifies on boiling. The yellow component is yolk.

Question 6.
What are the functions of the cell membrane ?
Answer:

• It protects the cell.
• It provides shape to the cell.
• It allows materials to enter and leave the cell through the tiny holes.

Question 7.
Give the functions of the following :

1. Endoplasmic reticulum
2. Golgi complex
3. Ribosomes

Answer:

1. Endoplasmic reticulum being a network of membranes, provides a large surface area for life functions to take place.
2. Golgi complex collects and distributes the substance made in the cell and it synthesises and secretes many materials.
3. Ribosomes is the site of protein synthesis.

Question 8.

1. What is a cell ?
2. Who discovered the cell ?
3. Name one microscopic organism.

Answer:

1. Living things are made up of tiny living parts known as cells.
2. Robert Hooke, an English scientist in 1665 discovered the cell.
3. Amoeba is a microscopic organism.

NCERT Solutions for Class 8 Science Chapter 8 – 5 Mark Questions and Answers

Question 1.
Differentiate between plant and animal cell. [NCT 2011]
Answer:
Differences:

Question 2.
Indicate whether the following statements are True (T) or False (F). [NCERT]

1. Unicellular organisms have one-celled body.
2. Muscle cells are branched.
3. The basic living unit of an organism is an organ.
4. Amoeba has a irregular shape.

Answer:

1. T
2. F
3. T
4. T

Question 3.
Write short notes on the following : [NCERT]

1. Cytoplasm
2. Nucleus of a cell

Answer:

1. Cytoplasm is the fluid present between the cell membrane and the nucleus. Organelles of cells are present in the cytoplasm. These are mitochondria, Golgi bodies, ribosomes, etc. Cytoplasm is made up of basic elements like C, H, O, and N. They are found in the form ef carbohydrates, proteins and water.
2. Nucleus of a cell is generally spherical and located in the centre of the cell. Nucleus is separated from the cytoplasm by a membrane called the nuclear membrane. Nucleus also contains nucleolus and chromosomes. Nucleus helps in inheritance and acts as control centre of the activities of the cell.

Question 4.
Describe the variations in shape and size of cells.
Answer:
Cell size. Some cells are very small and visible only with a microscope. Smallest cell is of bacterium, PPLO. An ostrich egg is the largest animal cell. In plants an alga, Acetabularia has a single cell of about 10 cm in length.
Cell shapes are very diverse. Some cells like those of Amoeba and white blood cells continuously change their shape. Most cells, however, maintain their constant shape. The shape of the cell is related to its function.

Question 5.
Name the different parts of the nucleus and give the function of each part.
Answer:

1. Nuclear membrane — It separates the nucleus from the cytoplasm. It allows the exchange of substances between the nucleoplasm and the cytoplasm.
2. Nucleoplasm — Chromosomes and nucleoli are present in the nucleoplasm.
3. Chromosomes — Play an important role in the inheritance of characters from one generation to another.

Question 6.
Give the functions of the following parts of the cell:

1. Vacuoles.
2. Centrioles.
3. Cellulose.
4. Plasma membrane.
5. Nucleus.

Answer:

1. Vacuoles store the chemicalproducts which accumulate within the cell due to the various life functions taking place inside the cell.
2. Centrioles present in animal cells are concerned with cell division.
3. Cellulose is present in plant cell and provides rigidity and protection to the cell.
4. Plasma membrane protects the cell and allows materials to enter and leave through the tiny holes.
5. Nucleus controls everything that takes place in the cell.

Question 7.
Explain the mode of cell division in Amoeba. [KVS 2006, 2007, 2008]
Answer:
The cell divides and splits into two parts known as daughter cells. The daughter cells are identical to the parent cell. The nucleus of the parent cell divides into two, followed by the division of cytoplasm. Finally, the two daughter cells are formed.

Question 8.
In multicellular organisms, how does growth take place ? [NCT 2005 ; MSE (Chandigarh) 2008]
Answer:
In multicellular organisms, the cells divide for reproduction and also multiply for growth. The increase in the number of cells is brought about by cell division. The cells so produced undergo a change in size and shape and the whole organism shows over all growth.

NCERT Solutions for Class 8 Science Chapter 8 MCQs

Question 1.
The structure which Robert Hooke observed under his self-designed microscope was
(a) cell wall
(b) cell membrane
(c) both (a) and (b)
(d) living cell
Answer:
(a)

Question 2.
Which of the following is covered by a single membrane ?
(a) Mitochondria
(b) Vacuole
(c) Lysosome
(d) Plastid
Answer:
(b)

Question 3.
Kitchen of the cells is known as
(a) mitochondria
(b) endoplasmic reticulum
(c) chloroplast
(d) Golgi apparatus.
Answer:
(c)

Question 4.
Cell theory was given by
(a) Schleiden and Schwann
(b) Virchow
(c) Robert Hooke
(d) Haeckel
Answer:
(a)

Question 5.
The only cell organelle seen in prokaryotic cell is
(a) mitochondria
(b) ribosomes
(c) plastids
(d) lysosomes
Answer:
(b)

Question 6.
Organelle without a cell membrane is
(a) ribosome
(b) Golgi apparatus
(c) chloroplast
(d) nucleus
Answer:
(a)

Question 7.
Which organelle is known as the store house of the ! cell ?
(a) Mitochondria
(b) Vacuole
(c) Ribosomes
(d) Golgi complex
Answer:
(d)

Question 8.
Green plastids are also called
(a) chromoplasts
(b) chloroplasts
(c) chromatin
(d) none of these
Answer:
(b)

Question 9.
Which of the following is not unicellular ?
(a) Euglena
(b) Paramecium
(c) Chicken
(d) Amoeba
Answer:
(c)

Question 10.
Thread-like body that lies in the nucleus of the cell is
(a) cytoplasm
(b) chromosome
(c) nucleoplasm
(d) mitochondrion
Answer:
(b)

Question 11.
Which of these cells will have cell wall around them ?
(a) Cheek cells
(b) Nerve cells
(c) Onion peel cells
(d) Blood cells
Answer:
(c)