Exploring Forces
Chapter 5 · Curiosity — Science Textbook for Grade 8 · Complete Solutions
This page provides step-by-step, detailed answers to every in-text (Probe & Ponder, Activity reflection, character speech-bubble) question and every end-of-chapter exercise question from Chapter 5 — Exploring Forces (NCERT Curiosity, Grade 8). Original figures from the chapter are used wherever relevant, alongside recreated diagrams for additional clarity.
A
In-Text Questions
Probe and Ponder · Page 62
1. Why does it feel harder to pedal a bicycle when going uphill than on flat ground?
Answer
When cycling uphill, we have to work against both friction and an additional component of the gravitational force pulling the bicycle (and rider) back down the slope. On flat ground, only friction needs to be overcome. Since gravity now also opposes the forward motion while going uphill, we must apply a much greater muscular force on the pedals to keep moving, making it feel harder.
2. Why is it easier to slip on a wet surface?
Answer
Friction depends on the nature of the surfaces in contact — rough, dry surfaces have more interlocking irregularities and therefore more friction. Water (or any liquid) fills up and smooths over these tiny irregularities between our feet/footwear and the floor, drastically reducing the friction between them. With much lower friction available to oppose motion, our feet slide more easily, making it easier to slip on a wet surface.
3. Why do we feel ‘light’ or like we are ‘floating’ just after our swing reaches its highest point and begins to come down?
Answer
At the highest point of the swing, our upward motion has just stopped and we are about to begin moving downward under the pull of gravity. For a brief moment, the seat of the swing is not pushing up on us as strongly as before (since we are momentarily in a state similar to free fall combined with the swing’s motion), so the usual feeling of the seat’s support pressing against our body reduces suddenly. This momentary reduction in the felt support force gives the sensation of being ‘light’ or ‘floating’, even though the gravitational force is still acting on us throughout.
Activity 5.1 — Let us Explore · Page 63
Try moving the box in as many different ways as you can think of. Did you move the box in any other way than shown in Fig. 5.1?
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Fig. 5.1: Moving a box in different ways — (a) Pushing; (b) Pulling; (c) Lifting and carrying
Answer
Besides pushing, pulling, and lifting/carrying (as shown in Fig. 5.1), the box could also be moved by other methods such as: rolling it on its edge, dragging it along the ground using a rope tied at an angle, tilting and sliding it down a ramp, or even throwing it a short distance. In every one of these methods, a push or a pull has to be applied to the box to make it move — confirming that all forms of moving an object essentially involve the application of a force (push or pull).
Activity 5.2 — Let us Analyse · Page 64
What do you conclude from these examples? Does a force cause a moving object to stop? Can it change speed, or direction of motion, or change the shape of an object?
Answer
Yes, to all three. From the examples in Table 5.1 (and others like opening a drawer, stretching a rubber band, kicking a football, applying brakes), we can conclude that a force applied on an object can:
- Stop a moving object or slow it down (e.g., a friend pulling a moving bicycle to a stop).
- Change its speed if it is already moving (increase or decrease it).
- Change its direction of motion (e.g., hitting a moving ball with a bat redirects it).
- Change its shape (e.g., pressing an inflated balloon or stretching a rubber band).
- It can also start the motion of an object that was at rest.
In-text Bubble · Page 65
“Does this mean that whenever there is a change in speed or direction, or change in shape, a force is acting on the object?” — “Yes, none of these take place without the action of force.”
Explanation
This confirms the central idea of the chapter: a force is the underlying cause behind every change in an object’s state of motion (starting, stopping, speeding up, slowing down, or changing direction) as well as every change in an object’s shape. If none of these changes is occurring, it does not necessarily mean no force is acting — it could mean that the forces acting are balanced and cancel each other out (as explained in the “A step further” box on page 65).
In-text Bubble · Page 67
Is there any other contact force (besides muscular force)?
Answer
Yes — another important contact force is the force of friction. Unlike muscular force, which results from the action of our muscles, friction arises whenever one surface moves or tries to move over another surface that it is touching. Like muscular force, friction also requires physical contact between the two objects/surfaces involved, which is why it too is classified as a contact force.
Activity 5.3 — Let us Investigate · Page 67–68
Gently push a flat-based object and observe. Does it stop after travelling some distance? Is there a force acting on it which brings it to rest?
Answer
Yes. When the flat-based object (such as a lunch box or notebook) is gently pushed on a table or floor, it slides for some distance and then gradually comes to rest on its own, even though no one is touching it anymore. This happens in both directions of pushing. Since a force is required to change the speed of a moving object (and bring it to rest), there must be some force acting on the object in a direction opposite to its motion. This force is the force of friction, which arises between the surfaces of the object and the table/floor that are in contact, and it opposes the sliding motion until the object stops.
In-text Bubble · Page 68
Does this mean that the force of friction will be greater if the surfaces are rough?
Answer
Yes. Since friction arises due to the tiny irregularities present on the surfaces in contact (which lock into each other and resist sliding), a rougher surface has more and larger irregularities than a smoother surface. This means rough surfaces interlock more strongly, producing a greater force of friction compared to smoother surfaces, which is confirmed in Activity 5.4.
Activity 5.4 — Let us Explore · Page 68
Place the same object on different surfaces (glass, cloth, wood, ceramic tile, sand). Does the object stop after travelling the same distance on all surfaces?
Answer
No, the object does not stop after travelling the same distance on all surfaces. It travels the farthest on smooth surfaces like glass or ceramic tile (since friction is low there) and stops sooner (after a shorter distance) on rougher surfaces like cloth or sand (since friction is higher there). This shows that the force of friction depends on the nature of the surfaces in contact — friction is greater on rough surfaces and smaller on smooth surfaces.
In-text Bubble · Page 69
Is it essential for an object applying force on another object to always be in contact with it?
Answer
No. While forces like muscular force and friction do need physical contact (called contact forces), there are also forces that can act on an object from a distance, without any physical contact — these are called non-contact forces. Examples include magnetic force, electrostatic force, and gravitational force, all of which are explored later in this chapter.
Activity 5.5 — Let us Test · Page 69
Insert a second ring magnet above the first such that like poles face each other. Does the second magnet stay floating above the first? Reverse the poles — does it still remain floating?
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Fig. 5.7: Force between two ring magnets
Answer
Yes, when the like poles of the two ring magnets face each other, the second magnet stays floating above the first, supported in mid-air without touching it — because like poles repel each other, and this repulsive force balances the magnet’s weight. Gently pushing the floating magnet down, we feel a force pushing back against our hand, confirming the repulsion. When the poles of both magnets are reversed (so like poles still face each other, just the other pair), the second magnet still remains floating, since the repulsion is still between like poles. (However, if unlike poles are made to face each other, the magnets would instead attract and snap together, and the second magnet would not float.) This proves that a magnet can exert a force on another magnet even without being in physical contact with it — a non-contact (magnetic) force.
Activity 5.6 — Let us Experiment · Page 70
Rub a plastic scale/straw with polythene and bring it close to small pieces of paper. Do you notice something surprising?
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Fig. 5.8: Charged plastic scale attracting small paper pieces
Answer
Yes — surprisingly, the small pieces of paper get pulled towards the rubbed plastic scale/straw and stick to it, even though the scale never touches them. This happens because rubbing the plastic scale with polythene causes static electric charges to build up on the scale’s surface. This charged scale then exerts an attractive force — an electrostatic force — on the nearby uncharged paper pieces, pulling them towards it without any physical contact, confirming that electrostatic force is also a non-contact force.
Activity 5.7 — Let us Experiment · Page 70
Rub both balloons with woollen cloth and release them. What do you observe? Now bring the woollen cloth close to one of the rubbed balloons. What happens?
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Fig. 5.9: (a) Two uncharged balloons; (b) Two charged balloons repelling each other
Answer
After rubbing both balloons with the woollen cloth and releasing them, we observe that the two balloons move away from each other, as if they are repelling one another. This happens because both balloons get charged with the same (similar) kind of charge from the rubbing, and like charges repel each other.
When the woollen cloth (used for rubbing) is then brought close to one of the rubbed balloons, the balloon moves towards the cloth, as if attracted to it. This is because the balloon and the woollen cloth acquired opposite (unlike) kinds of charge during rubbing, and unlike charges attract each other.
When the woollen cloth (used for rubbing) is then brought close to one of the rubbed balloons, the balloon moves towards the cloth, as if attracted to it. This is because the balloon and the woollen cloth acquired opposite (unlike) kinds of charge during rubbing, and unlike charges attract each other.
In-text Bubble · Page 71
Does this indicate that the charge on the balloon is of a different kind from the charge on the woollen cloth? Does it mean that there are two kinds of electrical charges?
Answer
Yes. Since the two similarly-charged balloons repelled each other (like charges repel), but the balloon and the woollen cloth attracted each other (unlike charges attract), it confirms that the balloon and the woollen cloth carry opposite kinds of static charge. This shows that there are exactly two kinds of electrical charges, conventionally called ‘positive’ and ‘negative’ charge — when two objects are rubbed together, one of them gains positive charge and the other gains negative charge.
Activity 5.8 — Let us Observe · Page 71–72
Throw a ball vertically upwards. Does it come down? Throw it again, harder — does it still fall back down to the ground?
Answer
Yes, in both cases the ball always falls back down to the ground, no matter how gently or how hard it is thrown upward. This happens because of the gravitational force (force of gravity) exerted by the Earth, which constantly pulls every object towards itself. Even though the ball moves away from the Earth initially while going up, the Earth’s gravitational pull continuously acts on it, gradually slowing it down, stopping it momentarily, and then pulling it back down to the ground.
Bubble: Why do all objects fall towards the Earth? What exerts this force?
Since every object that is thrown up or dropped eventually falls towards the Earth, it means the Earth attracts (pulls) all objects towards itself. The force responsible for this is called the gravitational force, exerted by the Earth due to its mass. This force is also called the force of gravity, or simply gravity.
Bubble: Does the Earth pull every object with equal force?
No — as shown later in Activity 5.9, the Earth pulls different objects with different amounts of force, depending on their mass. This is confirmed by hanging different objects from a spring and observing that the spring stretches by different amounts for different objects.
Activity 5.9 — Let us Explore · Page 73
Hang different objects one by one from a spring. Is the stretch caused by each object the same?
Answer
No, the stretch caused in the spring is different for different objects — heavier objects cause a greater stretch in the spring, while lighter objects cause a smaller stretch. This happens because each object is pulled downward by the Earth’s gravitational force with a different magnitude (i.e., they have different weights), and the spring stretches more when a larger force (greater weight) is applied to it. This observation confirms that the Earth pulls different objects with different forces, and also forms the working principle of a spring balance, which uses the amount of stretch in a spring to measure the weight of an object.
Activity 5.10 — Let us Observe · Page 73
Look at the spring balance carefully. What is the maximum weight it can measure?
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Fig. 5.13: A spring balance and close-up of its scale
Answer
Looking at the Newton scale on the spring balance shown in Fig. 5.13, the highest marked value is 10 N. So, the maximum weight this particular spring balance can measure is $10\ \text{N}$, meaning it has a usable range of $0$ to $10\ \text{N}$.
Activity 5.11 — Let us Calculate · Page 74
Find the weight difference between two bigger marks, the number of small divisions between them, and the smallest value the spring balance can read.
Step-by-step solution
From the scale shown in Fig. 5.13:
- The weight difference between two consecutive bigger marks (e.g., between 0 and 01 N, or between 01 N and 02 N) is $1\ \text{N}$.
- The number of small divisions between these two bigger marks is $5$.
- So, the value of one small division is: $$\text{Smallest reading} = \frac{1\ \text{N}}{5} = 0.2\ \text{N}$$
Activity 5.12 — Let us Measure · Page 74
Suspend objects (like a pencil box, partially filled water bottle) from a spring balance one by one and record their weight.
Answer
This is a hands-on practical activity, so the recorded readings will vary depending on the actual objects used and the spring balance available. As a sample illustration, the weight of a pencil box might be measured as approximately 1.5 N, and a partially filled water bottle as approximately 3 N (these are only illustrative example values). To record actual readings: suspend each object from the hook one at a time, ensure the spring balance hangs freely and steadily, and carefully read the value where the pointer settles on the Newton scale, making sure your eye is level with the pointer to avoid reading errors.
In-text Bubble · Page 75
What is the difference between weight and mass?
Answer
For example, an object of mass $1\ \text{kg}$ has a weight of about $10\ \text{N}$ on Earth, but only about $1.6\ \text{N}$ on the Moon, even though its mass remains $1\ \text{kg}$ in both places.
| Mass | Weight |
|---|---|
| The amount of matter contained in an object. | The gravitational force with which the Earth (or another planet) pulls an object towards itself. |
| Measured in grams (g) or kilograms (kg). | Measured in newton (N), since it is a force. |
| Remains the same everywhere — on Earth, Moon, or any other planet. | Can vary from place to place, since gravitational force differs slightly across locations and significantly across different planets. |
Activity 5.13 — Let us Investigate · Page 76
Push an empty bottle (lid closed) into a bucket of water. Do you feel an upward push? Release the bottle — does it bounce up?
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Fig. 5.15: Bottle in water
Answer
Yes, when the bottle is pushed into the water, we feel a distinct upward push resisting our hand, and when released, the bottle bounces back up to the surface of the water. This shows that water (and liquids in general) exert an upward force on objects placed in them, called upthrust or the buoyant force. This force acts opposite to the downward gravitational force, and whether an object floats or sinks depends on which of these two forces is greater (or whether they are equal).
Bubble: Why don’t all objects fall to the bottom (sink) in water?
Although the Earth’s gravitational force pulls every object placed in water downward, the water simultaneously exerts an upward buoyant force on the object. If the buoyant force is equal to or greater than the object’s weight, the object floats; if the buoyant force is less than the object’s weight, the object sinks. So whether an object floats or sinks in water depends on the balance between these two opposing forces — not on gravity alone.
B
Exercise Questions (Keep the Curiosity Alive)
Question 1
Match items in Column A (Type of force) with the items in Column B (Example).
Answer
Lifting a school bag uses muscle action (muscular force); a compass needle aligns due to Earth’s/a magnet’s magnetic field (magnetic force); a rolling ball slowing and stopping on grass is due to friction; a fruit falling is pulled by Earth’s gravity (gravitational force); and a charged balloon attracting hair is due to static/electrostatic force.
| Column A | Matches With | Column B |
|---|---|---|
| (i) Muscular force | → | (b) A child lifting a school bag |
| (ii) Magnetic force | → | (e) A compass needle pointing North |
| (iii) Frictional force | → | (a) A cricket ball stopping on its own just before touching the boundary line |
| (iv) Gravitational force | → | (c) A fruit falling from a tree |
| (v) Electrostatic force | → | (d) Balloon rubbed on woollen cloth attracting hair strands |
Question 2
State whether the following statements are True or False.
(i) A force is always required to change the speed of motion of an object.
(ii) Due to friction, the speed of the ball rolling on a flat ground increases.
(iii) There is no force between two charged objects placed at a small distance apart.
Answer
(i) True — a force is needed to start, stop, speed up, or slow down an object; no change in speed can occur without a force acting on the object.
(ii) False — friction always opposes motion, so it causes the speed of a rolling ball to decrease gradually until it stops, not increase.
(iii) False — two charged objects placed near each other (even without touching) exert an electrostatic force on each other (attraction if oppositely charged, repulsion if similarly charged), since electrostatic force is a non-contact force that acts at a distance.
(ii) False — friction always opposes motion, so it causes the speed of a rolling ball to decrease gradually until it stops, not increase.
(iii) False — two charged objects placed near each other (even without touching) exert an electrostatic force on each other (attraction if oppositely charged, repulsion if similarly charged), since electrostatic force is a non-contact force that acts at a distance.
Question 3
Two balloons rubbed with a woollen cloth are brought near each other. What would happen and why?
Answer
The two balloons would move away from (repel) each other. This is because rubbing both balloons with the same woollen cloth causes them to acquire the same (similar) kind of static electric charge. Since like charges repel each other, the two similarly-charged balloons push each other apart through the non-contact electrostatic force, even without touching — exactly as observed in Activity 5.7.
Question 4
When you drop a coin in a glass of water, it sinks, but when you place a bigger wooden block in water, it floats. Explain.
Answer
Both the coin and the wooden block experience two opposing forces in water: the downward gravitational force (weight) and the upward buoyant force (upthrust) exerted by the water.
For the coin (made of dense metal), its weight is much greater than the buoyant force the water can exert on it (because of its small size and high density), so the net force is downward and the coin sinks.
For the wooden block, wood is much less dense, so for its size, its weight is relatively small, while it displaces a larger volume of water (especially being bigger), creating a large buoyant force that is equal to or greater than its weight. Hence the net force balances out (or pushes it up), and the wooden block floats.
In short: an object sinks when its weight is greater than the buoyant force acting on it, and it floats when the buoyant force is sufficient to balance its weight.
For the coin (made of dense metal), its weight is much greater than the buoyant force the water can exert on it (because of its small size and high density), so the net force is downward and the coin sinks.
For the wooden block, wood is much less dense, so for its size, its weight is relatively small, while it displaces a larger volume of water (especially being bigger), creating a large buoyant force that is equal to or greater than its weight. Hence the net force balances out (or pushes it up), and the wooden block floats.
In short: an object sinks when its weight is greater than the buoyant force acting on it, and it floats when the buoyant force is sufficient to balance its weight.
Question 5
If a ball is thrown upwards, it slows down, stops momentarily, and then falls back to the ground. Name the forces acting on the ball and specify their directions: (i) During upward motion (ii) During downward motion (iii) At its topmost position.
Answer
In all three cases, only the gravitational force (force of gravity) due to the Earth acts on the ball (we ignore air resistance here), and this force always acts vertically downward, towards the centre of the Earth.
- (i) During upward motion: The gravitational force acts downward, opposite to the ball’s upward motion, continuously decelerating (slowing down) the ball.
- (ii) During downward motion: The gravitational force still acts downward, but now it is in the same direction as the ball’s motion, so it accelerates (speeds up) the ball as it falls.
- (iii) At the topmost position: The gravitational force continues to act downward on the ball, even though the ball’s speed is momentarily zero at this point — it is this continued downward force that causes the ball to start moving downward again.
Question 6
A ball released from point P moves along an inclined plane and then a horizontal surface, stopping at point A. Think of a way so that when released from the same point P, the ball stops (i) before point A (ii) after crossing point A.
(i) To make the ball stop before point A
We need to increase the friction on the horizontal surface (or part of it) between the bottom of the incline and point A. This can be done by changing the surface to a rougher material (e.g., placing a cloth, sandpaper, or rough mat on the horizontal track) or by sprinkling sand on it. Greater friction will oppose the ball’s motion more strongly, causing it to lose speed faster and come to rest before reaching point A.
(ii) To make the ball stop after crossing point A
We need to decrease the friction on the horizontal surface, by making it smoother — for example, by using a polished/glass surface, or by lubricating/oiling the track. With reduced friction, the ball will face less opposition to its motion, retain more of its speed for longer, and travel past point A before finally coming to rest.
Question 7
Why do we sometimes slip on smooth surfaces like ice or polished floors? Explain.
Answer
We slip on smooth surfaces like ice or polished floors because such surfaces have very few irregularities, and therefore offer very low friction between our footwear and the surface. Normally, when we walk, friction between our feet and the ground provides the necessary grip that allows us to push off and move forward without our feet sliding. On smooth/slippery surfaces like ice or a polished floor (especially if wet), this friction is greatly reduced, so our feet are unable to get a firm grip and tend to slide unexpectedly in the direction of any small force we apply, causing us to lose balance and slip.
Question 8
Is any force being applied to an object in a non-uniform motion?
Answer
Yes. Non-uniform motion means the speed and/or direction of the object is continuously changing. Since a force is required to bring about any change in an object’s speed or direction of motion, an object undergoing non-uniform motion must definitely have a force acting on it (such as friction, gravity, an applied push/pull, or a combination of forces) that is responsible for this continuous change.
Question 9
The weight of an object on the Moon becomes one-sixth of its weight on the Earth. What causes this change? Does the mass of the object also become one-sixth of its mass on the Earth?
Answer
This change is caused by the difference in gravitational pull between the Moon and the Earth. The Moon is much smaller and less massive than the Earth, so its gravitational force of attraction is weaker — specifically, about one-sixth as strong as Earth’s gravity. Since an object’s weight is the force with which a planet/moon pulls it, a weaker gravitational pull on the Moon results in the object’s weight there being only $\frac{1}{6}$ of its weight on Earth.
However, the mass of the object does NOT become one-sixth on the Moon. Mass is the amount of matter contained in an object, and this amount does not change regardless of location — it remains exactly the same on the Earth, the Moon, or anywhere else in space. Only the weight changes, because weight depends on the local gravitational force, while mass is an intrinsic, unchanging property of the object.
However, the mass of the object does NOT become one-sixth on the Moon. Mass is the amount of matter contained in an object, and this amount does not change regardless of location — it remains exactly the same on the Earth, the Moon, or anywhere else in space. Only the weight changes, because weight depends on the local gravitational force, while mass is an intrinsic, unchanging property of the object.
Question 10
Three objects 1, 2, and 3 of the same size and shape but made of different materials are placed in water. They dip to different depths. If the weights of the three objects are $w_1$, $w_2$, $w_3$ respectively, then: (i) $w_1=w_2=w_3$ (ii) $w_1>w_2>w_3$ (iii) $w_2>w_3>w_1$ (iv) $w_3>w_1>w_2$
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Fig. 5.17: Objects 1, 2, and 3 dipping to different depths in water
Answer
Correct option: (ii) $w_1 > w_2 > w_3$
From the figure, object 1 is submerged the deepest, object 2 is submerged to a medium depth, and object 3 floats with the least depth submerged. Since all three objects have the same size and shape, an object that sinks deeper into the water needs to displace a larger volume of water to generate enough buoyant force to balance its own weight. This means the object that is submerged deepest must be the heaviest (greatest weight), since it requires the most buoyant force (and hence the most water displacement) to stay afloat at equilibrium.
Therefore, weight decreases in the order of depth submerged: object 1 (deepest, heaviest) has the greatest weight, followed by object 2, and object 3 (shallowest, lightest) has the least weight — giving us $w_1 > w_2 > w_3$.
From the figure, object 1 is submerged the deepest, object 2 is submerged to a medium depth, and object 3 floats with the least depth submerged. Since all three objects have the same size and shape, an object that sinks deeper into the water needs to displace a larger volume of water to generate enough buoyant force to balance its own weight. This means the object that is submerged deepest must be the heaviest (greatest weight), since it requires the most buoyant force (and hence the most water displacement) to stay afloat at equilibrium.
Therefore, weight decreases in the order of depth submerged: object 1 (deepest, heaviest) has the greatest weight, followed by object 2, and object 3 (shallowest, lightest) has the least weight — giving us $w_1 > w_2 > w_3$.
Reference Figure — Electroscope (Discover, Design and Debate · Page 79)
An electroscope is a device used to determine whether an object is electrically charged.
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Fig. 5.18: A homemade electroscope made using a jar, straw, copper wire, and aluminium foil
How it works
When a charged object (like a rubbed plastic scale) is brought close to the copper wire coil at the top, charge transfers down the wire and straw to the aluminium foil strips hanging inside the jar. Since both foil strips receive the same kind of charge, they repel each other and spread apart — this visible movement of the foil strips indicates that the object brought near is electrically charged. The greater the spread of the foil, the stronger the charge on the object being tested.