Push or pull on the object is called as “Force”.

Direction of force is defined as the direction in which object is pushed or pulled.
A force is that cause which changes or tends to change the state of rest or motion of a body.
That is the inertial state of body.

Unit of force: in SI system it’s expressed as newton or kilogram weight.

Effects of force:
Application of force brings the change.
• It can bring movement of stationary object
A force applied on stationary object, result into the change of its original position.
Example: pulling a toy tied with thread
• It can stop a moving object.
On application of force into opposite direction of the movement of object, stops motion of
object.
Examples: stopping a moving bicycle by usage of hand.
• It change the speed of an object
Example: if two rolling marbles collide each other then there speed decrease as compared to
their initial speed before the collision.
• It can change the direction of motion of an object.
The moving ball touched by bat changes into direction.
• It change the shape and size of object
Squeezing the toothpaste pack brings the changes in the actual shape of it. Same explanation
goes with shape of spring changes on pulling.

Classification of force:

Contact force:
To bring about change, contact forces require physical contact between the objects.
Push, pull, twist, friction etc. are best examples of such force.

Mechanical force:
Force which is exerted by machine is termed as mechanical force. The movement of water
wheel by simply movement water is the mechanical force.

Frictional force:
Surfaces which are in contact, the force acts between these two surfaces is called as friction
force.

Muscular force:
In daily life, man push and pull many things. In case of human and animal,the force being
apply by muscles is called as muscular force.
This force acts on the bodies in contact, that’s why termed as type of contact force.
Non-contact Forces:
These kind of forces do not require actual physical contact between two bodies on which it
act to bring about changes.

• Electrostatic force :
The force exerted by an electrically charged object on uncharged objects termed as
electrostatic force.
Example: plastic comb get electrically charged when rubbing on dry hair which exerts force
on pieces of small papers.

• Magnetic force :
The force exerted by magnet on magnetic material or bodies is called as magnet.
Toys inbuilt magnet attracts or repel each other.

• Gravitational force :
Object fall towards the pull exerted by earth is called as gravitational force .Theses force is
experienced by all the bodies within the gravity region of the earth

Weight:
The gravitational pull of the earth on a body is called its weight.
And it also defined as the force with which earth attracts a body towards its centre is termed
as weight.
W= m x g
Weight = mass x acceleration due to gravity
Greater weight indicates that earth is exerting greater pull on that particular body.
Spring balance is used to measure the weight of an object. In which we can see the extension
of spring is depends on the heaviness of object because heavy object get greater pull by earth
with greater force.

Measuring the forces:
Unit of measurement of force is newton in SI system.

1. Friction wears away the soles of our shoes:
   The force of friction between soles of shoes and road brings this change. In same
   case tyres of cars also wear out gradually.
2. A person may fall due to less friction:
   Slipping off while walking due to banana skin or water spill on tiles is because their
   presence reduces the friction between two surfaces.
3. Friction wears out rubbing machine parts:
   Due to friction between rubbing parts there is gradual damage to machine parts.
4. Friction produces heat:
   Whenever there is moving parts in machine contact each other facilitate the
   generation of heat.
5. Friction shows down motion:
   Moving things are slows down because of excessive friction.
6. Reduces the efficiency of machine:
   Part of applied force is wasted to overcome the frictional force which ultimately
   result into decrease in efficiency of machine.
   Advantage of friction:
7. Friction enables us to walk without slipping on road as gives the firmness to stand.
   In case of tyres of vehicles it prevents skidding.
8. Friction ensures the proper contact of pencil and paper so we can write down easily.
9. Matchstick get burn because of frictional force between the head of stick and side of
   matchstick which brings heat.
10. Cutting of wood with saw.
11. The breaks of vehicle work on the principle of frictional forces between shoes and
    rim of the wheel.
    Short Notes
    Class 6 ICSE

Types of friction:

1. Static and sliding friction :
   (a)Whenever the force applied is balanced by force of friction called as static friction.
   There is no net movement of object. This force end with beginning of sliding of object at
   rest.
   (b) Sliding force is defined as minimum amount of force require to just slide over the
   object. It is less than the static friction.
2. Rolling friction :
   The force of friction between the rollers on a body and the surface is called as the rolling
   friction. It is much lesser than sliding friction.
   Examples: ball bearing, cylindrical bearing rubber tyre
   Methods of increasing friction:
   • Grooves are made in tyres of vehicles to increase friction between tyre and road which
   used to avoid the skidding.
   • Spikes are provided in shoes for better grip on ground fro athlete.
   • To drive machine wheel , belt over the wheels are made of special type which increase
   the frictional force.
   Methods of reducing friction :
   • Application of lubricants brings lesser friction between two surfaces which are in
   contact, applying oil or grease is termed as lubrication.
   • Use of ball bearing in machine reduces the friction , in this case the axle is not in direct
   contact with rubbing surfaces so less friction is generated.
   • By using wheels to move object as it lesser down the direct contact so with it can be
   pull down.
   • Friction can be reduced by streamlining the body that included the bodies with
   pointed or sharp front portion of the moving object
   Limiting friction:
   The friction force can be increase up to the limit only, this consider as limiting friction.
   The maximum value of the friction that can occur between two surfaces is called as limiting
   friction.