Concepts

Class 8
Chapter 11 Class 8 - Force And Pressure

## Muscular force

Force which is exerted by muscles of the body is called Muscular force

All our body activities like lifting, walking, running, bending is because of muscular force

It is a contact force because muscular force can only be exerted on physical contact

Example 1

When we lift a book by our hand,it is because of muscular force of our hand

Our hand must be in contact with book,only then we are able to lift it

Example 2

When we walk,it is because of muscular force of our legs

Our legs must be in contact with the ground,only then we are able to walk

Example 3

To lift heavy weights, we use muscular force

### Muscular Force by Animals

Animals like bulls and horses use their muscular force to move bullock carts and horse carts

## Friction

The force which opposes motion of one body over another body is called friction

It is a contact force because it is exerted only when this force is exerted by one body on another body

Example 1

A ball which is moving comes to rest after some time

This is due to force of friction

This friction opposes motion of ball on the ground and brings it to rest.

Note-

Friction is a contact force because ball should be in contact with ground only then force of friction occurs and ball stops

Example 2

Boat comes to rest if we stop rowing

This is due to force of friction

This friction opposes motion of boat on the river and brings it to rest.

Friction is a contact force because boat should be in contact with the river only then force of friction occurs and boat stops

Example 3

A car comes to rest when engine is switched off

This is due to force of friction

This friction opposes motion of car on the road and brings it to rest.

Friction is a contact force because both car and river shall be in contact with each other,only then force of friction occurs and boat stops