### Theory:

Can you push or lift a book lying on a table without touching it?

Books

Generally, to apply a force on an object, your body has to be in physical contact with the object. The contact may also be with the help of a stick or any other object.
The force resulting due to the action of muscles is known as the muscular force. Muscular force can be applied only when there is contact with an object. It is also called as a contact force.
Example:
• Opening and closing a door
• Lifting up the bucket
• Animals also make use of muscular force to carry out various activities and other tasks. Animals like bullocks, horses, donkeys and camels are used to perform various activities for us.
Friction:
Now let us look at an example,

Imagine you are pushing a ball. Then the ball starts moving. Will you think it will move continuously?
No, it will not move continuously. It will move to a certain distance, and then it comes to rest.

Why does the ball come to a halt? Do you have any idea?

Let us introduce a new term called, friction. Friction is a force that acts opposite to the moving body, and it tries to stop the motion of a moving body. Similarly, when we stop pedalling a bicycle, it gradually slows down and finally comes to rest. It is also due to friction between the road and the tyres.

Friction can be classified into two types, static friction and kinetic friction. Kinetic friction can further be classified into sliding friction and rolling friction.

Friction can be classified as:

Static friction: The friction experienced between static bodies or bodies at rest. A box of heavy load is very difficult to lift or move, which is due to static friction.

Kinetic friction: The friction between two or more moving bodies. Kinetic friction can be classified as sliding friction and rolling friction.
• Sliding friction: If a body slides over the surface of another body, the friction acting between surfaces in contact can be termed as sliding friction.
• Rolling friction: If a body rolls over the surface of another body, the friction between the surfaces is called rolling friction.