Explain the concept of frictional force.

1. Whenever the surface of one body slides over another, each body exerts a certain amount of force on other body. 

2. These forces are tangential to the surfaces. The force on each body is opposite to the direction of motion between two bodies. 

3. It prevents or opposes the relative motion between two bodies. 

4. It is common experience that an object placed on any surface does not move easily when a small force is applied to it. 

5. This is because of certain force of opposition acting between the surface of the object and the surface on which it is placed. 

6. To initiate any motion between pair of surfaces, we need a certain minimum force. Also, after the motion begins, it is constantly opposed by some natural force. 

7. This mechanical force between two solid surfaces in contact with each other is called as frictional force.