Friction

Friction is a force opposing the motion of an object when the object tends to move relative to the other. Therefore, it is an example of contact force.

Friction is zero when a block lies on a flat surface. If an external force is applied on the block and increased slowly, the friction between the block and the flat surface increases almost linearly. This friction is called static friction. The magnitude of this friction is always less than the product of the coefficient of friction between the two surfaces and the normal reaction on the block. The block starts sliding when the force reaches its threshold value. This friction (the friction at which the block is about to start) is called the limiting friction. The magnitude of this friction is always equal to the product of the coefficient of friction between the two surfaces and the normal reaction on the block. Once the block starts sliding, the friction drops and remains almost constant. This is called the kinetic or dynamic friction. Therefore, the maximum value of friction is the limiting friction.

Friction is a necessary evil. Friction causes the loss of energy into a non-renewable form (thermal energy), but at the same time we can not do several important things without friction. For example, walking is impossible without friction between our feet and the ground. 

The study of friction is called tribology.

Test Yourself: 

(1) A horse has to apply more force to start a wagon than to keep it into motion. Why?