# Kinetic Friction

## Kinetic Friction

Friction is a force that tries to oppose motion between the two surfaces in placed contact. When we throw a ball on the floor it starting with some velocity. But once it is rolled ideally no force is acting in the direction of motion and according to Newton’s first law the ball should keep on rolling but this does not happen. The ball stops after moving certain distance, so a force must be acting on it. That force is known as Friction.

What is a Kinetic Friction?

The friction force act on the body when the body is moving is called the Kinetic friction. It always acts in opposite direction of the motion, not like static friction which act in the direction of motion. Kinetic friction is slightly less than the maximum static friction. Kinetic friction depends on the kinetic coefficient. Force that resists motion of one body over another with which it is in contact, it is denoted by fk. As motion starts static friction (fs) vanishes and Kinetic friction (fk) appears. It is independent of the area of contact and velocity of the body. It varies with Normal reaction (N).

fk α N

fk = Constant x N

fk = μk x N

Where,

μk = Co-efficient of Kinetic Friction.

There are three cases can arise in a body’s motion.

Case 1: When applied force > fk

Fa > fk

(Fa – fk) = ma

$$Acceleration(a)\,\,=\,\,\frac{\left( {{F}_{a}}-{{f}_{k}} \right)}{m}$$.

Case 2: When applied force = fk

Fa = fk

∴ a = 0

The body moves with uniform velocity.

Case 3: When applied force = 0

Fa = 0

$$a\,\,=\,\,-\frac{{{f}_{k}}}{m}$$,

No motion occurs, the body stops.