Whenever the surface of a body slides over that of another, each body exerts a force of friction on the other, parallel to the surfaces. The force of friction on each body is in a direction opposite to its motion relative to the other body.
The force of friction comes into action only when there is a relative motion between the two contact surfaces or when an attempt is made to have it.
It is a self adjusting force, it can adjust its magnitude to any value between zero and the limiting (maximum) value i.e
0 ≤ f ≤ fmax
The frictional force acting between any two surfaces at rest with respect to each other is called the force of static friction. And the frictional force acting between surfaces in relative motion with respect to each other is called the force of kinetic friction or sliding friction.
