KINETIC FRICTIONAL FORCE:

If the body begins to slide on the surface, the magnitude of the frictional force rapidly decreases to a constant value fk kinetic friction.

It has value equal to μkN.

where μ k = coefficient of kinetic friction and N = normal force.

Kinetic friction is of two types:

(a) Sliding friction
(b) Rolling friction

As, rolling friction < sliding friction, therefore it is easier to roll a body than to slide.

Kinetic friction (fk) = μk R

where μk = coefficient of kinetic friction and R = normal reaction.

Angle of repose or angle of sliding It is the minimum angle of inclination of a plane with the horizontal, such that a body placed on it, just begins to slide down.

If angle of repose is a. and coefficient of limiting friction is μ, then μs = tan α