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 frictionAs, 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
