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