Concept and application of limiting friction:

Sol.: The maximum value of frictional force that comes into play, when the body just begins to slide over the surface of the other body, is called as limiting friction. Consider solid body which is placed on horizontal plane surface.

W = Weight of body acting through center of gravity downwards.

R = Normal reaction of body acting through center of gravity downwards.

P = Force acting on the body through center of gravity and parallel to the horizontal surface.
F = Limiting force of friction

If 'P' is small, the body will not move as the force of friction acting on the body in the direction opposite to 'P' will be more than 'P'. But if magnitude of 'P' goes on increasing a stage comes, when the solid body is on point of motion. At this stage, force of friction which is acting on body is called as LIMITING FORCE OF FRICTION (F).

R = W; F = P

If the magnitude of 'P' is increased further the body will start moving. The force of friction, acting on body is moving, is called as KINETIC FRICTION.