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A solid block of mass m is resting on rough surface as shown in Fig. 5.1 (a).


We try to pull the block starting from low force and increasing slowly. We find that the block does not move. It means that the applied force is counter balanced by a force acting in the opposite direction of the applied force. This force developed to oppose the motion is called frictional force. When P increases frictional force F increases. When P reaches maximum, the block tends to slide over the resting surface. Let us draw the free body diagram of the block (Fig. S.l(b)).


The block is in equilibrium

Ʃ F x  =0

P-F =0

F =p

Ʃ f y =0

N –mg =0

N =mg