Find the work done by the frictional force in drawing a circle of radius r by a pencil of negligible mass with a normal pressing force N ( coefficient of friction `mu_(k)`).

Work done by a frictional force is

Work done by frictional force

Work done by the force of friction

If the normal force is doubled, the coefficient of friction is

Work done by force of static friction .

Block of mass m are released from rest and they slide down the inclined surface as shown in figure. Show that the work done by the frictional forces on the blocks are same in all the cases. Also calculate the speeds with which the blocks reach point B. The coefficient of friction for all the surface is mu_k .

A : Work done by the frictional force can’t be positive. R : Frictional force is a conservative force.