A block B of mass m is kept on the rough surface of a plank p, also of mass m. The plank is placed on a smooth horizontal floor. The block B is projected to the right with velocity V0 on the plank. Work done by force of friction on B is WB and that on P is WP. Then

Force of friction on B is fB and that on P is fP and they are equal in magnitude. fB is directed to the left and fP is directed to the right. When the block is projected, after initial slipping, B and P start moving with same velocity and the force of friction reduces to zero. During the course of slipping displacements of B and P relative to the ground are dB and dP respectively and also dB>dP. Now WB=fB→.dB→=−fBdB=−f.dB and WP=fP→.dP→=fP dP. Since dB>dP we can write WB>WP and WB+WP is negative