Magnitude of frictional force acting on the block is given as f=µr× Reaction force =µrR Work done against friction force, =f.s=fs‌cos‌θ where, s is the displacement and θ is the angle between the frictional force and displacement. Here, θ=180°,s=d ⇒W=µRRd‌cos‌180°=−µRRd[∵cos‌180°=−1] where, negative sign implies that the displacement is in the opposite direction to that of the frictional force.