AMU Engineering 2017 solved paper Solved Paper

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.