A block of mass m = 2 kg is placed in equilibrium on a moving plank accelerating with acceleration a = 4 m/s2. If coefficient of friction between plank and block μ = 0.2. The frictional force acting on the block is

Driving force w.r.t platform = ma= 2 × 4 = 8 NAnd resisting force = flim = μ N= 0.2 × 2×10 = 4 NHere resisting force sufficient to keep the block at rest w.r.t plank hence the block will slide on plank and friction force will be 4 N.