A block of mass m = 2 kg is placed in equilibrium on a moving plank accelerating with acceleration a = 4 $\mathrm{m}/{\mathrm{s}}^2$. If coefficient of friction between plank and block $\mathrm{\mu } = 0.2.$ The frictional force acting on the block is

Driving force w.r.t platform = ma

= $2 \times 4 = 8 \mathrm{N}$

And resisting force = ${\mathrm{f}}_{\lim } = \mathrm{\mu } \mathrm{N}$

= $0.2 \times 2\times 10 = 4 \mathrm{N}$

Here 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.