Here, $m = 15\,\text{kg}$; $a = 0.5\,\text{m}\,\text{s}^{-2}$, $t = 20\,\text{s}$ and $m = 0.18$Force on the block due to the motion of the trolley,$F = ma = 15 \times 0.5 = 7.5\,\text{N}$Force of limiting friction on the block,$f' = mR = \mu m g$ $= 0.18 \times 15 \times 10 = 27\,\text{N}$(a) To a stationary observer on the ground, force F on the block acts so as to cause the motion and the force f opposes the motion of the block. Since $f > F$, the block will continue to remain stationary. In fact, the force of limiting friction f adjusts itself to be equal to the force F.(b) The motion of the trolley is an accelerated one. Therefore, the trolley is a non-inertial frame of reference and the observer moving with the trolley is in a non-inertial frame. The laws of mechanics are no longer valid in such a frame.