Q 11-05-178JEE MainJEE Main 2019 (10 Jan, Shift 1)Medium
A block of mass $m$ is kept on a platform which starts from rest with a constant acceleration $g/2$ upwards. Work done by normal reaction on the block in time $t$ is
Answer: (D) $\dfrac{3mg^2t^2}{8}$
Newton's second law for the block: $N - mg = m\dfrac g2$, so $N = \dfrac{3mg}{2}$.
Displacement in time $t$: $s = \tfrac12\cdot\dfrac g2\cdot t^2 = \dfrac{gt^2}{4}$, in the direction of $N$.
$$W = Ns = \frac{3mg}{2}\cdot\frac{gt^2}{4} = \frac{3mg^2t^2}{8}$$
Solution by Sreeraj P, M.Sc Physics