Q 11-04-197JEE MainJEE Main 2018 (15 Apr, Shift 2)Medium
A body of mass $2\ \text{kg}$ slides down with an acceleration of $3\ \text{m/s}^2$ on a rough inclined plane having a slope of $30^\circ$. The external force required to take the same body up the plane with the same acceleration will be: $(g = 10\ \text{m/s}^2)$
Answer: (D) $20\ \text{N}$
**Sliding down:** $mg\sin30^\circ - f = ma$
$$f = 2\times10\times\tfrac12 - 2\times3 = 10 - 6 = 4\ \text{N}$$
**Pushed up** (friction now acts down the plane, same magnitude), taking the force along the incline:
$$F - mg\sin30^\circ - f = ma$$
$$F = 10 + 4 + 6 = 20\ \text{N}$$
Solution by Sreeraj P, M.Sc Physics