Q 11-07-184JEE MainJEE Main 2019 (9 Apr, Shift 1)Easy
A solid sphere of mass $M$ and radius $a$ is surrounded by a uniform concentric spherical shell of thickness $2a$ and mass $2M$. The gravitational field at distance $3a$ from the centre will be
Answer: (B) $\dfrac{GM}{3a^2}$
The shell extends from $a$ to $3a$, so a point at $3a$ is just outside all the mass, $M + 2M = 3M$. It acts as if concentrated at the centre:
$$g = \frac{G(3M)}{(3a)^2} = \frac{GM}{3a^2}$$
Solution by Sreeraj P, M.Sc Physics