Question

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 :

• A. $\frac{2GM}{9a^2}$
• B. $\frac{GM}{3a^2}$
• C. $\frac{GM}{9a^2}$
• D. $\frac{2GM}{3a^2}$

Answer 1

Answer: (B)

$\frac{GM}{3a^2}$

Answer 2

We use gauss's Law for gravitation
$g. 4 \pi r^2$ = (Mass enclosed) 4$\pi$G
$g = \frac{3M4 \pi G}{4 \pi (3a)^2}$
$= \frac{MG}{3a^2}$
Option (2)