Question:
Two concentric spherical shells have masses $M_{1}, M_{2}$ and radii $R_{1}, R_{2}\left(R_{1}
Solution:
Here, $d=\frac{R_{1}+R_{2}}{2}$
The gravitational force of mass $\mathrm{m}$ due to shell of $M_{2}$ is zero as gravitational field inside shell is zero.
Gravitational field due to shell of mass $M_{1}$ outside the shell at mass $m=\frac{G M}{d^{2}}$
So, force $F=\frac{G M m}{d^{2}}$
$F=\frac{G M m}{\left(\frac{R_{1}+R_{2}}{2}\right)^{2}}$
$F=\frac{4 G M m}{\left(R_{1}+R_{2}\right)^{2}}$