A shell of mass M and radius R has another point mass m placed at a distance r from its centre (r > R). The force of attraction between the shell and point mass is.

F = $\frac{GMm}{r}$ F = $\frac{GMm}{r^2}$ F = zero None of above

F = $\frac{GMm}{r^2}$

F = $\begin{cases} \frac{GMm}{r^2} \quad r ≥ R \text{(outside and at surface)} \\ ~~0 ~~~~~\quad r < R \text{(inside)} \end{cases}$