Q: A point mass m is placed inside a spherical shell of radius R and mass M. at a distance R/2 from the centre of the shell. The gravitational force exerted by the shell on the point mass is
(a) $\frac{G M m}{R^2}$
(b) $-\frac{G M m}{R^2}$
(c) Zero
(d) $ 4 \frac{G M m}{R^2}$
Ans: (c)