Two concentric shells of mass `m_(1)` and `m_(2)` are situated as shown. Find the force on a particle of mass `m` when the particles is located at `(a) r=r_(1),(b)r=r_(2),(c)r=r_(3)`. The distance `r` is measured from the centre of the shell.

Answer

(a) `Gm(M_(1) + M_(2))//a^(2);` (b) `(GMm//b^(2))`; (c ) `0`