Two concentric spherical shells have masses `m_(1)`, `m_(2)` and radii `R_(1)`,`R_(2) (R_(1) lt R_(2))`. Calculate the force by this system on a particle of mass `m`, if it is placed at a distance `((R_(1) + R_(2)))/(2)` from the centre.

Answer

`[(4GM_(1)m)/(R_(1)+R_(2))^(2)]`