Question

A thin hollow disc has mass M, inner radius R and outer radius 2R. P is a point on its axis a distance 2R from centre O. Then the intensity of gravitational field at point P is

Answer 1

REF.Image

At the surface of sphere, apply superposition :-

$E=\frac{GM}{(2R{)}^2}-\frac{GM/8}{R^2}$ (cut out part would've mass M/8 as its volume is V/8 of complete sphere)

$=\frac{GM}{4R^2}-\frac{GM}{8R^2}$

$\Rightarrow E=\frac{GM}{8R^2}$ at P