A  spherical  cavity  is  created  in  a  uniform  solid  sphere  of  mass  M  and  radius  R  by removing a part of sphere. The radius of the cavity is r & its centre is at a distance ‘a’ from the centre of the sphere.

Consider superposition of complete sphere of mass M and radius R  and for cavity take negative mass MR3r3 and radius r. g→=GMR3a→+r→−GMR3r3r3r→=GMR3a→ Gravitational field is independent of location of point in cavity or we can say the field is uniform inside the cavity.