Inside a hypothetical spherical shell of uniform mass distribution, the gravitational field at any interior point is:

1. By the shell theorem (a corollary of the inverse-square law), the contributions of all elements of a uniform shell cancel at every interior point. 2. So the gravitational field inside a uniform shell is identically zero. 3. Outside the shell, the field is the same as if all the mass were concentrated at the centre. 4. This is why g decreases LINEARLY with depth d inside a uniform Earth (only the mass within r = R − d attracts). _Source: syllabus Physics Ch 7 "Gravitation", §7.5–7.6 (shell theorem implication)_