Newton's Shell Theorem (two parts):

1. A uniform spherical shell attracts a particle outside it as if the entire mass were concentrated at the centre.
2. A uniform spherical shell exerts zero net gravitational force on a particle inside it.

Consequences:

• For a solid sphere, only the mass enclosed within radius r (the sphere of radius r) contributes to the gravitational field at distance r from the centre.
• g inside Earth (at depth d): $g_d$ = g*$\frac{R-d}{R}$ (only inner sphere matters)
• g outside Earth: g = $\frac{GM}{r}$^2 (entire mass acts from centre)