Gravity of a concentric hollow sphere
= Gravity of a concentric hollow sphere
{title2=$g(r)=GM(<r)/r^2$}
For constant <mass density> $\rho$ on $a<r<b$, the <shell theorem> gives zero <gravitational field> for $r\le a$, inward magnitude $4\pi G\rho(r^3-a^3)/(3r^2)$ for $a<r<b$, and $4\pi G\rho(b^3-a^3)/(3r^2)$ outside. These formulas agree continuously at both boundaries. The empty cavity has zero <gravitational field> but generally nonzero constant <gravitational potential>.