Using the approximations that the Earth is perfectly spherical and of uniform density ρ ρ , and the fact that within a uniform hollow sphere there is no gravity, the gravitational acceleration a experienced by a body within the Earth is proportional to the ratio of the distance from the center r to the Earth's radius R . This is because underground at distance r from the center is like being on the surface of a planet of radius r , within a hollow sphere which contributes nothing.