Shell Theorem

The shell theorem, proved by Isaac Newton, is a result in classical mechanics that simplifies the gravitational effect of a spherically symmetric body. It states that such a body attracts objects outside it exactly as if all of its mass were concentrated at a single point at its center, and that a spherically symmetric hollow shell exerts no net gravitational force at all on an object located anywhere inside it. A consequence of these two facts is that, inside a solid sphere of uniform density, the gravitational force on an object varies in direct proportion to its distance from the center, falling to zero exactly at the center of mass. The theorem is applied throughout astronomy whenever the gravitational effect of a star, planet, or other roughly spherical body needs to be calculated.