>>11489966Suppose you have a hollow sphere of mass.
An object inside the sphere won't experience any net force from gravity. Thus as you dig into a solid sphere of mass, decreases linearly, as the mass shell above you no longer contributes to the net force. At the center of the Earth you would experience no net gravitational force.
Newton's Law holds only for point masses, the fact that we can model the external gravitational field from earth as a point mass field is something we take for granted nowadays. Newton had some crazy geometric proof for this.
