I read the proof that for a thin spherical shell with mass M, the potential energy U for a point mass m inside the shell is constant (depending only on the radius R of the shell) and thus the gravitational force on m is 0 at any point inside the shell. Then they talk about a point mass inside the earth: they say that thanks to what we discovered about the energy and force on m inside a shell - we can conclude that the gravitational force on m inside the earth (A uniform sphere) is only due to the mass that's "locked-up" inside the sphere with radius r, that represents the distance between the center of the earth and m, and all the earth's mass "outside" of r doens't contribute to the gravitational force at all. Also - the earth's mass that's "inside" exerts gravitational force as though it was all concetrated at the center of the earth. I don't understand - what's the difference between a shell and the earth? why in one we have 0 force and on the other it's not 0? and how did they conclude all of the above (mass "outside" m exerting no force, mass "inside" m exerting force as if it was all in the center of the earth) from the spherical shell proof?