I was reading this very small section in my physics book and cant seem to come to grips with its concept. It starts off with Newtons shell therem being applied to a particle located inside a uniform shell and that the shell exerts no 'net' gravitational force on a particle inside it. It then uses the earth as an example and says (if earth's mass were uniformly distributed) for a particle that somehow traveled inward from earths surface that the gravitational force would tend to 1) increase because the particle is moving closer to center of Earth and 2)decrease because the thickening shell of material lying outside the particle's radial position would not exert andy net force on the particle. *What I'm understanding so far is: By using [F=GMm/r2] case1) is saying that r is decreasing here, thus increasing F and for case2) that M is decreasing, thus decreasing F? THEN it says for a uniform earth the first case would prevail but for our NON uniform earth the "force on the particle actually increases as it begins to descend, and then the force reaches a maximum at a certain depth, and then decreases as the particle descends farther." *What? is this just saying that for the earth, we cant apply the general rules, because the earth is special and has some max force. Why does the book confuse me with this fact if this "particle inside the earth" example is impossible anyways? OR am i misunderstanding the last statement completely? Thank you.