CompuServe Messages

Hi, Joe– The thing to remember is that Earth's gravitational effect drops as one approaches the center. Imagine a hollow spherical shell of uniform composition (well, the composition can vary with radius, but not asymmetrically). Its outer boundary is a sphere, and its inner boundary is a smaller sphere. If you are outside, it will pull on you depending on its mass and distance. If you are inside, it has no net gravitational effect on you; the pull from nearby matter on one side of you is balanced by the pull of more matter farther away on the other side. You are weightless as far as the shell has it. Inside the earth, everything farther away from the earth's center than you are constitutes such a shell, and might as well not be there as far as gravity is concerned. Only the matter closer the center has a net effect on you (hence my imaginary asteroid of radius "r", where "r" is your distance from the earth's center). If you are at the center, there is no matter closer to it than you are, so there is no net gravity. No reason to choose any direction as "up" or "down" (left and right are still there, since they are defined by your body, not gravity) except maybe you want to think of "up" as the direction from your feet to your head, which will change as you move about. A little bit away from the center, there is gravity, and an "up" independent of your body, but not a very important gravity. –Doug