Why does the Earth bulge at the middle?
It is because of rotational kinetic energy and centripetal force. The reason why Earth is a near-perfect sphere is because gravity pulls everything towards the center, and the sphere is the most compact. As you know, all points on the surface of a sphere are equidistant from a center point. The centripetal force pulls out. Say you go on a tire swing and have a friend spin you very quickly. If you relax your arms or legs, they are flung out. When things spin around, centripetal force pulls everything out.
