Problem: Calculate the distance from the center of the Earth to a point below the surface where the acceleration due to gravity is 0.50g, where g is the acceleration due to gravity at the Earth's surface. I am really stuck on this. I tried using T^2 = 4pi^2 / GMe *r^3 but it didnt work it could be because M is supposed to be the mass of the sun but I decided it should be the mass of the earth! But it wasnt, it didnt work. I just cant seem to find a way to relate Keplers Laws to this problem. Are there other equations I could use? Thanks in advance!