Hey, I have a test tommorow and I don't really understand gravitational potential energy or kepler's laws. It's not really a theory test, it's the math aspect, but I still don't understand it. I know of the formulas such as Fg=Gm1m2/r^2, Eg=Gm1m2/r, Ek=1/2 mv^2, and I think that's all. But somehow in the answer book for this question they come up with the equation C=GM/4(pie)^2 and C=r^3/T^2? This is the question if it helps: Using the mass of the sun and the period of revolution of Venus around the Sun, determine the average Sun-Venus distance. Also, I'm not really understanding a couple other questions as well, if anyone had something I could read that would help me out for this, it'd be amazing, thanks.