# Not really understanding Gravitational Potential energy or Kepler's laws?

1. Jul 14, 2011

### mayodt

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.

2. Jul 14, 2011

### Staff: Mentor

I'm not much help on Kepler's Laws, but the wikipedia entry looks to have some good info:

http://en.wikipedia.org/wiki/Kepler's_laws

.

3. Jul 14, 2011

### Bloodthunder

F = Gmm/r^2 for gravitational potential energy
F= mv/r = mr$\omega^{2}$for centripetal force.
$\omega$ = 2$\pi$/T

Reshuffle them.

Last edited by a moderator: Jul 14, 2011
4. Jul 15, 2011

### cupid.callin

only third law of kepler has mathematical questions at basic level
$T^2 \propto R^3$
tell me if you need its derivation

for Gravitational potential ...

the GPE of 2 masses m1,m2 separated by r is $U = - \frac{Gm_1m_2}{r}$

for more help please write back ...