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Homework Help: Not really understanding Gravitational Potential energy or Kepler's laws?

  1. Jul 14, 2011 #1
    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. jcsd
  3. Jul 14, 2011 #2


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    Staff: Mentor

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


  4. Jul 14, 2011 #3
    F = Gmm/r^2 for gravitational potential energy
    F= mv/r = mr[itex]\omega^{2}[/itex]for centripetal force.
    [itex]\omega[/itex] = 2[itex]\pi[/itex]/T

    Reshuffle them.
    Last edited by a moderator: Jul 14, 2011
  5. Jul 15, 2011 #4
    only third law of kepler has mathematical questions at basic level
    [itex]T^2 \propto R^3[/itex]
    tell me if you need its derivation

    for Gravitational potential ...

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

    for more help please write back ...
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