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Homework Help: Planetary Motion Formula

  1. Jan 20, 2009 #1
    I just need an explanation of a formula, and I think part of it is wrong, so here is the formula:


    so T is the period and [tex]T_A[/tex] is the period of planet A
    then below it is what I think is wrong, [tex]T_G[/tex] G is the Kepler's constant, I'm not sure what that really is
    [text]R_A[/tex] which is radius of planet A over radius of planet B cubed, this side is right, any help is appreciated thanks
  2. jcsd
  3. Jan 20, 2009 #2


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    Homework Helper

    I comes from Kepler's third law which states that

    [tex]T^2 \propto R^3[/tex]

    So if two planets have the same mass, in your case planet A and G.

    then [itex]T_A^2= k R_A^3[/itex] and [itex]T_G^2= kR_G^3[/itex]

    so just divide them and you'll get the formula
  4. Jan 20, 2009 #3
    I think I found the answer after some searching, which I did try before which brought me here, but I think it is [tex](T_A/T_B)^2=(R_A/R_B)^3[/tex]
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