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Kepler's third law

  1. Feb 1, 2007 #1
    I don't know but in school I learnt it in a different way. I mean in school the Kepler's third law was thought as if all the orbits were circular. But according to the definition its all elleptical, and in the school equation we won't add the planets mass, but here

    The planet mass is also added.
    So I am not sure what kind of equation I should use to solve sums?

  2. jcsd
  3. Feb 1, 2007 #2

    Doc Al

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    Staff: Mentor

    Two points:
    (a) A circle is special kind of ellipse; I'm sure you instructor approximated the orbits as circular just to keep the calculations easier.
    (b) The masses are added in turning Kepler's 3rd law from a statement of proportionality to an equality; it goes beyond Kepler's original law by adding Newton's model of universal gravity.​

    I don't know what you mean by "solve sums".
  4. Feb 1, 2007 #3

    Thanks alot for replying. We finished orbital motion just 3 days ago and he gave us a work sheet with problems to solve. I think your correct, he approximated the orbits as circular to keep thinngs simple. The equation that I am using is

    r- radius of orbit
    M - mass of the center object

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