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Homework Help: Kepler's law?

  1. Oct 17, 2008 #1
    Planet X orbits the star Alpha with a 'year' that is 200 earth days long. Planet Y circles Alpha at nine times the distance of planet X. How long is a year on planet Y?

    I know that I have to use the kepler's law : T^2 = (4pi^2/GM)r^3
    but I'm not too sure how to do solve this...
    Please help!
    Thanks.
     
  2. jcsd
  3. Oct 17, 2008 #2

    LowlyPion

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    Homework Helper

    So T12/r13 = T22/r23
     
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