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Homework Help: Planetary Motion? I think?

  1. Nov 29, 2008 #1
    1. The problem statement, all variables and given/known data
    "Yoda has just discovered planetary system consisting of star Dagobahr and its two planets: Rool and Krau. Planet Rool has average orbital radius 1.6 times as big as planet Krau. If planet Rool orbits Dagobahr in 2.9 Earth years, how long is a year for the resident of Krau? Give the answer in Earth years.

    2. Relevant equations
    Trool^2/rRool^3 = TKrau^2/rKrau^3

    3. The attempt at a solution

    TRool =?
    rRool = 1.6*rKrau

    I have no idea where to go from here...
  2. jcsd
  3. Nov 29, 2008 #2


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    Staff Emeritus
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    Try re-arranging your equation so that the orbital radii are on the same side. What happens if you substitute your values into the equation?
  4. Nov 29, 2008 #3
    Kepler's Third Law can be rewritten as R = kT2/3, where k is the constant of proportionality. When R increases by 1.6, by what factor does T increase?
    Last edited: Nov 29, 2008
  5. Dec 1, 2008 #4
    Does T increase by 1.6, too?
  6. Dec 1, 2008 #5
    That is incorrect. T would increase by 1.6 if T and R are directly proportional. However, according to Kepler's Law, R is proportional to T^2/3.
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