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Homework Help: Find Pluto's year length using Kepler's third law.

  1. Sep 17, 2012 #1
    1. The problem statement, all variables and given/known data

    Pluto is 40 times further from the Sun than we are. How long is a year on Pluto? (Use Kepler's third law.)

    2. Relevant equations


    3. The attempt at a solution

    Te^2(Earth period)=x*r^3

    Tp^2(Pluto period)=x*(40r)^3

    I don't know what to do exactly from here. I'm not even entirely sure that I'm going about this the right way.

    Any help would be appreciated.
  2. jcsd
  3. Sep 17, 2012 #2
    Hint: For every planet [which goes around the sun] in our solar system R^3/T^2=constant.
  4. Sep 17, 2012 #3
    I've already made x a constant (T^2/r^3), in the attempt at a solution. I just don't realise what to do from there.
  5. Sep 17, 2012 #4
    I don't know what you mean by:
    First of all you need to understand why for every planet which goes around the same star R^3/T^2 is constant.
    Then don't forget what you already know about the difference in R between earth and Pluto.

    I'm already gave you the answer, actually...
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