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Homework Help: Orbital radius

  1. Nov 29, 2008 #1
    1. The problem statement, all variables and given/known data
    see problem 9 attachment

    2. Relevant equations
    see problem 9 attempt attachment

    3. The attempt at a solution
    I come up with a solution that is much smaller than any of the choices. I do not know if I am to assume the orbital is circular. I do not know if there is another error besides that in my work.

    Attached Files:

  2. jcsd
  3. Nov 29, 2008 #2

    You need not assume that the orbit is circular. Use one of Kepler's Laws (which applies for both circular and elliptical orbits).
  4. Nov 30, 2008 #3
    ok if I use kepler's third law would I start by using moon II orbital period and radius to solve for the mass of the planet?
  5. Nov 30, 2008 #4
    You don't need to solve for the planet's mass. Remember that R^3 is proportional to T^2, and that must hold for all objects orbiting a body. Hence R^3/T^2 = constant.
  6. Nov 30, 2008 #5
    ok so I can find that ratio from moon II data and then multiply that ratio by T^2 of moon III?
  7. Nov 30, 2008 #6
    yup, that is right
  8. Nov 30, 2008 #7
    thank you for your help
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