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Two moons

  1. Oct 20, 2009 #1
    1. The problem statement, all variables and given/known data

    Two moons orbit a planet in nearly circular orbits. Moon D has orbital radius r, and moon E has orbital radius 4r. Moon D takes 20 days to complete one orbit. How long does it take for moon E to complete one orbit


    2. Relevant equations

    None - I think


    3. The attempt at a solution

    I am hoping that this problem is not complicated. Since it takes 20 days for moon D with radius r, it would take 80 days for moon E with radius 4r?

    Is my reasoning correct ??
     
  2. jcsd
  3. Oct 20, 2009 #2

    D H

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    Staff Emeritus
    Science Advisor

    No, it isn't. What do Kepler's laws have to say about this problem?
     
  4. Oct 20, 2009 #3
    Is it just 20 days the same as moon D?

    A line drawn from the sun to any planet sweeps out equal areas in equal time intervals.

    Does the size - radius matter then?
     
  5. Oct 20, 2009 #4
    Ok the above doesn't make sense.

    Then I don't know, really.
     
  6. Oct 20, 2009 #5
    It's not just 20 days.

    Keplers second law is only valid for the same planet or moon (and it's trivial for a circular orbit)
    Try the third law.
     
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