1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: 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

    User Avatar
    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.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook