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Homework Help: Circular Motion (ratio of velocities)

  1. Dec 11, 2012 #1
    1. The problem statement, all variables and given/known data

    Two small planets are moving in circular orbits around the same star. If the radius of the orbit of planet A is 4 times the radius of the orbit of planet B, find the ratio of their speeds vA/vB.

    2. Relevant equations

    Not really sure but
    UCM T=2*pi*r/v

    3. The attempt at a solution
    I honestly have no idea how to do this problem. I know the answer is .5 I just want to know how to get there!
  2. jcsd
  3. Dec 11, 2012 #2


    User Avatar

    Staff: Mentor

    Have a look at Kepler's third law. It may give you a starting point.
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