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Double Star Mass

  1. Aug 20, 2009 #1
    1. The two components of a double star are observed to move in circles of radii r_1 and r_2. What is the ratio of their masses? (Hint: Write down their accelerations in terms of the angular velocity of rotation, w)

    The answer is m1/m2 = r2/r1.

    How does one cancel the velocity from the problem to get this result?

    2. Relevant equations

    For an isolated two body system

    where m are the body masses and a are the accelerations

    angular velocity equals velocity divided by radius

    angular acceleration

    3. The attempt at a solution
    m1.dw1/dt = m2.dw2/dt

    therefore m1.w1=-m2.w2

    m1/m2 = -w2/w1

    m1/m2 = -(v2/r2)/(v1/r1)

    m1/m2 = -v2r1/v1r2
    Last edited: Aug 21, 2009
  2. jcsd
  3. Aug 20, 2009 #2


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    Do the stars rotate about any which point or about a specific point? What are this point's properties?
  4. Aug 21, 2009 #3
    Unfortunately all information is contained in the question. This question is taken directly from chapter 1 problem 2 of Classical Mechanics by Kibble and Berkshire. I'm not sure if this is a good book yet or not. From your reply it seems the question does not contain enough information to find the solution given. Can one infer the conditions of the system from the result given (m1/m2 = r2/r1)?
  5. Aug 21, 2009 #4


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    I am not familiar with the textbook, but it does not matter. Does "center of mass" ring a bell?
  6. Apr 4, 2010 #5


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    I know this is an old post but for anyone looking up this problem, here's how I got to that answer:

    Using Newtons law for 2 isolated bodies

    m1a1 = -m2a2

    The bodies are rotating ( here i guessed at the same velocity).
    The motion is rotational so you can use tangential acceleration

    aT = w2r and substitute this into above to get

    m1 w2r1 = -m2w2r2

    cancelling out w2 on both sides, move m's to left and r's to right to get

    m1/m2 = -r2/r1
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