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Homework Help: Plaskett's binary system

  1. Nov 18, 2008 #1
    i've been having trouble understanding a part in this question. i'll just map out the basics to the question: 2 stars with a velocity of 220km/s are orbiting at equal distance. the masses of the stars are equal. period = 14.4 days it asks to find the mass of each star.

    i understand that u can use kepler's law to get the answer.

    GMm/r2 = mv2/r

    -which is what i used to come up with my answer seeing that i found the mass relative to a center mass. but according to the solution it's suppose to be:

    GMm/(2r)2 = mv2/r

    my question is, why is r denoted 2r in order to arrive at a correct answer?
  2. jcsd
  3. Nov 18, 2008 #2


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    Staff Emeritus
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    Homework Helper

    On the left hand side, you use the distance between the 2 stars.

    On the right hand side, you use the distance from a star to the center of the circular orbit.
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