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Homework Help: Help with Kepler's Laws problem PLEASE!

  1. May 8, 2008 #1
    ok so here's the problem
    A binary star system has two stars, each with the same mass as our sun, separated by 8.00 x 10^11 . A comet, far away from both stars is essentially at rest. Slowly but surely, gravity pulls the comet toward the stars. Suppose the comet travels along a straight line that passes through the midpoint between the two stars.
    What is the comet's speed at the midpoint?

    here's what i tried (but it was wrong)
    pe initial(comet)= ke intial = 0
    pe final (comet)= ke final= -GMsun/r
    since 2 masses --> 2(GMsun/r)

    ke = 1/2mvr^2---> v= sqrt(4(GMsun/r))
    it says i have the wrong answer...
    Is my method completely wrong? What can I do to fix it? Any help is greatly appreciated!:confused:
  2. jcsd
  3. May 9, 2008 #2
    why do you have 1/2mvr^2 for the kinetic energy? You do end up with the correct expression for v. What did you use for r? r is half the distance between the stars.
  4. May 9, 2008 #3
    Method looks fine to me, if r is the distance from midpoint to star.
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