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