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Homework Help: Circular Motion and Gravitation of a star

  1. Dec 11, 2007 #1
    1. The problem statement, all variables and given/known data
    Two equal-mass stars maintain a constant distance apart of 8.0 x10^10 m and rotate about a point midway between them at a rate of one revolution every 12.6 yr
    (a) why don't the two stars crash into one another due to the gravitational force between them
    (b) what must be the mass of the stars
    2. Relevant equations

    F = G*m1*m2/r^2
    F(r) = ma(r)
    a(r) = v^2/r
    v = 2*pi*r/T

    3. The attempt at a solution
    part b i said m1 = m2 = m
    F = ma(r)
    F(12) = m*(2*pi*r/T)^2 / r
    F(12) = G*m^2 / r^2

    substitution and solving for m gave me

    m = ((2*pi*r)^2 * r )/ (G*T^2)

    substituting i got 2.4 *10^26 kg

    answer in book is like 9.6*10^26 kg

    can someone point out my error(s) to account for this discrepancy?
  2. jcsd
  3. Dec 11, 2007 #2
  4. Dec 11, 2007 #3
    oh snaps i figured it out

    if anyone cares....

    my mistake was that in the equation for graviational attractive force (F = G*m1*m2/r^2) the r is actual distance between the two center of masses and my mistake was that i thought it was half of that distance (the radius)

    so yeah...cheers?
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