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Homework Help: Balls exerting gravitational force on eachother

  1. Dec 7, 2008 #1
    1. The problem statement, all variables and given/known data

    Two balls, each with a mass of 0.790 kg, exert a gravitational force of 8.39 x 10^-11 N on each other. How far apart are the balls? The value of the universal gravitational constant is 6.673 x 10^-11 N m^2/kg^2. Answer in units of m.

    2. Relevant equations
    F = (G * m1 * m2) / r²

    3. The attempt at a solution

    8.39 * 10^-11 = (6.673 * 10^-11 * 0.79 * 0.79) / r²
    r² = 2.0145899
    r = 1.42m

    The physics program that I am using says 1.42 m is incorrect. I have checked my work, but I still get 1.42 m. Help?

    edit: Sorry, I messed up the topic title :(
  2. jcsd
  3. Dec 7, 2008 #2


    User Avatar
    Homework Helper

    Re: Need

    This isn't quite right. You haven't isolated r2 properly. What you have here is really 1/r2. You need to flip it so you are solving for r2.
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