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Gravitational force problem

  1. May 23, 2006 #1
    Two point masses each equal to 1 kg attract one another with a force of 1 kg.wt. What is the distance between the 2 point masses?
    I solved it in the following way:
    Here F = 1 kg.wt = 9.8 Newton, m1 = m2 = 1 kg, G = 6.6 x 10^(-11) N.m^2/kg^2
    F = [G(m1)(m2)]/r^2
    r^2 = [G(m1)(m2)]/F
    By solving I get,
    r = 0.00025 cm
    But the book answer is 8 cm. Please say which one is right. Here the symbol ^ represents power.
  2. jcsd
  3. May 23, 2006 #2

    Doc Al

    User Avatar

    Staff: Mentor

    Your answer looks OK to me. (G is closer to 6.67 x 10^(-11) N.m^2/kg^2)
  4. May 23, 2006 #3
    Your work looks correct F = Gm1m2/r^2

    In order for the book to be right you must have

    Gm1m2/.08^2 = F

    This means 6.67e-11*1*1/0.0064 = F

    Evaluating gives....

    F = 6.67e-11/0.0064 = 1.0422e-8 N

    This number is both ugly and way too small for 1kg.wt to be equal to 1.0422e-8 N. I would argue that there is yet another typo in your book. You may want to double check the referencing in the solutions portion of your text.
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