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A question about Newton's Law of Universal Gravitation

  1. Dec 18, 2007 #1
    1. The problem statement, all variables and given/known data
    The gravitational force between two shperes is 2.50x10^-8. Their centers are 105 cm apart. The larger sphere has a mass of 8.20 kg. Find the mass of the smaller sphere.

    2. Relevant equations

    3. The attempt at a solution
    d² x F=Gm1m2/d² x d²

    m2=((2.50x10^-8 N)(105² cm))/((6.67x10^-11 Nm²/kg²)(8.20 kg))
    m2=5.04x10^5 kg

    That can't be right because the missing mass is supposed to be smaller than 8.20 kg. Did I move the equation around right?
  2. jcsd
  3. Dec 18, 2007 #2


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    Staff Emeritus
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    Gold Member

    Are you mixing the units cm and m inappropriately?

    - Warren
  4. Dec 18, 2007 #3
    Okay, I forgot to convert cm to m.

    But using d=1.05 meters, I get 50.4 kg as my answer. It's closer, but still too large. :frown:
  5. Dec 18, 2007 #4


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    I agree with your result.

    By "smaller" sphere they may have meant smaller in size, not necessarily smaller in mass.
  6. Dec 18, 2007 #5
    So far though, we've only used mass for those problems.
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