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Homework Help: Weight of the Earth on the moon?

  1. Aug 28, 2007 #1
    Hi everyone!
    Ok, so here's the problem:
    What would be the weight of the earth if it were on the moon (just try and imagian that :). The masses of the moon and the earth, and their radiuses, are given.
    I cannot see what's the trick here.
    There are four possible options.
    The one that seemed the most logical is :

    where m - moon mass, M - Earth mass, R - Earth radius, r - moon radius.

    But the material it's supposed to be related to is torques and moments of force... I don't see how it's related :-\

    I'd be really thankful for help on how to approach this problem.

  2. jcsd
  3. Aug 28, 2007 #2
    Is it the same as the weight of the Moon on the Earth?
  4. Aug 28, 2007 #3
    Weight = gravitational force between two object.

    P.S. I wonder if the earth is sitting on me, or I am sitting on the earth?
  5. Aug 28, 2007 #4


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    Your idea and equation are correct cosmic_tears.
  6. Aug 29, 2007 #5
    Thank you all!
    I simply thought I'm missing something! I thought maybe I should somehow concider the fact that the far side of the earth is much further away from the moon than the close side, (as opposed to now, when there's hardly any difference).
    It made me think about the fact that if you walked into a black hole your legs will probably get torn cause of the different gravity on your body :).
    Thanks again!
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