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Lagrange point

  1. Oct 31, 2005 #1
    So im supposed to find the distance from the moon where the force on a particle from the earth and sun is equal.

    I get this expression (earth-moon distance is 400 000km):
    yMm/(r^2) = yMm / (400000km-r)^2

    But from that I get r=200 000km and the answer should be 40 000km...

    whats wrong with the expression?

  2. jcsd
  3. Oct 31, 2005 #2
    mass of the moon != mass of the earth
  4. Oct 31, 2005 #3
    but the product of the masses is the same on both sides right? dont they cancel...?
  5. Oct 31, 2005 #4


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    No, they are not the same and they can't cancel if they are not the same.
  6. Oct 31, 2005 #5
    then what is the expression...?
  7. Oct 31, 2005 #6


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    There should be only one mass variable on each side of your equation.
  8. Oct 31, 2005 #7
    but newtons gravitation law is yMm/r^2...
  9. Oct 31, 2005 #8


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    Yes. But do you KNOW what M and m are?
  10. Oct 31, 2005 #9
    M=mass of earth
    m=mass of moon

  11. Oct 31, 2005 #10
    oh, heh, I don't know what caused me to be so slow... I forgot the mass of the particle at the Lagrance point...

    Bah, sorry for the trouble :P
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