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Homework Help: 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?

    thanks
     
  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

    Tide

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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

    DaveC426913

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    Gold Member

    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

    Tide

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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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