1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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?

  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


    User Avatar
    Science Advisor
    Homework Helper

    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


    User Avatar
    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


    User Avatar
    Science Advisor
    Homework Helper

    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
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook