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Law of Universal Gravitation: gravitational attraction of Earth and Moon is canceled

  1. Feb 28, 2009 #1
    1. The problem statement, all variables and given/known data
    The mass of the Moon is 7.36 x 10 ^ 22 kg. At some point between Earth and the Moon, the force of Earth's gravitational attraction on an object is canceled by the Moon's force of gravitational attraction. If the distance between Earth and the Moon (center to center) is 3.84 x 10 ^ 5 km, calculate where this will occur, relative to Earth.


    2. Relevant equations

    Fg = Gm1m2 / r ^ 2



    3. The attempt at a solution

    Given:
    Mass of Earth = 5.98 x 10 ^ 24 kg
    Mass of Moon = 7.36 x 10 ^ 22 kg
    Radius between Earth and Moon = 3.84 x 10 ^ 5 km = 3.84 x 10 ^ 8 m

    So at this point, Fgmoon = Fgearth, but I need another object to calculate the point.
    So assuming the object is 1kg. Fg between Moon and object = Fg between Earth and object.
    I need to find the r between Earth and object. This distance is the entire distance between Earth and Moon minus distance between object and Moon.

    Gm(earth)m(test) / (distance between moon and sun - r) ^ 2 = Gm(test)m(moon) / r ^ 2

    I don't know either r.
    I'm not sure how to find r with this equation.
     
  2. jcsd
  3. Feb 28, 2009 #2

    Delphi51

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    Homework Helper

    Re: Law of Universal Gravitation: gravitational attraction of Earth and Moon is cance

    The two r's are identical so you have only one unknown.
    The form of your equation is like a/b = c/d.
    Your first step should be to cross multiply to ad = bc, which eliminates all fractions.
    Cancel what you can. Express the quadratic equation in standard form
    ar^2 + br + c = 0
    Use the quadratic solution formula to find r.
     
  4. Feb 28, 2009 #3
    Re: Law of Universal Gravitation: gravitational attraction of Earth and Moon is cance

    Ok, so I have:
    (G)[m(earth)][m(test)](r ^ 2) = (G)[m(test)][m(moon)][(distance between moon and sun - r) ^ 2]
    (9.8N/kg)(5.98 x 10 ^ 24 kg)(1kg)(r^2) = (9.8N/kg)(1kg)(7.36 x 10 ^ 22 kg)(3.84 x 10 ^ 8 m)

    Wait, so the (G) and the [m(test)] can cancel out yes?
    So then,

    [m(earth)](r ^ 2) = [m(moon)][(distance between moon and sun - r) ^ 2]
    (5.98 x 10^24kg)(r^2) = (7.36 x 10^22kg)(3.84x10^8m - r)^2
    (5.98 x 10^24kg)(r^2) = (7.36 x 10^22kg)(14.75x10^16m^2 - 7.68x10^8mr - r^2)
    Is this correct?
    Can I now just take (14.75x10^16m^2 - 7.68x10^8mr - r^2) to find r or do I need to solve the rest of the equation? Sorry my algebra is very bad.
     
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