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Gravitational force = 0 between two planets?

Which is the answer?

Poll closed Mar 4, 2008.
  1. ((M-m)/(M+m))R

    0 vote(s)
  2. (M+m)/(M-m)

    0 vote(s)
  3. (((M^.5)+(m^.5))/(M-m)) * (M^.5) R

    0 vote(s)
  4. (((M-m)/(M+m))^.5) * R

    0 vote(s)
  5. (((M^.5)-(m^.5))/(M-m)) * (M^.5) R

    0 vote(s)
  1. Mar 3, 2008 #1
    1. The problem statement, all variables and given/known data

    A planet of mass M has a moon of mass m in a circular orbit of radius R. An object is placed between the planet and the moon on the line joining the center of the planet to the center of the moon so that the net gravitational force on the object is zero. How far is the object placed from the center of the planet?

    2. Relevant equations


    I used the variable d to describe how far the object is placed from the center of the planet

    3. The attempt at a solution

    G(M(mass of object))/d^2 = G(m(mass of object))/(R-d)^2

    G and mass of object cancel out

    M/d^2 = m/(R-d)^2
  2. jcsd
  3. Mar 4, 2008 #2
    find d.?
  4. Mar 4, 2008 #3


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    Science Advisor
    Homework Helper

    Looks good to me!

    Hi Hayden! Welcome to PF :smile:

    Looks good to me!

    Now all you have to do is to solve the quadratic equation … :smile:
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