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How to solve for mass given gravitational force and radius ?

  1. Jul 22, 2012 #1
    how to solve for mass given gravitational force and radius ???

    1. The problem statement, all variables and given/known data
    The gravitational force between two identical objects 2.00m apart is 3.50x10^-6N. determine the mass of each object.
    the answer should be [458kg]

    2. Relevant equations

    i know that Fg= GmM/r^2
    so gravitational force is equal to the gravitational constant times mass one and mass two over the radius squared

    3. The attempt at a solution
    i tried manipulating the formula to solve for the masses
    i went
    Fgr^2/G=mM

    i did (3.50x10^-6N)(2.oo^2m)/(6.67x10^-11)

    which does not give me the answer of 458kg
    please help!
     
  2. jcsd
  3. Jul 22, 2012 #2
    Re: how to solve for mass given gravitational force and radius ???

    You calculated Fgr^2/G, which you said was equal to mM.

    They didn't ask for mM though, so you haven't finished the problem yet.
     
  4. Jul 22, 2012 #3
    Re: how to solve for mass given gravitational force and radius ???

    but mM is the masses though.
    so would i need to solve for mass one in order to solve for mass two ?
     
  5. Jul 22, 2012 #4
    Re: how to solve for mass given gravitational force and radius ???

    mM is the product of the masses. You need to find each of them, m and M.

    What is the relationship between m and M?
     
  6. Jul 22, 2012 #5
    Re: how to solve for mass given gravitational force and radius ???

    im not sure on how to solve for each m individually.
    am i manipulating the formula incorrectly?
     
  7. Jul 22, 2012 #6
    Re: how to solve for mass given gravitational force and radius ???

    No, it was correct to solve it for mM.

    What do they tell you in the problem about the two masses that might help you solve for one?
     
  8. Jul 22, 2012 #7
    Re: how to solve for mass given gravitational force and radius ???

    they are both identical .
    haha ohkay so if i calculate one they should both be the same.
     
  9. Jul 22, 2012 #8
    Re: how to solve for mass given gravitational force and radius ???

    Exactly.

    Mathematically, m = M.
     
  10. Jul 22, 2012 #9
    Re: how to solve for mass given gravitational force and radius ???

    Thanks a million .
    i must read the questions more carefully hah.
     
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