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Homework Help: Help with gravitation of forces

  1. Jan 19, 2009 #1
    1. The problem statement, all variables and given/known data
    Two objects attract each other with a gravitational force of magnitude of 1*10^-8 N when separated by 20 cm. if the total mass of the two objects is 5kg, what is the mass of each.

    2. Relevant equations

    3. The attempt at a solution
    i know everything that has to plugged into the equation, but the only part that is hard for me is that it gives you the total sum of the two objects and in the equation, you have to use the product of the masses. how can i change the formula to make it so that i can use the total sum instead of the product?
  2. jcsd
  3. Jan 19, 2009 #2


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    Homework Helper
    Gold Member

    You seem to be stating it the wrong way round. You got the product, (you tell us)! You are given the sum!

    From those two you should be able to work out m1 and m2.
  4. Jan 19, 2009 #3
    ok i see what you are saying but something just ain't clicking yet. i used what i was given to find the product of m1 and m2:


    so now i have the sum of the two masses and the product of the two.
    so if i took the square root of 5.997 you get 2.49 kg. if you add 2.45+2.45 you get 4.9. which is close to 5(prob just cause of round issues) so is that the right way?
  5. Jan 19, 2009 #4

    Doc Al

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    Staff: Mentor

    What you call "mtotal" is really m1*m2. So m1*m2 = 5.997 is one equation.

    But you also know that: m1 + m2 = 5.

    Combine those two equations.
  6. Jan 19, 2009 #5


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    I haven't checked your calculation, but if and when it is right you have got the product. And you already had the sum.

    Write down what these two things are as two equations and you may see how to proceed.
  7. Jan 19, 2009 #6
    alright i figured it out guys. thanks a lot
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