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Homework Help: How to find mass with gravitational attraction.

  1. Oct 17, 2007 #1
    Hello, I'm trying to solve a problem, but I'm doing something wrong.

    This is the problem out of my Physics book, but the problem I'm working on has different numbers, so any help will not be cheating, I just need to know the process.

    Two objects are attracted to each other gravitationally with a force of 2.5e-10 N when they are 0.25 meters apart. Their total mass is 4.0 kg. Find their individual masses.

    I know the answer is m1=3.9kg, and m2=0.1kg. But I don't know how they got this answer.

    Help please.
  2. jcsd
  3. Oct 17, 2007 #2
    You can calculate the product of the masses with the given force and distance. Then because you know the sum, you have a system of 2 equations with 2 unknowns
  4. Oct 17, 2007 #3
    I know that, but I'm still confused.

    Since the equation is: Fg = G m1m2/r^2

    if I plug in the numbers I'm given:

    I have: 2.5e-10 = 6.67e-11 m1m2 / 0.25^2.

    I solve for m1m2, which is: m1m2 = (Fg*r^2)/G , right?

    Doing that, I have m1m2 = (2.5e-10*0.0625)/6.67e-11 = 0.23426

    Now what do I do?
    I know m1=3.9, and m2=0.1, so shouldn't (m1m2=0.39) ?

    The sum of the masses: (m1+m2=4), but how do I use this in the (m1m2=0.39) ?

    I'm doing something wrong, but what?
  5. Oct 17, 2007 #4
    I don't see anything wrong in your calculations but are you sure you copied the numbers right?

    If you have two equations: m1 + m2 = 4 and m1*m2 = #

    Solve for m1 in one equation like m1 = 4 - m2
    And then plug that into the other equation m1*m2 = (4 - m2)*m2 = #
    Once you have m2, go back to m1 + m2 = 4 to solve for m1.

    The idea is to solve one equation for a variable. Then replace the same variable in the second equation so there's only one unknown.
  6. Oct 17, 2007 #5
    Thanks, I figured it was that, but for some reason, I could not figure out (4-m2)*m2=#.
    Major brain fart.

    Thanks a lot!
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