1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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.

    Problem:
    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.

    Answer:
    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:
    Fg=2.5e-10
    G=6.67e-11
    r=0.25

    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!
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: How to find mass with gravitational attraction.
  1. Gravitational Attract (Replies: 6)

Loading...