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Homework Help: Gravitation multiple particle system

  1. Jul 21, 2009 #1
    1. The problem statement, all variables and given/known data

    The three spheres in Fig. 13-45, with masses ma=80g ,mb=10g and mc=20g, have their centers on a common line, with L=12 cm and d=4cm. You move sphere B along the line until its center-to-center separation from C is d=4cm. How much work is done on sphere B (a) by you and (b) by the net gravitational force on B due to spheres A and C

    2. Relevant equations

    3. The attempt at a solution
    I figured if W=Fd....then I started with trying to figure out the Force on particle b.
    F= Gm1m2/rr so...
    -G(.01)(.08)/(.04^2) + G(.01)(.02)/(.08^2) which gave me -3.13e-11...i then multiplied this by the distance d=.04....this gave me = -1.25e-12 J...

    however the answer is .50 pJ

    that's for net gravitational force, i'm not really sure how they mean for me to calculate the force by me???
    Last edited: Jul 21, 2009
  2. jcsd
  3. Jul 22, 2009 #2


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    Homework Helper

    No, I don't believe that will work here. That is the formula for work done by a constant force, but here the force on the moving particle is changing as it moves.

    I would suggest using an energy approach here. What is the change in energy of the moving particle? And think about these questions: How is that energy change related to the work done? What is the work done by a conservative force?
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