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Homework Help: Work done by internal forces in gravitation

  1. Feb 22, 2010 #1
    1. The problem statement, all variables and given/known data
    Two bodies of mass M and m separated by a distance 'R' are released and they move towards each other under the influence of gravity . What is the work done by internal forces (considering the 2 bodies as a system) when they are at a distance 'x' apart ?

    3. The attempt at a solution

    When they are at a distance R apart their potential energy = -GMm/R . When they are at a distance x apart their P.E = -GMm/x. So change in P.E = GMm(1/x-1/R) . That should be the change in K.E = Work done by the internal conservative force. But isn't work done by internal force always zero . Maybe , I am confused .
  2. jcsd
  3. Feb 22, 2010 #2
    No you use the Work Energy theorem where work of internal forces is taken into consideration.
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