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Homework Help: Free body diagram of atwood machine

  1. Jun 21, 2011 #1
    1. The problem statement, all variables and given/known data

    I need to find the forces on this modified atwood machine:

    http://imageshack.us/photo/my-images/36/atwood.jpg/

    3. The attempt at a solution

    What I think:

    - On mass M there are two forces, Mg and -T.
    - On the mass above the spring there are two more forces, -2mg and T.
    - On the mass below the spring there are also two forces, -mg and kx.

    Is any of this correct?
     
  2. jcsd
  3. Jun 22, 2011 #2
    You seem to have got it right.
     
  4. Jun 22, 2011 #3
    Thanks.
     
  5. Jun 22, 2011 #4

    Doc Al

    User Avatar

    Staff: Mentor

    Careful: The only force of gravity on that mass is mg. (But there's a spring force on it.) So there are three forces acting on that mass.
     
  6. Jun 22, 2011 #5
    Ooh, you're right, I was thinking there might be a 2mg plus a kx after you mentioned the spring force, but I think it's only mg and kx.

    I knew the spring was going to make it trickier than I had thought.
     
  7. Jun 22, 2011 #6
    Are the forces on the mass above the spring then T and -mg and - kx?
     
  8. Jun 22, 2011 #7

    Doc Al

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    Staff: Mentor

    Yes.
     
  9. Jun 22, 2011 #8
    Thanks
     
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