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Hanging masses

  1. Mar 20, 2014 #1
    1. The problem statement, all variables and given/known data
    A hanging mass M1 =1 kg is attached by a light string that runs over a frictionless pulley to a mass M2=2 kg that is initially at rest on a frictionles table .
    What is the magnitude of the acceleration a of M2 ?


    2. Relevant equations
    I used for M1 :
    Fg1 + T =M1a (with T tension of string)
    and for M2 :
    N+Fg2+T=M2a

    3. The attempt at a solution
    NOW I'm stuck , I can't solve it , any help ??
     
  2. jcsd
  3. Mar 20, 2014 #2

    Doc Al

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    For M2 you have added forces that are perpendicular to each other. Don't do that! Instead, consider horizontal forces separately. (Assuming the table is horizontal, that's the direction of the acceleration.)

    Be careful with signs.
     
  4. Mar 20, 2014 #3
    What I have written are vectors :
    for M1 without vectors :
    _m1g+T=-m1a
    and for M2 :
    N+T-m2g=m2a

    I still didn't get your point
     
  5. Mar 20, 2014 #4

    Doc Al

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    Good. (Note that the forces of gravity and tension all act vertically.)

    This combines vertical forces (N, mg) with horizontal forces (T). Don't do that.
     
  6. Mar 21, 2014 #5
    I thought T was a vertical force too
     
  7. Mar 21, 2014 #6
    On mass 1 it is a vertical force. On m2 it is pulling from the side. Remember that since your pulley is massless and frictionless, the tension along the string will be completely constant.
     
  8. Mar 21, 2014 #7

    Doc Al

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    Not when it acts on M2, which slides along a horizontal table. (A picture would help.)
     
  9. Mar 21, 2014 #8

    haruspex

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    The OP doesn't actually state the orientation of the string between pulley and M2. Yes, it's probably meant to be horizontal, but in principle could be anything.
     
  10. Mar 21, 2014 #9
    so what am I supposed to do ? Cause that's all what is available in the problem statement , there is no picture : Should I work with it as a horizontal force ?
     
  11. Mar 21, 2014 #10

    haruspex

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    Yes.
     
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