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Homework Help: Momentum-impulse vs. rectilinear motion equations

  1. Mar 2, 2007 #1
    Here we are again...We have apully system as seen on page 1 of the attachment. I tried using the rectilinear motion equations, assuming a=constant, but the book's solution used the momentum-impulse equations and got a different answer.

    I dont understand why my approach is incorrect... Thanks.
     

    Attached Files:

  2. jcsd
  3. Mar 2, 2007 #2

    Doc Al

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    Nothing wrong with using kinematics with constant acceleration. But how did you solve for the acceleration?
     
  4. Mar 2, 2007 #3
    I just assumed the acceleration was gravity...I'm guessing there's something wrong with that assumption...but I dont know what...
     
  5. Mar 2, 2007 #4
    Its not gravity because its not free falling. I cant believe I made that mistake. I would have to use Sum of forces=ma and find the acceleration if i wanted to use kinematics, right?
     
  6. Mar 2, 2007 #5

    Doc Al

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    The "acceleration due to gravity" applies to a freely falling object--where the only force acting is gravity. But these objects are attached to ropes and pulleys--they are not freely falling! Solve for the acceleration like in any other pulley problem.
     
  7. Mar 2, 2007 #6

    Doc Al

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    Exactly. (I knew you'd snap out of it. :smile: )
     
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