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Homework Help: Massless Pulley Problem

  1. Feb 27, 2013 #1
    1. The problem statement, all variables and given/known data

    A light rope is attached to a block with mass 4.00 kg that rests on a frictionless, horizontal surface. The horizontal rope passes over a frictionless, massless pulley, and a block with mass m is suspended from the other end. When the blocks are released, the tension in the rope is 10.0N.
    a.What is the acceleration of either block?
    c.Find the mass m of the hanging block.

    2. Relevant equations


    3. The attempt at a solution

    My attempt at solving the problem is attached. Can someone tell me where I went wrong?
    (It got cut off at the bottom, it just says that my answer of 0.813kg is wrong, but I don't know why. THE CORRECT ANSWER IS: m=1.37kg
  2. jcsd
  3. Feb 27, 2013 #2
    Sorry, My attempt at the solution is now attached.

    Attached Files:

    • HELP.pdf
      File size:
      151.1 KB
  4. Feb 27, 2013 #3


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    Science Advisor
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    Gold Member

    You've been inconsistent in your choice of directions. Which way is the mass m accelerating?
  5. Feb 27, 2013 #4
    Recheck your free body diagram. Where are the blocks accelerating? Specifically, on what axis as I am seeing two different axis here and that is not correct.

    EDIT: Sorry, didn't see haruspex's post when I responded, I did the same problem.
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