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Block and tackle system

  1. Jan 28, 2014 #1
    1. The problem statement, all variables and given/known data

    The boy is lifting himself using the block and tackle shown. If the weight of the block and tackle is negligible, and the combined weight of the boy and the beam he is sitting on is 120 lb, what force does he have to exert on the rope to raise himself at a constant rate? (Neglect the deviation of the ropes from the vertical.)

    I drew a FBD for the bottom of the block with the weight going down and 4 tensions pulling up, each with a force of 30lb, but the book has 5 tensions. Which one is correct?

    mxCJ7kV.png
     
  2. jcsd
  3. Jan 28, 2014 #2

    haruspex

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    Do you dispute that the rope the boy is holding is also at tension T? Do you dispute that all five tensions act on the boy+beam system?
    It might be clearer to you if you consider the lower pulley block and the boy+beam separately. What is the tension in the rope connecting them?
     
  4. Jan 28, 2014 #3
    oh, so the tension occurs 5 times b/c 4 times at the lower block, but then also from the boy like the picture because the whole thing is one system, and the block is only part of it.

    thanks for the explanation to clear it up!
     
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