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Extremely confusing dynamics question

  1. May 5, 2013 #1
    Hi guys, this is so confusing as there are lots of assumption needed (I think). I can't do this even though I got the answer and tried working backwards.

    A painter in a crate which hangs alongside a building, weighs 1000N.When the painter pulls on the rope, the force he exerts on the floor of the crate is 450N. If the crate wieghs 250N, find the acceleration. Answer: 2.0ms^-2

    Firstly, we don't know whether the system is even accelerating upwards or downwards..and I also realized that the tension exerted may be just the normal force of 450N exerted back onto the painter (since he exerts 450N on the floor) does that mean he is apparently weightless? That is quite strange though, because if we consider in terms of forces acting on the painter, then it's 450N from the floor and 450N from the rope, both acting upwards. And I'm asking this because some answers I found online say the weight of the painter can be neglected although I don't know why...pls help!! I'm stuck in this one question for a long long long time...
     
  2. jcsd
  3. May 5, 2013 #2

    mfb

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    Phrased like that, the problem statement should have some sketch attached.

    The answer is right if a rope goes from the crate over some fixed anchor (at the top of the building) back to the painter.
     
  4. May 5, 2013 #3

    haruspex

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    Note this means the painter weighs 1000N, not painter plus crate.
    You shouldn't need to predict that - it will come out in the answer.
    The tension in the rope equals the normal force from the floor? Why should it?
    The weight of the painter is given, and certainly cannot be neglected.
    Have you drawn free body diagrams for crate and painter, including an unknown tension? What equations do you get?
     
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