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Impossible by definition?

  1. Mar 26, 2009 #1
    Here is an interesting question I thought about:

    If a man carries a load x/(g*d) above his head, and his arms do work x, and his legs now have the added work x, over d meters, why does the load remain at the same height?
  2. jcsd
  3. Mar 26, 2009 #2


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    Gold Member

    Simply put, the arms and legs do no actual work in the vertical direction and since [tex]W = \int_{x1}^{x2} F \cdot dx[/tex] , the work done by or to the load is 0 and thus won't move in height.
  4. Mar 27, 2009 #3
    But if the load is to remain stationary wouldn't they have to oppose the weight? Isn't energy expended when the man is stationary? You see this is what I don't get in physics, some basic things don't match up. For example, a 0.5 kg fridge magnet stuck on a fridge for years is supposed to do no work but if we were to cling to a rock face for years and years we would expend energy. How is that explained?
  5. Mar 27, 2009 #4


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    Science Advisor

    You expend energy all the time no matter what you do, just to keep your body running, etc. But in terms of purely mechanical work, no work is being done. You're confusing human effort with work. A table has no problem holding something up either. Do you think tables are performing work?

    Same answer: Stop anthropomorphizing.
  6. Mar 27, 2009 #5

    Doc Al

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    Staff: Mentor

    This question comes up a lot. Here's one thread that might help you sort it out: https://www.physicsforums.com/showthread.php?t=119026
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