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Basic question about defining work

  1. Jun 5, 2015 #1
    I know this will involve a simple answer but I just can't get it atm.

    work = force X distance and has units of Joules ie energy

    when I hold a bucket of water elevated so arm is parallel to the ground I have done some work.

    I now hold it out there and my arm muscle burns and fatigues fighting gravity ie I am metabolising energy to hold the bucket yet the bucket has moved no distance hence W = Fx0 =0 but I am definitely expending energy????
  2. jcsd
  3. Jun 5, 2015 #2


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    Staff Emeritus
    Science Advisor

    You are expending energy because the human body is an imperfect machine. The energy is spent contracting you muscles fibers over and over again and is eventually dissipated as heat. That's how your muscles work and why it is impossible to hold something perfectly steady. If you replaced your arm with a metal bar, then no energy would be expended.
  4. Jun 5, 2015 #3
    so W=FxD still holds if you calculated the movement of the muscles?

    I am not suggesting anyone would do the calculation but just saying?
  5. Jun 5, 2015 #4


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    Staff Emeritus
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    I believe so, but I don't know how to apply a work equation to a stretching muscle fiber.
  6. Jun 5, 2015 #5
    elastic energy equation?
  7. Jun 5, 2015 #6


    Staff: Mentor

    You would have to go down to the level of the actin and myosin. It uses the energy in ATP to trigger a little "ratchet" movement which exerts a force over a distance on a molecular scale.
  8. Jun 5, 2015 #7
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