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B Why we consume energy while holding something statically?

  1. Mar 11, 2016 #1
    Hello.

    When we are lifting up weight using arm, there is work done by us (we give energy for the weight to move upward and so we feel tired), and by law of conservation of energy, we can write mgΔh= mΔ(v^2)/2.

    However, when we are holding the weight statically (arm does not move at all), we also feel tired some time later. It seems like we are still consuming energy even when we do not have any work done on the weight and our muscle does not move at all. Why? How a contracted muscle that is static consumes energy? Non-living things that support weight does not need energy but living things does! Why?

    Your answer will be appreciated. Thanks!
     
  2. jcsd
  3. Mar 11, 2016 #2
  4. Mar 12, 2016 #3

    Drakkith

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    At the cellular level your muscles actually aren't static. The muscle fibers alternate between fully contracted and fully relaxed, which consumes energy even when your overall muscle is "static".
     
  5. Mar 12, 2016 #4
    Thanks for sharing knowledge!
    I should've written E_used= mgΔh+mΔ(v^2)/2.
     
  6. Mar 12, 2016 #5

    sophiecentaur

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    Otoh, a table transfers no energy, internally or externally when it supports a brick. That's because there is no motion within it - apart from thermal motion and that will be in equilibrium with its surroundings
     
  7. Mar 12, 2016 #6

    Nidum

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  8. Mar 12, 2016 #7

    russ_watters

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    I've heard that before, but I'm not sure if I've ever seen a good reference - do you have one?

    In either case, it is worth noting that our bodies are so inefficient that even when in a dead sleep, we still put out about the same heat (about 70-80 watts) as when at rest and awake. Our muscles don't need to be vibrating to consume energy.
     
  9. Mar 12, 2016 #8

    Drakkith

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    I can't seem to find one that directly says that, but I swear I've seen one before, and everything I've read leads me to understand that this is the way it works. I'll let you know if I find a good reference.
     
    Last edited: Mar 12, 2016
  10. Mar 13, 2016 #9

    A.T.

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    Last edited: Mar 13, 2016
  11. Mar 13, 2016 #10

    sophiecentaur

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    Figure 8 in that reference is interesting.
     
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