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Work applied to a spring by my finger muscles

  1. Aug 6, 2015 #1
    I take a spring and expend work to compress it with my fingers thereby increasing the potential energy of the spring. After it is at its final compressed state, am I doing work?
     
  2. jcsd
  3. Aug 6, 2015 #2

    Doc Al

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    Once you stop compressing the spring, you stop doing mechanical work on it.
     
  4. Aug 6, 2015 #3
    I agree, but it is a strong spring and all the work I am expending holding it compressed is getting me tired!
     
  5. Aug 6, 2015 #4

    Doc Al

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    Sure! It takes energy for you to maintain that tension. But that's got nothing to do with the spring. You could be replaced by a rock! The rock won't get tired. :smile:
     
  6. Aug 6, 2015 #5
    Hey, I am a rock! Well, maybe not...

    Expending energy or expending work? My thought is when I hold it, my muscles vibrate resulting in a force over a distance but I don't get the energy back when the spring oscillates as I act as a dampener.
     
  7. Aug 6, 2015 #6

    Doc Al

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    Not sure what you mean by "expending work". But internal to you, your muscles are doing work contracting and relaxing even when your fingers do not move the spring. You require chemical energy in order to maintain that tension on the spring with your fingers.
     
  8. Aug 6, 2015 #7
    My muscles are doing work and I am using chemical energy maintaining the tension on the spring, but mechanical work is not being applied to the spring?
     
  9. Aug 7, 2015 #8

    Doc Al

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    Sounds good.
     
  10. Aug 7, 2015 #9

    Nidum

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    As an extreme example of large amounts of energy being used to generate a static force you should have seen a Concorde doing a ground engine test .

    Over 140 000 pounds of thrust being developed by four 593's on a non moving aircraft .
     
    Last edited: Aug 7, 2015
  11. Aug 8, 2015 #10
    Where is the work performed by my muscles and the chemical energy being transferred to?
     
    Last edited: Aug 8, 2015
  12. Aug 9, 2015 #11

    Nidum

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    (1)

    Finger pressing against non moving object , Concorde on test , solenoid holding up a load , motor producing stall torque against a load are all example of the same thing - in loose definition they are machines of zero efficiency .

    In each case all energy being used to produce a force against a non moving object is dissipated as heat .

    (2)

    Consider three possible conditions :

    Non moving load .
    Accelerating load .
    Load moving at constant velocity .

    Same 'engine' has different values of efficiency for energy transfer in each condition .

    Zero efficiency is the default case of load not moving .
     
  13. Aug 9, 2015 #12

    Doc Al

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    All these goings on are within your body. It all ends up as increased internal energy ("heat").
     
  14. Aug 9, 2015 #13

    Doc Al

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    Exactly.
     
  15. Aug 9, 2015 #14
    Sorry for being dense.

    After compressing the spring with my fingers, I wedge my fingers between a crack in a big rock. I do this for many days until I die and decompose and obviously am no longer utilizing any chemical energy, yet still the spring is compressed. My index finger and thumb bones both have an equal and opposite force on both sides, and so does both sides of the crack in the rock.

    Is something now creating heat?
     
  16. Aug 9, 2015 #15

    Doc Al

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    No.

    There's nothing intrinsic to the compressed spring that "creates heat". Recall that you could compress the spring with a rock and there will be no additional work done or energy transferred once the spring is fully compressed. It's only when you (your biochemical system) are actively creating tension or muscular effort that you "create heat".
     
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