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Negative efficiency?

  1. Mar 26, 2007 #1

    adrenaline

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    strange question efficiency

    if a human does pushups , it is an oscillatory motion and no work was done. efficiency which is defined as e= useful energy output/energy input is 0. But since you burn a lot of atp and calories, can the efficiency coefficient be a negative value?
     
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  3. Mar 26, 2007 #2

    adrenaline

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    I posted this in the biology section but since engineers are more familliar with efficiency I was wondering what you thought.

    If a human does pushups it is a oscillating motion where he returns back to his starting position, the useful work accomplished is 0. Since his useful work is 0 so technically his efficiency is 0. However, since he is burning calories ( supplying atp to the muscles for contraction, moving his lungs for oxygen uptake etc.) can his effiency be in the negative value? afterall his overall energy level is a deficit? or am i completely way off base with the definition of efficiency?
     
  4. Mar 26, 2007 #3

    Danger

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    I would be inclined in this case to base it upon what the body is actually doing rather than the net movement. On the downward part of the push-up, the muscles are still supporting the weight of the body, but to a lesser degree than during lifting. That is to impart a far lower earthward acceleration than would be caused by un-resisted gravity.
    The actual efficiency starts with the initial chemical conversion process of digestion and oxygen metabolism, ATP production and use... all of that stuff at every level of cellular activity, not just in the muscles themselves.
     
  5. Mar 26, 2007 #4

    berkeman

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    I don't think you can go below zero. The useful work done divided by the total work done -- you can have zero useful net work done as in your pushup example, but how can you have negative work done?
     
  6. Mar 26, 2007 #5

    adrenaline

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    i guess from a mechanical efficiency point of view it is zero but the person is burning calories and will lose weight so from a biological biochem point of view he is using up calories ( assuming he does not take any food or calories in that day) and does 200 pushups. the mechanical efficiency is still 0 but boy has he used up a lot of energy! Anyway it looks like it is not possible to get a negative value for efficiency?
     
  7. Mar 26, 2007 #6

    russ_watters

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    How's this for negative efficiency: your car sliding down a hill you are trying to climb in the snow.
     
  8. Mar 26, 2007 #7
    I think you are mixing up things here. The mechanical efficency cannot really be compared to the chemical efficency. The efficency of the pushups is more like energy to do pushups/total energy expended.
     
  9. Mar 26, 2007 #8

    adrenaline

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    OK ,THANK YOU
     
  10. Mar 26, 2007 #9

    jim mcnamara

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    First off, a pushup is two separate sets of motions - extending your arms under load, then bending them under load. Both require energy. Going up turned muscle energy expenditure into potential energy. Doing down used energy to prevent splatting your nose on the floor. But it did result in a total decrease in potential energy.

    Just because you wound up in the same postion you started with the same potential energy does not mean that no energy was expended.

    The basic energy equation:
    energy in = energy out + entropy

    In this case there was large a entropy component. Is that what you mean?
     
  11. Mar 26, 2007 #10

    berkeman

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    << Thread from Biology merged into this thread >> adrenaline, please do not multiple post the same question in different threads. Next time, please PM the appropriate Mentor (or use the REPORT post button), and ask that the original thread be moved. We don't want the clutter of multiple similar-post threads going on at the same time in the PF. Thank you.
     
  12. Mar 26, 2007 #11

    adrenaline

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    I've read that efficiency can only be a of 0 to 100% ( i gather only a perpetual motion machine will have 100% efficiency) so I was wondering in the case of the pushups, can it be a negative value due to the energy expanded and the increase in entropy ?( heat emitted from body, sweat etc.) Or am I just totally confusing the definition of efficiency?
     
  13. Mar 26, 2007 #12

    Q_Goest

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    Hi adrenaline,
    In mechanical engineering, the efficiency of a pump or compressor is also known as the isentropic efficiency which is equal to the amount of energy an isentropic process would require to compress or pump some fluid, divided by the actual amount of work required. (isentropic means the process has no change in entropy)

    Similarly, the efficiency of an expander is also known as isentropic efficiency which is equal to the amount of energy or work that is removed from a fluid by an expander, divided by the amount of work removed if the process were isentropic.

    Here's a reference that may help: http://www.taftan.com/thermodynamics/ISENEFF.HTM

    There are other types of efficiency in engineering, but I think these are the most applicable to what you're asking. They apply to a mechanism and its thermodynamic efficiency.

    The point is that in order to calculate efficiency, you need a definition. There is nothing intrinsic about the two definitions given above, they are simply definitions that we can all use and makes the most sense. It doesn't mean there really is such a thing as an absolute efficiency. For example, an isothermal compression process will require less work than an isentropic one, so such a compressor would actually have an isentropic efficiency greater than 100%! If a compressor ended up putting a lower pressure out than the pressure in, the isentropic efficiency would be less than 0%. It all depends on how you define efficiency.

    So to answer your question, I think it doesn't make sense to ask what the efficiency of a person doing a pushup is. One needs to define efficiency in order to measure it and give it any quantifyable value. Depending on how you do that, efficiency can be anything if the process isn't somehow intrinsic to physics.

    Side note: some processes are intrinsic, such as the overall increase in entropy in the universe given any process, but I don't know that really applies here.
     
  14. Mar 26, 2007 #13

    adrenaline

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    Thankyou very very much.
     
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