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Why is it that if you hold a weight in a position

  1. Aug 10, 2009 #1
    i have asked many lecturers and students this question and have never received a logical sounding answer. Why is it that if you hold a weight in a position (lets just say at the midpoint of a bicep curl) that you are using energy yet there is no net work being done as there is no distance that the force is going through. i have heard answers such as pertubations around this point account for the energy loss. it seems as though the energy is lost as heat (this is why the muscle heats up) the question is more directed to the governing equation behind this. The muscle is technically acting as a structure. i was thinking something along the lines of a Qh(heat input) is the input and Wout(work output) is zero this makes this system a zero efficiency system.
     
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  3. Aug 10, 2009 #2

    Doc Al

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    Re: work

    While no mechanical work is being done on the weight (it doesn't move, so the work done on it is zero), your muscle fibers are continually contracting and relaxing in order to maintain the force that holds up the weight. That requires energy.
     
  4. Aug 10, 2009 #3
    Re: work

    Energy is being transferred but no work is being done on the weight itself.Energy is the ability to do work and to give a different example if the weight were sitting on a shelf it would have gravitational potential energy by vrtue of the fact that work can be done if it were say nudged from the shelf.I am not knowledgeable enough to describe the energy changes involved in holding the weight in position.
     
  5. Aug 10, 2009 #4

    Doc Al

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    Re: work

    Just to be clear: While the weight is being held stationary, no energy is being transferred to the weight.

    Bottom line: You and your arm are biological systems. Lots of energy-consuming activity is going on behind the scenes to allow you to support that weight. As opposed to resting the weight on a table top, which would not require any energy input to maintain.
     
  6. Aug 10, 2009 #5
    Re: work

    Work is simply the transfer of energy. It is not necessarily related to what you can see or not see.
     
  7. Aug 10, 2009 #6
    Re: work

    I know students are puzzled by this discrepancy between theory and experience (including myself when I was in school).

    A nice way to illustrate that it's a biological thing is to take the bicep away and replace it with a table. How much energy does it take for a table to hold up a weight? Apparently none at all. You don't need to feed the table to keep the weight lifted. You can die and centuries pass, and as long as nothing in the external environment upsets the table or the weight, it will stay put.

    Similarly, if you hold a weight with a rigid part of your body, it takes no effort. For example, holding a weight in your lap, as long as you don't need to keep it balanced, and albeit it might be uncomfortable.
     
  8. Aug 10, 2009 #7

    FredGarvin

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    Re: work

    Some energy is expended in strain energy for the slight deformation of the table. This could only be eliminated if the table were perfectly rigid, which nothing is.
     
  9. Aug 10, 2009 #8
    Re: work

    First year physics students could care less about your crazy notions of rigidness.
     
  10. Aug 10, 2009 #9

    daniel_i_l

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    Re: work

    I know many first year physics students that would disagree with you.
    And what do you mean by "crazy"? You're saying that perfectly rigid bodies exist?
     
  11. Aug 10, 2009 #10
    Re: work

    If a student can appreciate the distinction between a rigid and nonrigid body, then they should have no problem accepting the original argument -- that holding up a weight requires no expenditure of energy.

    Talking about the rigidness of the table in my example simply distracts from the main point. And while there is no "perfectly rigid body", for a sufficiently rigid body, you can ignore it all together.
     
  12. Aug 10, 2009 #11

    Doc Al

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    Re: work

    While it's true that when you place the object on the table, the table will deform and gravitational PE will be transformed into strain energy, but so what? No input of energy is required for the table to support the object. I just don't see the relevance of this point to the issues raised in this thread.
     
  13. Aug 10, 2009 #12

    russ_watters

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    Re: work

    Note that people have not been differentiating between work/energy and power here. A table most certainly absorbs energy from a book when a book is placed on it and releases that energy when the book is lifted off. What it doesn't do is absorb or expend energy continuously.

    The difference isn't really critical until both situations are part of the discussion and the op understands the issue just fine: this is an example of a system with zero mechanical efficiency. Other examples include a car balancing on the upslope of a hill using its engine and clutch and a hovering helicopter.
     
  14. Aug 12, 2009 #13
    Re: work

    this was the response i was looking for. Just want some confirmation on this... so the complete energy transfer is from chemical/metabolic to heat. being zero efficient would just mean that no work is being done. also i think a good response to this question would be to answer the following scenario "how much energy is required to hold a 5kg dumbell for 1 minute being 60cm from the pivot point (elbow)". i just dont see how time is a parameter when it comes to this question.
     
  15. Aug 12, 2009 #14
    Last edited: Aug 12, 2009
  16. Aug 12, 2009 #15

    Doc Al

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    Re: work

    Yep. (No work is being done on that weight.)
    Since you burn chemical energy continuously to maintain the tension in your arm, why wouldn't time be a factor? The longer you hold the weight, the more energy you must burn. (Just don't expect to answer that question quantitatively in any simple way; it's best viewed as a biology problem, not simple physics.)
     
  17. Aug 12, 2009 #16
    Re: work

    That's because you have completely ignored the human body as a biological system. The way in which the body uses stored energy changes significantly between http://en.wikipedia.org/wiki/Anaerobic_exercise" [Broken] energy usage.
     
    Last edited by a moderator: May 4, 2017
  18. Aug 13, 2009 #17
    Re: work

    ok, but lets just say we change the system. an electric drill drilling into something that is too high in friction. the batteries get drained the coils heat up and no work is being done. could this scenario be quantitatively measured using electrical theory.
     
    Last edited by a moderator: May 4, 2017
  19. Aug 13, 2009 #18
    Re: work

    what i said must have sounded stupid, what i meant was that because the system seems to be static no time derivatives moving parts etc it appears as time would not be a parameter even though we know that it is because you need to supply energy continuously. this might be a little philosophical but i'm wondering why it is that when there is no motion that the system is zero efficient, why does nature choose to assign it a zero efficiency when it is static. i still cant get my head around this general concept
     
  20. Aug 13, 2009 #19
    Re: work

    When you say that a system is zero efficient, then you have to ask the question, zero efficient at what? Is your body efficient at converting the energy from the food you eat into holding an object up against gravity? No. Is your body efficient at converting the energy from the food you eat into heat? Yes. You cannot compare your body (arm holding a weight) to an inanimate object (wood 2x4 holding a weight). Your (live) body is like a machine with a fuel source that is always running (similar to the helicopter example). The 2x4 does not have a fuel source so it cannot be compared efficiency wise.
     
  21. Aug 13, 2009 #20
    Re: work

    I think you need to review the concept of conservation of energy a bit.

    Edit: You still need to solve the arm problem. I know the answer, but I'm not telling you. I'll let you figure it out on your own. Also, let's all stop using the word efficiency. No where does the answer to this question have anything to do with efficiency. In fact, talking about it is wrong.
     
    Last edited: Aug 13, 2009
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