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Energy expended, no work done?

  1. Apr 13, 2007 #1
    this has been something that has been bothering me for a long time:

    say that you are holding a big box in your arms. You are constantly expending chemical energy to keep that box in the air. energy is measured in watts, which equal joule/seconds (right?). now say you hold it for 10 seconds, and you are using 10 watts (arbitrary values). 10 joules/second * 10 seconds = 100 joules. According to this, you have done 100 joules of work on the box, correct? but, work = force* distance. while you have exerted a force, distance =0, so work equals 0, according to this logic.

    so have you done 100 joules of work on the box, or 0?

    I am not talking about net work done on the box. I am talking about the work you have done on the box.
     
  2. jcsd
  3. Apr 13, 2007 #2
    You did no work on the box, no net work on the box, nothing on the box.
     
  4. Apr 13, 2007 #3
    why?

    where is my first line of logic flawed?

    (I agree that it must be [textbooks generally are not flat out wrong, and i have taking physics] but i dont know where or why)

    Another way you could look at it is this. Say that at the end of that 10 seconds i am exerting 10 watts. However, I then stop exerting energy, and it falls. When it is falling, i am exerting no force, so therefore no work. So if i was doing 0 joules of work both times, why did it fall the second time? what is the difference between 0 joules and 0 joules?
     
    Last edited: Apr 13, 2007
  5. Apr 13, 2007 #4
    Because dispite not moving the box anywhere, the fibers in your muscles constantly expand and contract when you hold the bod at a fixed position.
     
  6. Apr 13, 2007 #5
    so if i could somehow keep those fibers from expanding and contracting, i could hold the box with no effort?
     
  7. Apr 13, 2007 #6
    No, thats how your muscles work. You wouldnt be able to move. Contraction and expansion of the muscles is what gives you your mobility.

    But theoretically, yes. (Your joints and body would have to be one rigid solid piece).

    Interestingly enough, I saw on TV that there are some animals that have the ability to stay perfectly still for hours, so I suspect their muscles are doing something funny. (I think it was lizards)
     
    Last edited: Apr 13, 2007
  8. Apr 13, 2007 #7
    thank you!

    alright, thanks a lot!

    and what i meant was that if you could stop them from moving temporarily, and then start moving again whenever you wanted.

    like a lizard, i guess.... but thats trivial.
     
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