Energy expended, no work done?

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Discussion Overview

The discussion revolves around the concept of work in physics, particularly in the context of holding an object stationary. Participants explore the relationship between energy expenditure, force, and the definition of work, questioning how energy is used when no displacement occurs.

Discussion Character

  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant asserts that while holding a box expends energy (measured in watts), the work done on the box is zero because there is no displacement, leading to a contradiction in understanding energy expenditure versus work done.
  • Another participant agrees that no work is done on the box, emphasizing that there is no net work since the box does not move.
  • A participant questions the logic behind the zero work done, seeking clarification on the difference between energy expenditure and work when the box falls after stopping the exertion of force.
  • One participant introduces the idea that muscle fibers must contract and expand to hold the box, suggesting that this biological process is distinct from the physics definition of work.
  • Another participant speculates on the theoretical possibility of holding the box without muscle movement, questioning the implications of such a scenario on mobility.
  • A later reply mentions animals that can remain still for extended periods, hinting at unique biological adaptations that may relate to the discussion.

Areas of Agreement / Disagreement

Participants generally agree that no work is done on the box in the physics sense due to lack of displacement, but there is ongoing debate about the implications of energy expenditure and muscle function.

Contextual Notes

The discussion highlights the distinction between energy expenditure and the physics definition of work, with some assumptions about muscle function and biological processes remaining unresolved.

mr200backstrok
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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.
 
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You did no work on the box, no net work on the box, nothing on the box.
 
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 don't 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?
 
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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.
 
so if i could somehow keep those fibers from expanding and contracting, i could hold the box with no effort?
 
No, that's how your muscles work. You wouldn't 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)
 
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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 that's trivial.
 

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