Work: Holding something up

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Pengwuino

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Something i've always wondered is why does it feel like work is being done when you're just holding something up. According to the definition of work, 0 work is being done if no displacement occurs... but no one can say with a straight face that you aren't doing some form of work somewhere when you hold a 100lb bag up or something. Maybe this is a biology question but where exactly is energy being consumed/used when a human being holds something up in the same position? Or is it even possibly just a mental trick that it feels like some work is being done (maybe its just neurological signals)?
 

eep

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I'm no expert but here's my guess. In order to keep the object from falling you have to exert an upwards force to counteract gravity. Our muscles have to tense in order for this to happen. This is a biological process which uses energy, but I'm not quite sure what the process is.
 
Ya im not 100% sure what the best answer to that would be but i would say its got more to do with the definition of work. Theres 0 work done on the bag because it dosent move. But you arm is definitly using energy (ATP and all that chemical energy in the body). And if you think about it your arm is creating the normal force, so basicaly your applying as much force as a table would on the bag. Im not sure if that realy answered the question at all but it was worth a try.
 

jtbell

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When you're holding something up (stationary), your muscle cells are alternately contracting and relaxing and moving back and forth. So they're indivdiually doing work, but they're fighting each other so the work probably gets dissipated as heat.

For details, you'll have to ask a biomechanics or biophysics expert.
 

Pengwuino

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No it does not answer my question skigamemaker. I think jtbell is right in directing me to a biologist or biphysicist.
 
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The example at right is another standard paradox in the definition of work. If the box is being carried at constant velocity, then no net force is necessary to keep it in motion. The force exerted by the person is an upward force equal to the weight of the box, and that force is perpendicular to the motion. If there is no motion in the direction of the force, then no work in done by that force. Yet you certainly feel like you are doing work if you carry a heavy box. The resolution of the paradox is similar to the first example - your muscles must maintain an extra tension to stay upright under the load. This requires a greater amount of internal contraction and release of our muscle fibers, and hence internal work in our bodies. But the work done on the box is zero since by moving in a straight line at constant speed, it's energy is remaining the same.
http://hyperphysics.phy-astr.gsu.edu/hbase/work2.html

Rather than type, I will just copy and paste. Jtbell already gave you the answer.
 

Pengwuino

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So its the contraction and release of muscles... well i have no knowledge of that so i'll go see how that all works. Thanks guys.
 

russ_watters

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Pengwuino said:
Something i've always wondered is why does it feel like work is being done when you're just holding something up. According to the definition of work, 0 work is being done if no displacement occurs... but no one can say with a straight face that you aren't doing some form of work somewhere when you hold a 100lb bag up or something. Maybe this is a biology question but where exactly is energy being consumed/used when a human being holds something up in the same position? Or is it even possibly just a mental trick that it feels like some work is being done (maybe its just neurological signals)?
You did answer your own question in the question, but consider this: why is it easier to stand up straight than to do a "wall sit" (leaning back against a wall in a seated position)?
 
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It's just a net work of zero on the system because work is being done by gravity downward while you do work in the opposite direction. Take away gravity and you will start pushing the object up.
 

Doc Al

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PatPwnt said:
It's just a net work of zero on the system because work is being done by gravity downward while you do work in the opposite direction.
No work is being done on the object (by you or by gravity) since the object doesn't move. The work being done to maintain muscle tension is internal to your body.
 

Hootenanny

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PatPwnt said:
It's just a net work of zero on the system because work is being done by gravity downward while you do work in the opposite direction. Take away gravity and you will start pushing the object up.
Mechanical work is not being done. No displacement, hence no work. There was a thread similar to this in the intro physics sections not solong ago, I'll try and see if theres anything useful in there.

~H

[edit]Sorry, Doc you beat me to it again :wink:
 

Tide

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Work is being done by the heart in pushing blood through arteries, veins and capillaries -there is force and there is displacement in the direction of the force. However, as Russ indicated, no work is being done on the object which should be evident from the fact that its kinetic and potential energies are fixed.
 

pervect

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Muscles convert chemical energy into mechanical energy. Probably the best way of looking at the issue is to say that your muscles are operating at 0% efficiency when you hold a weight over your head, or do isometric exercises.

Chemical energy is still being used by your body (the ADP/ATP metabolism involving your mitochondria) - but none of that energy is doing any actual work. You feel like you are doing work because your body is actually using up chemical energy stores (the ATP molecule). Unfortunately, the efficiency of work output is 0% in the case of isometric exercise or when you hold a weight "over your head".

You can see that no mechanical work is being done by considering a stretched spring, for instance. The spring is stretched, and exerting a force, but it (the spring) never "gets tired" or "runs out of energy".

Similar remarks can be made about tables - they can hold up weights, they also do not "get tired" or "run out of energy".
 
You're confusing the physics definition of work with the colloquial meaning of work.
 

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