Impossible by definition?

  • Thread starter lewis198
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Here is an interesting question I thought about:

If a man carries a load x/(g*d) above his head, and his arms do work x, and his legs now have the added work x, over d meters, why does the load remain at the same height?
 

Pengwuino

Gold Member
4,854
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Simply put, the arms and legs do no actual work in the vertical direction and since [tex]W = \int_{x1}^{x2} F \cdot dx[/tex] , the work done by or to the load is 0 and thus won't move in height.
 
But if the load is to remain stationary wouldn't they have to oppose the weight? Isn't energy expended when the man is stationary? You see this is what I don't get in physics, some basic things don't match up. For example, a 0.5 kg fridge magnet stuck on a fridge for years is supposed to do no work but if we were to cling to a rock face for years and years we would expend energy. How is that explained?
 

alxm

Science Advisor
1,841
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But if the load is to remain stationary wouldn't they have to oppose the weight? Isn't energy expended when the man is stationary?
You expend energy all the time no matter what you do, just to keep your body running, etc. But in terms of purely mechanical work, no work is being done. You're confusing human effort with work. A table has no problem holding something up either. Do you think tables are performing work?

You see this is what I don't get in physics, some basic things don't match up. For example, a 0.5 kg fridge magnet stuck on a fridge for years is supposed to do no work but if we were to cling to a rock face for years and years we would expend energy. How is that explained?
Same answer: Stop anthropomorphizing.
 

Doc Al

Mentor
44,765
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But if the load is to remain stationary wouldn't they have to oppose the weight? Isn't energy expended when the man is stationary? You see this is what I don't get in physics, some basic things don't match up. For example, a 0.5 kg fridge magnet stuck on a fridge for years is supposed to do no work but if we were to cling to a rock face for years and years we would expend energy. How is that explained?
This question comes up a lot. Here's one thread that might help you sort it out: https://www.physicsforums.com/showthread.php?t=119026
 

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