Just a simple thought experiment

AI Thread Summary
The discussion centers on the confusion surrounding the Work-Energy Theorem, particularly regarding why exerting force against an immovable object, like a wall, leads to fatigue despite no work being done on the object. Participants explore the analogy of an electro-hydraulic piston that applies force without movement, questioning where the energy goes when it eventually fails. The conversation shifts to the biological aspect of muscle fatigue, noting that energy is expended even when muscles are tensed without movement. It is suggested that the real work occurs within the body, as muscles require energy to maintain tension. Understanding the relationship between physical exertion and muscle function is emphasized as a key takeaway.
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When I first learned the Work-Energy Theorem, I was a little confused. After all, it implied that no work was done when you push against a wall that doesn't move, but I know that I still feel tired after pushing against a wall.

So, suppose you have an electro-hydraulic piston bolted to the ground near a wall. When you turn it on, nothing moves. Surely, the piston won't keep applying the force forever (not all the electric energy will be transferred. I.e. the piston is not doing work, but since the piston will eventually die, where does the energy go? I tried this thought experiment with a car pushing on a wall, but concluded that the chemical energy from the gasoline gets converted into the kinetic energy of the wheels and noise + heat created by the wheelspin.

I can't find the answer to the piston problem, and I figure this is more similar to my arm pushing on the wall. Perhaps it's just that I don't understand how they work :P
 
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Be a little more old-fashioned and think of holding up a big pile of books in front of a bookshelf. You get pretty exhausted pretty quickly. Shove them on the shelf and presto: the weight is carried by the shelf that doesn't get tired, ever.
Your fancy piston clouds the issue. Levers, weights and pulleys and such are complicated enough already.
You are right re the car experiment.
 
So then the question "why do we get tired" is the question I should be asking... I'm getting the idea that the answer isn't physics related. Do you know the answer? Perhaps my biology teacher could answer tomorrow :P
 
Excellent idea. I'm a physicist and I have a hard time understanding it, let alone explain it!
 
There's no work being done on the wall, because its not moving, rather the work that's being done is inside your body. i.e. your muscles..
 
MostlyHarmless said:
There's no work being done on the wall, because its not moving, rather the work that's being done is inside your body. i.e. your muscles..
Hmm, this actually makes sense. I remember something in early general science courses about muscles expanding and contracting to move. Not just through the length of a joint, but laterally as well.
 
I'm no biologist, but I don't think your muscles actually have to be moving in order for you to use energy, just keeping them flexed and exerting a force is enough to tire you out.
 

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