Homework Help: Just a simple thought experiment

1. Jan 26, 2014

Fifty

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

2. Jan 26, 2014

BvU

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.

3. Jan 26, 2014

Fifty

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

4. Jan 26, 2014

BvU

Excellent idea. I'm a physicist and I have a hard time understanding it, let alone explain it!

5. Jan 26, 2014

MostlyHarmless

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..

6. Jan 26, 2014

Fifty

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.

7. Jan 26, 2014

MostlyHarmless

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.