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Yeah, I have been thinking a lot about potential energy, and it just doesn't quite add up. There is something missing.

If I push up against a huge massive block of cement, the wall is pushing back at me with the exact same force I push it to neutralize the forces and equal them out. Now here is my problem: I am using potential energy to push against that wall, but it doesn't move at all. I don't move either. NOTHING moves. Where is that energy going to? Sure, "some" of it turns into heat, a microscopic, no, close to none of that energy at all deforms the cement block. Where is that energy going to? Where is that block getting it's energy from to push back at me?

If you are in a space ship, and you push against the wall, the energy will accelerate you the way you push yourself, and the space ship will be affected by your force. But on earth I don't get pushed back. I just stand there and push, and have no idea where the energy has gone.

Lets go to another aspect. I put a glass onto the table. That glass is pushing down onto the table, because of gravity, and the table is pushing up at the exact same amount. Where is the energy coming from? I can't figure it.

How about a wound up spring? I can leave that spring wound up for ever if the materials held. Where is the energy coming from that keeps that spring wound up? I don't know.

Can anyone explain?

TheComet

It comes from the energy you stored up originally. To wind up a spring, you have to put a lot of energy into it, energy that is not released until you let it go. The same thing for the wall. You aren't actually putting any energy into it, even though you think you are, because you have no actual effect on the wall.

EDIT: Great, that same stupid Evony ad squeezed itself in there! Arghhhh!

So when I push against the wall, I am actually pushing against myself? But there is some force holding that spring open, isn't there?

TheComet

I'm going to try and remember physics class from last year, even though it was with the worst teacher I've had to date.


I know there was a good explanation of why this happens, but I can't remember it so I'll make something up that makes sense in my head. The energy you exert does generate thermal energy, as does any other motion of your body. Hence sweating when you do strenuous activities like pushing on cement blocks for science. But when you say you are pushing on the block in one direction, you have to be pushing off of something else(in this case, the ground). Imagine trying to push that cement block on an icy pond? It would be much more difficult due to the lack of friction. Anyway, to your question(s): the energy formed by metabolism and expended by your limbs is exerted in two directions, the block and the ground. The block does not "get its energy" from anywhere, but your inability to push it is due to the resisting forces of friction between it and the ground which is greater because of its greater mass.


This is much the same. The force of friction due to gravity keeps you from moving, even though you are exerting energy.


You say the table is pushing up on the glass. However, is it not then pushing down on the ground, and through it, the Earth? Gravity caused by the Earth pulls everything towards it, but obviously a glass cannot be pulled through a table. I think the answer you're looking for here is the electrical resistance between electrons in the bottom surface of the glass and the top surface of the table.


I'm not sure what you mean by "if the materials held."? Please elaborate.


Hope that helped a bit.

this is true... but equally important is where that energy goes.

yeah... this is something that really confuses me too. I think that some of the confusion comes from the differences between energy and work. Energy is the ability to do work, not work itself. A confusing thing is that this would then make potential energy, the potential ability to do work.

I think that part of the answer to the "Glass on a table" thing, is that it also takes work to pressurise matter. The potential energy that the glass has would have been from whenever the universe started, however it started (at the big band for example, the force seperating matter, whatever it may have been). When the glass pushes down on the table, it pressurises the table. This pressure is stored as potential energy. So, that much is understandable. It can be kind of understood why these two objects would remain in equilibrium.

What i think is going on then, is that, even though there are two forces opposing eachother, no work is being done because nothing is moving (since work=force*distance). Since no work is being done, the potential ability to do work stays the same in both objects (keeping in mind that during the time the glass was set on the table, work was done to pressurise the table). So that pretty much explains the glass on the table problem.

Now, lets consider a person pushing on a wall. Because pushing on a corner involves friction and other forces that keep you in place, i think a different situation would be easier to understand. Imagine that your in the middle of nowhere. There is a metal plate below you, and above you. Each of the corners of one plate is tethered with elastic to each of the corners of the opposite plate. Picturing that in your head? Ok. Now, when you push on the top plate, you do work to seperate the two plates (cuz your also pushing on the bottom plate). While your pushing the plate up, work is being done (gravitationally seperating the two plates, and stretching the elastic, as far as we're concerned). When you've reached the length of your body, with your hands over your head, the forces are in equilibrium. When you stop exerting the two forces (on pushing down on the bottom plate, one up on the top plate), then the potential energy is released from the elastic, accelerating the two plates down towards eachother.

But your still actively doing something while the two plates are apart, and you couldn't hold those two plates apart indefinitely, because your using energy.

This is where i'm not sure, and i thinkk that this is the real issue, but heres my guess as to what's going on: All that energy is being turned into heat. If you were standing on earth, and threw a rock, which managed to break free from earths gravitational field, you wouldn't be as hot as if you had pushed with the same force, on something attatched to whatever you were standing on. That's my only guess - that when your muscles undergo the chemical reaction making them contract, some Potential Energy is turned into heat, and some into kinetic energy. If that kinetic energy is canceled out, it turns back into heat (maybe... the kinetic energy goes into breaking bonds in muscles, which releases chemical potential energy as heat?)

Yeah... even in the simplest cases of forces interacting, theres alot going on...

Venge, makes sense to me. Good explanation. I think he meant that the metal in a spring would decay over time; partly because of natural decay and party because the stored up energy takes a toll and stresses the metal. Then when that material get's week enough, the energy overpowers the resistance and releases. But, I guess we'll wait for him to clarify what he meant eh?

You got it. That's exactly what I meant.



Sure, but that still doesn't tell me where my energy went... The block does have a greater mass and is held down by gravity, but me pushing up against it doesn't change it at all.

Let's say I'm holding a six-pack of beer. Here I am, in my back yard, just standing there, holding the beer. I'm not moving, the beer isn't moving, nothing is moving, but yet I am wasting energy to keep that beer above the ground. That makes a bit of sense, because I'm fighting against gravity. But no one can tell me where that energy is going. I am not doing any work, as Neuro Fuzzy said, but yet I am using energy. This all doesn't make sense. Because for me to hold that pack of beer in the air for ever, I would need an infinite amount of energy, which means gravity has an infinite amount of energy. BUT no one can use gravity to produce energy.

If I however put it down on a table, the table must have an infinite amount of energy to stop that beer falling down onto the ground. Where is that energy coming from?



That can't be. You could run some maths on that to see how many joules of energy you use in 10 minutes of pushing, and covert that into heat, and you will quickly notice that not all of it goes to heat.

TheComet

Potential energy isnt a measure of energy gained or lost, but a measure of energy that could be gained or lost.

eg. Gravitational Potential energy.
Chair on the ground has (lets keep it simple) 0 GPE.
If I lift it above my head, it gain GPE, but it doesnt actually gain energy. It has the potential to gain energy (when I drop it, it will fall gaining Kinetic energy and "losing" its GPE).

As for pushing against the wall, I dont think youre using potential enegy, I think youre using Chemical energy, ie energy stored by in your muscles. This energy does go somewhere. Work = force*distance and the wall does move a very small distance so you do transfer as little bit of energy to the wall. You lose more as heat and a little bit as sound. My best guess as to where the energy majority goes is trying to overcome the electro-magnetic forces between you and the wall.

You have a point though, it doesnt seem satisfactory to say all the energy is lost as heat.

Edit: Some inspiration from NF's post, to hold it still, you must be giving it Kinetic Energy equal to that that gravity gives it. This cancels out the forces so the beer stays still. Also, its quite a lot of energy that you will lose, thus accouting for all the chemical energy you lose.

but chemical energy is another form of potential energy

Actually, i'm more and more confident that that energy goes to heat. I mean, in that situation i posted above, the entire system is just you, two metal plates, and the elastic. You have lost chemical potential energy, and there isn't much else that could have gone to.

My science teacher did this demo. She had a student hold a piece of paper in the air, and she picked up two giant metal balls, one in her left hand, one in her right hand. Then, she slammed the two metal balls together on the same spot (on opposite sides) on the paper.

This collision burned a hole in the piece of paper. This was due to two things- the huge amount of pressure applied to the paper, but the kinetic energy also left a huge amount of heat there (the area on the paper was still hot after the collision had ended). Almost all of the energy had gone to heat, and the rest to sound.

blegh, i tried doing some math to solve this problem, but i can't without knowing how many calories are burned when your arm exerts a force (which can't be determined without knowing the angle of the arm, and alot of other stuff). basically though, here is what i know for sure is going on in the example seperating the two plates tied to eachother with elastic.

1. as you push up and seperate the two plates, you give them elastic potential energy.

2. as you maintain the seperation, the potential energy of the two plates and the elastic does not change. However, you (standing inbetween the two plates) are losing chemical potential energy maintaining your muscles in that position. This is the part we can't agree upon. I think that all the chemical potential energy you lose, you (and the plates) gain in heat.

3. as you release the plates and allow them to go back together, that energy is then turned into heat and sound.

I didn't read the thread (sorry, maybe later) but in short - FORCE does not equal ENERGY. The only energy involved in the wall scenario is the chemical energy you are giving your muscles to generate the force against the wall - and 100% of that energy is converted to heat as your muscles apply the force.

No energy is involved in holding the spring compressed - only force.

That does sound a bit weird, doesn't it? Why should I give off tons of heat and energy between those two elastic plates, but a brick doesn't? I just can't imagine that all of that chemical energy is being converted to heat and sound, but the brick doesn't do anything...

I am trying to believe everyone's arguments, but I still can't see the whole picture...

TheComet