All of StephensCreek's Comments + Replies

What keeps the earth from collapsing is not electrostatic energy (well, I guess you could probably describe it so it sounds that way, but I don't think that's the best way to describe it) and no, it isn't used up. At a microscopic level, things do move around in various ways even when they're part of objects we think of as being in equilibrium. Heat is the name we give to this sort of jiggling about, and it's a form of energy. But in equilibrium the net force acting on those atoms is zero, so the jiggling isn't moving things against an opposing force. ("But wait, say one of those atoms jiggles just a tiny little bit. Then it's no longer in its equilibrium position, which means that any further jiggling will be against an opposing force." Ah, but further jiggling may move it the other way, in which case the atom is gaining energy instead of losing it. On average it stays in the same place and the net energy change adds up to zero.) In the very long term the jiggling will reduce, not because energy is being used up holding things apart but because heat tends to spread itself out uniformly (the jiggling spreads to other nearby things) and the average temperature of the universe (temperature is a measure of heat per available way-of-moving, which is kinda like heat per molecule) is rather small -- so in the long run everything gets pretty cold. In the short term, the dumbbell is at about the same temperature as the other stuff around it, and the flow of heat out of it is matched by the flow of heat into it. But the key thing here is that exerting a force, as such, does not consume energy. The earth is not radiating away energy as it exerts a gravitational pull on everything else in the universe. An electron is not radiating away energy as it exerts an electrostatic attraction or repulsion on every other charged object in the universe. This may be counterintuitive if you think of energy as that stuff that everything needs in order to do anything, but the moral is tha

This is a good question. The answer is that it shouldn't take any energy to hold something in place, but your arms are very inefficient. When you keep one of your muscles contracted the individual cells in that muscle are all contracting and relaxing repeatedly. This burns energy. So for a human holding a dumbbell takes energy. But this is just an unfortunate consequence of the way muscles work. If the human body had some way to "lock" the skeleton into place then you would be able to hold a dumbbell for as long as you wanted.

No, the dumbbell-on-a-table is not using any energy to keep from falling down. What takes energy, in general, is not exerting a force but moving things against an opposing force. Getting the dumbbell up into the air takes energy; letting it fall again will release some energy; but that's a one-off, and there isn't any ongoing energy cost to keeping it in the same place. (Why does the stuff on the surface of the earth not all fall inward and the earth collapse into a black hole? Because as well as the inward force those things experience due to gravity, there are also outward forces, mostly arising from electrostatic repulsion between the electrons on nearby atoms, and when something is sitting motionless on the ground that's because those forces balance one another. This is also what's happening to the dumbbell on the table.) So maybe the real question here is not "how can the table hold the dumbbell up without expending energy?", but "how come my arm needs to use energy to hold the dumbbell up?". The answer to that is: while there are structures, like a table, that can hold a dumbbell up without using energy, your arm is not built one of them: because it's made from flexibly-articulated bits, and it's only able to achieve the same effect the table can by less efficient means. (But in exchange for that it gets the ability to do things the table can't, like moving the dumbbell around.)