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A rock sitting on land, the ocean floor, or floating in space maintains its shape somehow. Gravity isn't keeping it together because it is too small, so I'm assuming it is chemical or nuclear bonds keeping it together as a solid. If not it would simply crumble apart. So, what type of energy maintains the shape of a rock, where did this energy come from, and is it slowly dissipating?

As a corollary, if a large rock is placed on top of a small rock, is the energy required to maintain the shape of the small rock 'used' at a greater rate?

knzhou
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CramerTV
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    Closely related: https://physics.stackexchange.com/questions/1984/why-does-holding-something-up-cost-energy-while-no-work-is-being-done – dmckee --- ex-moderator kitten Nov 30 '18 at 21:43
  • @dmckee, that's actually the analogy I used on a Worldbuilding question which, after thinking about things, prompted this question. Thanks for the link. – CramerTV Nov 30 '18 at 23:19

5 Answers5

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No, the exact opposite is true.

The molecules in a rock don't stay together because they're spending energy. They stay together because of attractive chemical bonds. The molecules have lower energy when they're together than when they're not, so you have to spend energy to break the rock apart, not to keep it together.

knzhou
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    From where does the energy come for the chemical bonds? Isn't "attractive chemical bonds" an exchange of electrons? Is this exchange lossless? – CramerTV Nov 30 '18 at 23:14
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    Where does the energy come from to roll downhill? There's an absolute potential energy at the bottom of the hill. Why doesn't it just roll uphill after a while sitting at the bottom when the energy runs out? – William Grobman Dec 01 '18 at 00:13
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    I'm not being snarky either. This is an exact analogy using gravity and macro matter contours instead of electric forces and bond shapes. – William Grobman Dec 01 '18 at 00:14
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    @CramerTV A chemical bond doesn't consist of firing electrons back and forth. It's simply the fact that electrons have some energy when they're bonded, and some energy when they're not, and the former is lower. – knzhou Dec 01 '18 at 09:52
  • @WilliamGrobman, gravity is an exchange of gravitons thus the energy 'pulling' on the object rolling downhill is a result of the graviton exchange. So what creates the 'electric forces' and maintains 'bond shapes'? Isn't the electromagnetic force 'created' by an exchange of photons? Is the graviton and photon exchange lossless or does it eventually result in a loss of energy that will in time release all chemical and even atomic bonds and yield a completely energy filled universe? – CramerTV Dec 03 '18 at 18:22
  • Energy isn't a thing, per se. Energy is basically an accounting scheme for motion and resisted forces' capacity to induce motion. As far as I know, force carrying particles don't run out of force after consuming it. I'm actually pretty sure force charges are conserved quantities but you'd have to ask another question there – William Grobman Dec 03 '18 at 18:31
  • If they did somehow lose their force carrying charge, it would essentially be a type of particle decay and they wouldn't have the same identity anymore. I believe the detailed mechanics of when particles decay isn't understood and is believed to be random (with a possibly predictable half life), but I don't think that is really an energy question and doesn't apply to fundamental force carrying particles – William Grobman Dec 03 '18 at 18:33
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    @CramerTV Sorry, but that's not at all how forces work. It turns out that to compute the force between two particles, you have to use diagrams called Feynman diagrams. The simplest Feynman diagram has a squiggle that goes between the two particles. Some people say that squiggle means one particle fires a photon to the other one, but that's not what actually happens. – knzhou Dec 03 '18 at 18:34
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    @CramerTV Unfortunately, almost every pop science source explains forces in this fake way because it's easier -- the real explanation is just too technical. There is no need for the firing and refiring of photons to be "lossless", because it doesn't actually happen, at all. – knzhou Dec 03 '18 at 18:36
  • @knzhou, Ok, wow. I do get how things are abstracted and 'dumbed down' so we laypeople can understand the generalities. So please point me to something to read about this. I've been under a false impression for a LONG time then. I'm not a physicist but an engineer working on various wireless comms gear for the last 20 years or so and I've always thought the Smith charts were actual photons (energy) leaving the antenna to be received by another antenna. Now you're saying it's not really like this. Just, wow. I'm excited to learn how it really works. Thanks. – CramerTV Dec 03 '18 at 19:15
  • @CramerTV A radio transmitter antenna does emit energy in the form of electromagnetic waves (or photons, if you prefer), and a receiver antenna likewise does receive energy. However, 2 atoms chemically bonded together aren't swapping real photons with each other, but it can be useful to model electromagnetic attraction (or repulsion) via the exchange of virtual photons. – PM 2Ring Dec 04 '18 at 03:48
  • @CramerTV The Smith charts are perfectly right, it just doesn't work that way for individual particles interacting -- there you have to use quantum mechanics. Unfortunately I'm not sure what the best way to learn the quantum explanation is. The easiest way in for engineers is probably cavity QED, which has real applications, but it still requires a strong knowledge of quantum mechanics, which you'll have to pick up from one of the standard books. – knzhou Dec 04 '18 at 12:04
  • @CramerTV There you will learn that there are ways of interacting at the quantum scale that don't look at all like a "particle exchange" mechanism. Unfortunately I don't know how to explain it in words alone. You really need the mathematics here to avoid misunderstandings. It is completely possible to learn quantum mechanics from good, free public sources alone, but it will not be easy; this is the kind of thing that requires a year of serious study. – knzhou Dec 04 '18 at 12:05
  • @knzhou, I will look into cavity QED and other sources of QM. Thanks! – CramerTV Dec 04 '18 at 17:35
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There are various mechanisms that keep solid things together, they all have one thing in common: They reduce energy to a minimum! When you want to break it apart, it costs you energy to do so!

Examples of bonds are:

Hydrogen-Bonds, which are very weak and come from an asymmetry of the electron around the proton, in such a way that it is energetically favourable to form bonds instead of repel each other.

Ion-bonds, which can be quite strong, but the materials are often recalcitrant (brittle). Materials having ion-bonds are not pure, they are a mixture of two different elements, one positively charged, another negatively charged and they form molecules together, mainly due to the Coulomb force.

There are many more!

kalle
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The amount of work done is equal to the distance moved times the force in the direction of motion. As the rock is staying the same shape it does not need to exert energy.

You may be thinking that the rock needs to expend energy in order to hold up its heavy mass in the same way our muscles do if we hold up a heavy weight. But muscles need to contract to lift a heavy weight and this requires continuous activity at the cellular level as explained in the answer to this question.

Virgo
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Consider an answer by contradiction:

Imagine the rock is in the vacuum of outer space with no energy able to be added to it. Suppose it does use energy to maintain shape. Then at some point, it will run out of energy and the shape will change. Now, since it is out of energy and can't change shape, isn't it now maintaining shape without energy?

lamplamp
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Once there was a paradox in physics: why electron doesn't fall "down onto" atom's kernel and they decided it was "rotating". But "rotating" it would emit radiation and thus would loose energy. So later then they decided that "rotating" electron looses no energy staying on its "enabled orbits" and only would emit radiation when changing orbits.

The rock is a "set" of atoms, actually. You should look into the root of things.

Gravity isn't keeping it together because it is too small

Gravity is huge on small distances (look up the canonical formula — it's having /R^2, actually). But it's being compensated by other forces. We might continue this for long, but obviously there's no reason to repeat well known sources: https://en.wikipedia.org/wiki/Fundamental_interaction

Back to your q-n: rock is a collection of atoms. Atoms poses colossal energy indeed but as electrons on theirs orbits don't loose theirs energy and there's no intensive radioactive decay occurring it's all being "more or less" balanced. And when it's balanced — you guess it.

poige
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  • Electrons in atoms don't really move in orbits, like planets orbiting a star. There are energy levels called orbitals, but it's misleading to think of an electron in an orbital moving with a classical trajectory, where at any given moment the electron has a definite position and a definite momentum. – PM 2Ring Dec 04 '18 at 03:41
  • Is light a wave or not? :) All those things are actually abstractions and in my explanation I've given abstractions that use "planet model of atoms". It doesn't mean if you scale your microscope you'd really see balls around balls. It's pretty obvious what you're saying – poige Dec 04 '18 at 04:39
  • That's, BTW, why some parts of mine answer are quoted like "down onto", and "rotating". It's not the essence actually but it helps in my opinion to come up with understanding of stability the overall system has and why it's considered stable. – poige Dec 04 '18 at 04:43