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Before answering the question, keep in mind that I am a second year Biology student, with no experience in studying Physics and I consider myself 'mathematically illiterate'.


An example of matter preventing other matter from reaching a lower potential energy state, is a table carrying an object, preventing it from falling due to gravity. However, I find it hard to understand what energy interactions are occurring between the matter involved.

After Marcello Fonda kindly explained the terminology and concepts relevant to my question, I have refined my question and clarified my understanding of the subject.

I understand that energy is,

a measure of an object's ability to interact with other objects, changing their state of motion

and that,

Holding an object in place maintains its potential for interaction (energy), it doesn't change it.

Initially, I suggested that the force present keeping the weight of the object (which I assume is a product of the amount of matter in the object and the 'strength' of gravity) at a higher potential energy, was possibly due to the repulsive and attractive forces between the atoms constituting the table.

Marcello Fonda has agreed that the static force is caused by atomic repulsion. This makes me wonder if increasing the weight of the object on top of the atoms constituting the table, will cause them to be 'squeezed' or compressed, thereby increasing their energy state?

If so, where would the source of energy pressing the atoms together come from?

Side note: it would be nice to know the formal name for these 'barriers', or more specifically, the name for the matter preventing the other matter from falling to lower potential energy states.

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  • $\begingroup$ It requires energy to lift a piece of (say) lead from the floor to the table top. But once the piece in on the table top it undergoes no more energetic changes. $\endgroup$
    – Gert
    Feb 4, 2023 at 13:25
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    $\begingroup$ I wish more physics students would ask this question. $\endgroup$
    – garyp
    Feb 4, 2023 at 13:34

2 Answers 2

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The concept of energy is a rather mysterious one. Feynman's explanation is probably one of the most beautiful, simple, and clear I've seen so far, you should try and check it for a good understanding.

The main point of the answer to your question, though, is that we like to define energy as being always related to the force used for moving objects, not for holding them in place. To keep an object still doesn't change its energy, although force may be needed: you can thus think of energy as a measure of an object's ability to interact with other objects, changing their state of motion, for example by colliding with them or by pulling or pushing them around. Holding an object in place maintains its potential for interaction (energy), it doesn't change it. If you release the object by removing the table, you can turn that potential for interaction into real interaction, for example by tying your object to a pendulum clock and have it move the dials while losing height (by the way, this is sometimes called a gravity battery).

A change in energy is called work, so for those forces which cause no change in an object's energy we say that they do no work. They are often called ideal constraint forces, so the table in this context would make for an ideal constraint.

The main reason all this stuff sounds counterintuitive is that we as animals consume energy even for static holds, but that is due to the fact that our muscles work by continual micro-contractions, not by a single hold. A static hold is never truly static for our muscles, rather is a very fast cycle of pull-and-release type oscillations (you might be aware of the actual mechanism) which actually does work at a microscopic level (it takes the energy from ATP bonds), though not at a macroscopic one. A table doesn't experience this kind of fatigue, since atomic repulsion is rather static in nature.

EDIT to address further question:

Increasing the weight will make the table bend and squeeze. Its atoms will then be at a higher energy state because of the stretching and squeezing of their bonds (which can be reasonably accurately be thought of as acting like microscopical springs). This increase of energy will come at the expense of the energy of the object, which will fall a little distance by bending the table downwards. What we expect is that the bending will eventually stop. This can be explained in terms of energy by the fact that, as you bend the table, the energy required to further bend it increases. You reach a point where lowering the object yields less energy than is required to bend the table, so the descent stops.

If no energy is dispersed in the process, lifting the weight will let the table go back to its initial, straight position. If there is energy dispersion (bonds are broken), then the table won't be able to get back to its initial position. If the dispersion is slow, the table will get saggy. If it is fast, the table will crack and break.

Note that all this is regardless of the table's height: unless the object is tied to the ground with some sort of spring, raising the table and the object together (like carrying them to the second floor) doesn't stress the table more. Otherwise we couldn't have desks in skyscrapers.

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  • $\begingroup$ Thank you @Marcello Fonda for clarifying the terminology relevant to the concepts I mentioned, and for clarifying the concepts themselves. I understand that, "Holding an object in place maintains its potential for interaction (energy), it doesn't change it". If the 'push back' against the weight of the object (which I assume is caused by the mass of the object and gravity) is caused by atomic repulsion between the atoms that are constituting the table, will increasing the weight of the object on top of the atoms, not 'squeeze' them together, thereby increasing their energy state. $\endgroup$ Feb 6, 2023 at 10:50
  • $\begingroup$ It certainly would, but that would imply movement of the object (the squeezing is a dynamic process, where the object goes down a little while compressing the table). If you consider the extreme example of the table being made out of gas (imagine a big syringe planted on the ground with the object sitting on its piston and the hole blocked), the object will compress the gas until the pressure is sufficient to hold it still, thereby increasing the temperature (energy state) of the gas. After the compression, though, no work is done. Feel free to accept the answer if you find it satisfying :) $\endgroup$ Feb 7, 2023 at 11:38
  • $\begingroup$ Side note: you usually don't even account for this kind of process, since it is neglectible for objects like tables (it is for gases). You call rigid bodies those for which you neglect this kind of dispersion of energy. I can incorporate this considerations in my answer if you want. $\endgroup$ Feb 7, 2023 at 11:50
  • $\begingroup$ I have one final question, it requires energy to keep these atoms at a higher energy state, when an object is on a table how is it keeping these atoms at a higher energy state without expending energy? $\endgroup$ Feb 7, 2023 at 12:07
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    $\begingroup$ It usually doesn't require energy to keep atoms at a higher energy state. It would, only if they were constantly falling back to lower energy states, by dissipating their surplus energy: the stretching of a bond at atomic level is reversible if small enough, i.e. no dissipation occurs. If the weight of an object becomes too big, the stretching is big enough to cause rupturing of some of the bonds, which dissipates energy: it's usually a slow process and you can see this in old shelves getting saggy (or broken ones, where the process was indeed fast). $\endgroup$ Feb 7, 2023 at 15:01
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A very partial answer to address a misconception, since this is a multi-part question. You wrote:

This makes me wonder if increasing the weight of the object on top of the atoms constituting the table, will cause them to be 'squeezed' or compressed, thereby increasing their energy state? If so, where would the source of energy pressing the atoms together come from?

Atoms and solids and gravitational fields are inconveniently complicated, so I'll start by replacing this complicated system with a very simple system.

Consider Alice and Billy playing on a see-saw. When Alice's separation from the planet decreases (the gravitational field does work on Alice), Alice doesn't need a rocket to keep her from smashing painfully into the ground - she just needs somewhere to put the energy that she's getting from the gravitational field. Alice puts the energy into the separation between Billy and the planet: when Alice goes down, Billy goes up.

Back to the weight on the table. If the table and the weight were both perfectly rigid (no amount of force can make either deform without breaking) then, if we started the weight resting on the table, the gravitational field wouldn't do any work because the weight wouldn't move, so the weight wouldn't need anywhere to put the energy - the table could be perfectly fragile in addition to being perfectly rigid, and nothing would happen. That, of course, is not how reality works: perfect rigidity is a frequently useful counterfactual, not a property of real objects made of atoms connected by inter- and intramolecular forces.

So we have a deformable weight on a deformable table, and we apply the force of gravity, which starts deforming the weight and the table. This allows the center of mass of the weight to go down: the gravitational field has done work on the weight.

Just like Alice and Billy, we don't need a rocket expending energy to push in the opposite direction to keep the weight from falling through the table. We just need somewhere to put the energy. For the weight and the table, the place where the energy can go is into the intramolecular force fields holding the solid together. Much like a spring (or, a bit closer to reality, like an incredibly complex system of countless, constantly jiggling, tiny interconnected springs), the solid has a higher energy state when it is deformed by an external force than when it is resting naturally.

The energy to cause the deformation comes from the gravitational field, which is a mathematical representation of the energy associated with the configuration of centers-of-mass in the system.

Stopping the deformation does not cost extra energy (unless you do it with a rocket). It demands a repository to put the energy into. The energy that comes from the configuration of centers-of-mass in the system goes into the configuration of interacting molecules in the system. If the configuration cannot change in such a way as to hold that much energy in that amount of time, either the table or the weight must break.

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  • $\begingroup$ @g s I think I understand the points you are making, which I believe re-contextualise the logic I was applying initially to the table and object situation, as well as, provide additional, necessary details. It is worth noting, I am unfamiliar with the formal definitions for energy or work, which could result in me unwillingly, misunderstanding the answer you have provided. $\endgroup$ Feb 8, 2023 at 12:24
  • $\begingroup$ Nonetheless, my interpretation is as follows, energy is spent moving an object toward the table or Billy (love the name) against gravity. Once that object is at a higher potential energy (in our two cases, the object is further from the ground, on the table and Billy is separated from the Earth), it will want to go to a lower potential energy state (who knows why). $\endgroup$ Feb 8, 2023 at 12:25
  • $\begingroup$ To do so, it will release energy, that will be transferred to other matter, raising its potential energy state (in our two cases, Billy's energy will be transferred to Alice, raising her potential energy state and the object will transfer its energy to the atoms of the table, by compressing or stretching them). In both cases, the energy came from gravity. $\endgroup$ Feb 8, 2023 at 12:25
  • $\begingroup$ To me, the overall crux of your answer seems to be that the energy that keeps the matter on the table at a higher energy state is gravitational fields (whose cause I only understand as being matter). I do not understand what gravitational field are, because the phrase "configuration of centres-of-mass in the system" was difficult to interpret for me. So I think matter itself influences how matter is arranged, somehow. $\endgroup$ Feb 8, 2023 at 12:25
  • $\begingroup$ @simon I think those would be better addressed as separate questions. $\endgroup$
    – g s
    Feb 8, 2023 at 18:24

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