> When a spring is compressed, the molecules within the elastic material are stretched apart from one another such that their final state is less tightly bound than their initial state, and the spring has gained mass.
Wait, isn't this the other way around? Why would molecules be less tightly bound when a string is compressed? They're all being pushed closer together.
"Less tightly bound" is true at the molecular level whether or not the spring is expanded or contracted from a neutral position. At a molecular level, the rest state is for the molecules is in a kind of bathtub, loosely analogous to the effect that a camlock has in a mechanical system. Strain in either the expansion or compression direction introduces an opposing stress.
Until the force exceeds enough of the bonds' strength to cause plastic deformation, both directions are "uphill" along the potential energy curve.
> both directions are "uphill" along the potential energy curve.
That makes more sense to me. So they're not necessarily physically "stretched apart" when compressed, but they're still held in a higher energy state, and they want to move back towards lower energy state.
I only looked this up based on the comment, and I am not versed in this, beyond standard undergrad physics an eon ago. Reader beware.
The GP is wrong; it's not because molecules are closer or further apart, it's because we have introduced potential energy into the spring. The spring would gain mass regardless if it is compressed (molecules closer together?) or extended (molecules further apart?).
I used question marks there because a coil spring is a wound wire. The coil-to-coil proximity is not happening at a molecular level and is irrelevant. It is the stretching of the bonds in the [circular] direction of the winding where they get closer/further. Being helical, when deformed one side is in tension and one side in compression, so there's a net zero molecular proximity. So I'm not convinced that spring compression is molecules getting closer, and that spring extension is molecules getting further.
There's also a torsion spring, which is a bar with the ends fixed in place, that is twisted axially for the spring effect. Often used in formula car applications, but widely elsewhere as well. Here it is clear to see that compression and extension have the exact same effect, and that "closer" and "further" both occur.
The introduced potential energy is equivalent to mass, hence the spring has gained mass (and the compressing/extending object has lost mass).
I agree that picking compression specifically was misleading.
> The GP is wrong; it's not because molecules are closer or further apart, it's because we have introduced potential energy into the spring.
The structure of the electron orbitals leads to a preferred bond length. So, pushing them closer or farther apart from that preferred bond length introduces potential energy into the spring. Its just that either direction is "up" the potential energy well.
Heat is kinetic energy, its just randomized. It isn't the conversion of kinetic energy to heat that makes the mass change observable, its the subsequent dissipation of the heat that does so.
The difference in mass is observable only when the binding energy you release gets outside of the measurement apparatus. Because the mass-energy equivalence goes both ways, any instrument that is set up to measure the mass won't register a change until that heat also dissipates. This equivalence is why physicists will frequently clarify what they mean by distinguishing "rest mass" from "mass", where "rest mass" is the mass that the body will have once it comes to rest.
Thought experiment: A stretched ideal spring sits inside an isolated enclosure with an instrument set up to measure the mass of the spring, enclosure and everything inside of it. You release the spring and it vibrates back and forth indefinitely. The instrument registers no change in mass at all.
Variation: The spring is not ideal; it has some damping. The enclosure remains ideal. You release the spring and it vibrates back and forth a little bit before damping slows it down and releases some heat. The heat stays inside the perfect enclosure. The instrument still registers no change in mass at all.
Variation: The enclosure isn't ideal, either. You release the spring, it vibrates back and forth a little bit before damping slows it down and releases some heat. The heat escapes the imperfect enclosure. The instrument registers a small change in mass, proportional to the energy released / speed of light squared.
Yeah, I meant as in "some heat escapes into the environment". That makes a lot of sense, thanks for the explanations as this stuff can be quite unintuitive at times :)
Wait, isn't this the other way around? Why would molecules be less tightly bound when a string is compressed? They're all being pushed closer together.