eli5: On a molecular level, what causes elastic things to return to their original shape after being bent?
In: Physics
When something is bent out of shape within its elastic bounds, the molecules retain their structure in relation to the other molecules. However, the intermolecular bonds are stretched. Whenever those bonds stretch beyond a certain point (elastic/inelastic transition point), the object can’t return to the exact original shape because some of those bonds break and shift to other molecules. Think of it like pulling a magnet on a string attached to a piece of metal. If you pull hard enough on that string, the magnet will pull off of the metal piece and jump to another.
It depends on what lengthscale you are referring to when you say molecularly. Chemical bonds have a length at which they are most stable, that is lowest energy. This has to do with the forces between the nuclei and electrons in the bonds. When you deform an object, you may be changing the length of the bonds. This would increase the potential energy such that when the force is released it will return to it’s most stable state and return to it’s original shape. However, I suspect that this is not what occurs most often. Forces between molecules probably dominate at the molecular level, rather than between nuclei and electrons. The principle will be the same though. There will also be larger still microscopic features that will be deformed well before molecular deformations will happen. For example, if you look at wood under the microscope it is quite porous. The shapes of the pores will change under deformation, leading to an increase in potential energy relative to the ground state that can propel it back to the ground state once the force is released. For materials that do not behave elastically, the deformation is able to cause a new stable ground state due to the movement of some micro features of the structure, similar to how when water is deformed the molecules will simply rearrange to be as stable as before and it won’t flow back towards it’s original location.
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