I refer to rubber bands and other elastic materials.
In: Engineering
The most basic forms of elasticity come from atomic bonds themselves stretching. This is a kind of elasticity present in all materials.
For rubbers specially, there’s another layer on top of this that allows for much greater stretching. Rubber molecules are very long and chain-like, i.e. the molecular can bend and fold in lots of places. When you stretch a rubber, you’re straightening out all of these chains, making the material much longer. But the natural state of the molecules is to be kinda jumbled up, so the material will want to return to that state by contracting.
This part is usually not explained well, so I’ll try my best. All materials have a natural thermal energy to them. This means all their molecules are moving and vibrating. When a long chain molecule moves and vibrates, it more or less stays as a bunch of somewhat jumbled chains, some get a little more jumbled, some a little less, so the material stays constant. But once you stretch it, there’s only way for the molecules to move, and that’s by folding and shortening. If the chain is already full straightened, it can’t straighten more. So the natural thermal energy in the rubber will make it contract.
Hope that helps!
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