- Every material has a certain degree of elasticity
- There is a point, called the elastic limit, where the material permanently deforms and can't be returned back to it's original shape (it gains a new elastic limit as well). This is also called plastic deformation or plasticity.
- Robert Hooke described the amount stretched as directly proportional to the force it would produce and vice versa (e.g. a bigger gravitational force acting down upon the elastic material - hanging a weight off of it - would produce a bigger stretch/extension)
However, I couldn't quite understand how this actually worked on a molecular level. It took me a while to find all the information, but I eventually understood (after much searching on Google!).
So here is the explanation I collected up (from various sources):
Basically, materials can stretch due to inter-molecular forces between atoms. There are different charges in different parts of the material and these attract. If you try to squash the material like charges repel too much and so the material tends to try to stretch out again. If you stretch the material the opposite charges attract too much and the material tends to attempt to go back to its original shape. This shape is the shape in which the material is balanced: there is an equal amount of attraction and repulsion and so this is the preferred state for the material.
However, there is also the aspect of the elastic limit. This works because beyond a certain point the atoms become to far apart or close together to be pulled back into their original positions. The atoms have so much force exerted on them that they displace. However, once they have displaced they will settle in their new positions and assume a new elastic limit. This explains why elastic bands snap: the atoms move so far apart from each other that the whole material gets pulled apart!
I hope this makes things slightly clearer. If you don't understand anything just comment below!
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