The extension of a spring is directly proportional to the load or force acting upon it.
F = ke
Mass / Volume
Force / Cross-sectional Area
Note: Cross sectional area = (pi x diameter^2) / 4
It is measured in Nm^-2 or Pa (Pascals)
Change in Length / Original Length
It has no units, it is just a number.
Stress / Strain
If you increase the force (load) on a material past the Elastic Limit, the material will become permanently elastically deformed, meaning it will no longer return to its original shape.
Limit of Proportionality
If you increase the Force (load) on a material past the limit of proportionality, then it will no longer obey Hooke's Law, however it will return to its original shape because the limit of proportionality is reached before the elastic limit.
Here the material suffers a large amount of plastic deformation without an extra force (load).
If a material is Brittle..
..It snaps suddenly when the force reaches a certain size, without plastically deforming.
Energy Stored in a spring/wire
0.5 x stress x strain
Ultimate Tensile Stress
The maximum stress a material can withstand.