Glossary — Materials

Elastic limit — the point on a force-extension graph beyond which a material does not return to its original length when the load is removed.

Elastic potential energy — the energy stored in a stretched or compressed material because of its change of shape.

Hooke's law — the extension of a spring or a wire is proportional to the force applied to it, provided the limit of proportionality is not passed. In symbols, $F = k\Delta L$.

Limit of proportionality — the point on a force-extension graph beyond which the extension is no longer proportional to the force, so the graph stops being a straight line.

Plastic deformation — a permanent change of shape that remains after the load is removed.

Spring constant — the force needed to produce unit extension. It is the gradient of a force-extension graph, and its unit is $\text{N}\,\text{m}^{-1}$.

Strain — the extension of a material divided by its original length. Strain has no unit.

Stress — the force acting per unit cross-sectional area. The unit is the pascal, $\text{Pa}$.

Tensile — describing a force or a stress that tends to stretch a material.

Young modulus — the ratio of tensile stress to tensile strain for a material. The unit is the pascal.