# Young Modulus

- The value of a
**Stiffness Constant**only applies to a certain sample. F_or example, a longer piece of the same material would have a lower stiffness constant._ The**Young Modulus**on the other hand is a**material property**, meaning it can be applied to all samples of the same material. It is calculated by the formula:

## Stress and Strain

**Stress**is a measure of the**force applied per unit cross-sectional area**of a material. It is measured in**Pascals**(**Pa**, or**Nm**, or^{-2}**Kgm**if you want to be pedantic). It is^{-1}s^{-2}calculated by the formula:*hence*

**Strain**is the**change in length relative to original length**. It has no units since it is the ratio of two lengths. It is calculated by the formula:

- A
**stress**on a material causes a**strain**.

- For a material, a stress-strain graph can be drawn. The gradient of this graph is then the Young Modulus. The Young Modulus is also measured in
**Pascals**. By finding the a**rea under a stress-strain graph**, it is possible to work out the**energy stored per unit volume**in a material.

- The Young Modulus, being a material property as it is, can be used to
**generalise the elastic properties of a material**.*This is very useful, for example, in working with seat belts, as it important to be able to calculate how far they will stretch for any length or thickness.*