Solids are materials that do not deform under tangential forces. Density is defined as mass per unit volume. One characteristic of interest in solids is elasticity, the deformation that results from the application of a force or torque. If the applied force or torque is not too great, the material will return to its original form when the force is removed. Beyond the elastic limit or yield strength, however, the material will retain the deformed shape. If further deformation is applied, the break point is reached, and the material ruptures.
The different measures of elasticity are called moduli. All of the moduli are ratios of stress to strain. In the case of Young's modulus, stress is defined as the perpendicular force applied per unit area, and strain is the elongation per unit length, or how much a given length will stretch. Mathematically,
