Elasticity

All deformations, no matter how complicated, can be described as the result of combinations of three basic types of stress. One is tension, which stretches an object along one direction only. Thus far, our discussion of elasticity has been entirely in terms of tension. Compression is the same type of stress, but acting in the opposite direction.

The second basic type of stress is shear stress. This results when two forces push on opposite ends of an object in opposite directions. Shear stress changes the object's shape. The shear modulus is the amount of shear stress divided by the angle through which the shape is strained.

Hydrostatic stress, the third basic stress, squeezes an object with equal force from all directions. A familiar example is the pressure on objects under water due to the weight of the water above them. Pure hydrostatic stress changes the volume only, not the shape of the object. Its modulus is called the bulk modulus.