ABSTRACT
By definition the relative positions of the points in a rigid body cannot change over time. For elastic bodies, on the other hand, these relative positions can change. Strain measures the relative deformation of the points in an elastic body when these deformations are "small". Strains in elastic bodies are the result of forces acting on the boundary of each volume element. The equations which govern the motion of "ideal" fluid with zero viscosity consist of one equation which is an expression of the law of "mass conservation" and three equations which express the law of momentum conservation. These are called Euler's equations. The equations that govern the motion of viscous fluid consist of the continuity equation and a set of three equations which express the law of momentum conservation. These equations are called Navier-Stokes equations. In many fluid problems one must consider the effects of temperature gradients on the flow.
