THE study of fluid mechanics is based, to a large extent, on the conservation laws of three extensive quantities:
1. Mass—usually total, but sometimes of one or more individual chemical species.
2. Total energy—the sum of internal, kinetic, potential, and pressure energy.
3. Momentum, both linear and angular.
For a system viewed as a whole, conservation means that there is no net gain nor loss of any of these three quantities, even though there may be some redistribution of them within a system. A general conservation law can be phrased relative to the general system shown in Fig. 2.1, in which can be identified: