Fundamentals of Fluid Flow

Principles relating to fluids at rest can be obtained with no ambiguity by purely rational methods. Some natural principles which are universally true are the basis of calculations. In flowing fluid, however, the laws that govern the motion are complex and most of the time defies mathematical expressions. Thanks to experimental data combined with mathematical theories which solved countless engineering problems in the past.
 

Time Rates

Time Rates
If a quantity x is a function of time t, the time rate of change of x is given by dx/dt.

When two or more quantities, all functions of t, are related by an equation, the relation between their rates of change may be obtained by differentiating both sides of the equation with respect to t.