In the field of applied mathematics, one often uses the approach described below:

*I)* Formulate a mathematical model of the situation, for instance by finding the governing equations of the problem.

*II)* Solve the equations by using some suitable analytic or numeric method.

*III)* Go back and verify that the obtained solution is consistent with the experimental observations.

**Remark:** We can get further assistance in the first step by using:

*a)* Dimensional analysis.

*b)* Scaling.

We will examine these two methods in the following parts.