To make this task easy, we address a number of questions and provide answers that help in overcoming these initial difficulties:

• What are the underlying laws and relations on which the model is based? • What type of equations result from the application of these laws and

relations? • What is the role of time, distance, and geometry in the formulation of the

model? • Is there a relation between the type of physical process considered and

the equations that result? • What are some of the simplifying assumptions that can be employed? • What type of information can be derived from the model solutions?