We here move to one of the major applications of differential equations both into this class and in general. Modeling is the process of writing a differential equation to explain a physical situation. Mostly all of the differential equations which you will use in your job as for the engineers out there in the audience are there since somebody, at several time, modeled a situation to come up along with the differential equation which you are using.
In this section is not intended to wholly teach you how to go regarding to modeling all physical situations. A complete course could be dedicated to the subject of modeling and even not cover everything! This section is implemented to introduce you to the method of modeling and demonstrate you what is included in modeling. We will seem three different situations in this section as: Falling Bodies, Population Problems and Mixing Problems.
In these all of situations we will be forced to create assumptions that do not correctly depict reality in most cases, but without them the problems would be extremely difficult and beyond the scope of such discussion and also the course in most cases to be truthful.