Modern physics, which had its beginnings in the inclined-plane experiments of Galileo, deals with the measurable aspects of the world about us. The laws and definitions of classical physics are, at least superficially, differential equations in which each variable represents the result of a particular kind of measurement. These variables are usually called physical quantites. Starting from a few general laws and definitions we can derive formally further relations between the physical quantities and their rates of change in space and time. If the original definitions are self-consistent and the original laws accurate, the relations we derive from them should enable us to predict what the numerical result of a certain measurement will be whenever certain other measurements have yielded specified numerical results.