Rotation and translational motion can be treated analogously to the theories of fluids and electricity presented in Chapter 1. This becomes particularly clear in single dimensional applications (motion along a straight line, rotation about a fixed axis; for the latter see Section 2.5). Momentum and spin (angular momentum) are stored in moving and rotating bodies, and they are exchanged with other bodies and the envi ronment in mechanical processes. Naturally, there are many differences in detail between theories of motion and those of other fields. However, I am interested in aspects of the conceptualization that demonstrate deep similarities. It is interesting to see how far we can go in mechanics using ideas developed for fluids and electricity.