What do the motions of a compact disc, a Ferris wheel, a circular saw blade, and a ceiling fan have in common? None of these can be represented adequately as a moving point; each involves a body that rotates about an axis that is stationary in some inertial frame of reference. Rotation occurs at all scales, from the motion of electrons in atoms to the motions of entire galaxies. We need to develop some general methods for analyzing the motion of a rotating body. In this chapter and the next we consider bodies that have definite size and definite shape, and that in general can have rotational as well as transnational motion.
Real-world bodies can be even more complicated; the forces that act on them can deform them-stretching, twisting, and squeezing them. We’ll neglect these deformations for now and assume that the body has a perfectly definite and unchanging shape and size. We call this idealized model a rigid body. This chapter and the next are mostly about rotational motion of a rigid body. We begin with kinematic language for describing rotational motion. Next we look at the kinetic energy of rotation, the key to using energy methods for rotational motion. Then in Chapter 10 we’ll develop dynamic principles that relate the forces on a body to its rotational motion.