ABSTRACT
In our first encounter with rigid motion in Chapter 2 it was shown that, in general, it contains three independent translations and three independent rotations. It was also observed at that time that a rigid body has a maximum of six degrees of freedom in threedimensional space. But sometimes it can have less. For example, a sphere seems to have only three degrees of freedom, owing to its spherical symmetry, which renders all rotations about its center irrelevant as far as its geometry is concerned. This has an important practical consequence because it affects the way we dimension the relative position of a sphere with respect to, say, a plane.
