Chapter 15 has a nice diagram about the forces on a pendulum mass, Fig. 15-3, I think. We treat a pendulum just as if it were a spring IF its amplitude of oscillation is only a few degrees left and right of vertical. I just activated a new page, "Why a pendulum behaves periodically, like a spring." It has a bunch of diagrams, for those of you who want some extra depth of knowledge. No trig, no calculus, just proportional right triangles. Look for this in the Lecture Files.
