Introduction To Non-Linear Dynamics — A Study Guide

“The solvable systems are the ones shown in textbooks. They behave. Confronted with a nonlinear system, scientists would have to substitute linear approximations or find some other uncertain backdoor approach. Textbooks showed students only the rare non-linear systems that would give way to such techniques. They did not display sensitive dependence on initial conditions. Nonlinear systems with real chaos were rarely taught and rarely learned. When people stumbled across such things-and people did-all their training argued for dismissing them as aberrations. Only a few were able to remember that the solvable, orderly, linear systems were the aberrations. Only a few, that is, understood how nonlinear nature is in its soul. Enrico Fermi once exclaimed, “It does not say in the Bible that all laws of nature are expressible linearly!” The mathematicians Stanislaw Ulam remarked that to call the study of chaos “nonlinear science” was like calling zoology “the study of nonelephant animals.” — James Gleick, Chaos: Making a New Science

Opening
I suppose you’re familiar with the simple pendulum. It’s an idealized model of a pendulum where the mass of the rod is neglected, the pivot is frictionless, and air drag is ignored.