The ideal pendulum has no dissipation or driving. This demonstrations starts off with an initial condition very close to the unstable "straight up" condition. You can start other trajectories by clicking on the running plot. Note that some trajectories orbit the stable fixed point at the origin, but some are "running" orbits, corresponding to the pendulum continuously whirling around without changing the sense of its motion. (The angle in this applet is not wrapped around to always lie between - and .) Remember that X corresponds to the angular position and Y of the angular velocity (proportional to the angular momentum).Return to discussion in pdf format.