Mechanics is more than equations of motion
With this activity you can explore the double double pendulum. Two double pendula are simulated with nearly identical initial conditions. The initial conditions of the first pendulum (the red one) are input. For the second pendulum (the blue one) the position of the lower pendulum bob is displaced by one micron. The left panel shows the evolution of both double pendula. Initially the blue pendulum is hidden by the red pendulum. The panel on the right shows the common logarithm of the difference between the angles of the lower bob for the two pendula.
With this activity you can explore surfaces of section for the driven pendulum. Input the parameters you wish to explore, then click "start". The evolution of the driven pendulum is shown in the left panel. The surface of section is shown in the right panel. Click on the right panel to choose initial conditions, and the simulation will begin. To try a different initial condition click the "restart" button.