This is a physics-based simulation of a double pendulum whose support point is moveable.

Students can change the support point and pendulum masses as well as gravity, damping and other parameters.

Some experiments to try:

• Try dragging the anchor point by clicking near it with your mouse. Whip it around quickly back and forth -- does the pendulum behave as you would expect?
• Turn on "show energy". When is kinetic energy high? When is potential energy high?
• Set damping to zero, get the pendulum moving rapidly, and turn on "show energy". You will notice the total energy decreasing slightly over time, although it should stay constant. Now try slowing down time by a factor of 3 or 4 (which gives more accurate computation). Is the energy still decreasing? What's going on?