The applet is on Page 2. It shows two planets in head-on motion. The applet displays the gravitational forces the planets exert on each other and shows how the speeds, momenta, and kinetic energies of the two planets, and the potential energy and total mechanical energy of the system vary during the motion.

Detailed information on how the applet functions is available under Applet Help on the applet's Help menu. A lesson on Newton's third law and related topics, with exercises to be performed with the applet, is available there as well.