Click on the picture to activate Active-X. The dotted border will disappear. Then drag a ball and release it. This operation can be done on any of the balls, and you may drag it either direction, left or right.
The animation displays the idealized behavior of the Newton's cradle apparatus, assuming nearly perfectly elastic, perfectly spherical balls of equal mass and composition. However, it does show slow loss of kinetic energy as the balls take some while to come to rest.
Flash animation by Bryan Heisey, used with permission.