I worked with this applet for about half an hour. Although the applet appearance is rather plain, it works well, and cleverly presents a topic not easily described by text. The physics appears to be correct (e.g., momentum is conserved, energy is conserved when it should be), even in the limits. Calculations are done only to about 5 significant figures, so the velocity values can be off somewhat after many collisions. There are minor grammatical problems in the text, particularly in subject/verb agreement. Overall this applet has a good potential to be effective in teaching the concepts of collisions in different reference frames. Its effectiveness for self-study could be substantially enhanced by including assignments or suggested items for students to look at.
The velocity of the center-of-mass is not displayed in the lab frame, and the velocity of the right barrier is not displayed in any of the other frames while running under Netscape 4.75. Each is displayed under IE 5.5.