Hello Professor Hwang I have been following these pages since its publication in january 2004, just happened to register today. I wonder how different will it be if we consider that the two colliding bodies are two circular discs. You might have heard of a board game called carrom , its very popular in India. Do take a look at it. Now we have tried to emulate the example you have put up here and applied it to the emulation of carrom. There are a few rough edges, but still the difference is not very clear. What changes will be necessary to make it work realistically? Regards Sujit Kane