Hi All, I am working on the problem of bead on wire and got stuck on some basic derivation detail. I took the same approach as Andrew Witkin used in his slide (page 13): http://www.cs.cmu.edu/~baraff/sigcourse/slidesf.pdf Here is the screenshot of the slide page 13: http://desmond.imageshack.us/Himg831/scaled.php?server=831&filename=pocs.png&res=medium [Broken] As we can see, C is the function of x and r is constant. x is a function of t as well. I got stuck on deriving the third line of the equation (the derivative of N with respect to t). My derivation result seems different from what the author proposed. Here is my step by step derivation: http://img269.imageshack.us/img269/8623/derivative.png [Broken] Could somebody point out my mistake? Any help will be greatly appreciated. Regards, John.