[SOLVED] Having problem in crossproduct, dotproduct. Sorry, i have a lil problem in solving for some math in physics equation. I'm doing simulation of rigid body collision reaction, and i applied the angular momentum, which is L = p x r = I ( w2 - w1); where p = mv is the impact of the collision, w1,w2 is the angular velocity. r be the vector from the center of body to the collision point. I'm wondering if the eq is p x r, in a 2D system, the third coordinate system is ignored. and in some circumstances, i let the equation to be reformed into something like p * n x r, where n is the reflect/normal vector at the collision point. since p x r is actually the cross-product, but some books are telling me to take dotproduct of it, i'm wondering and hoping to seek for help. Thanks in advance.