Physically based modeling is increasingly gaining acceptance within the computer graphics and mechanical engineering industries as a way of achiev ing realistic animations and accurate simulations of complex systems. Such complex systems are usually hard to animate using scripts, and difficult to analyze using conventional mechanics theory, which makes them perfect candidates for physically based modeling and simulation techniques. The field of physically based modeling is broad. It includes everything from modeling a ball rolling on the floor, to a car engine working, to a hang ing shirt...