Realistic synthetic motion is required in applications ranging from commercial entertainment to surgical training. However, generating realistic motion for complex objects is a difficult task because of the large amount of data that must be specified and because humans are very good at detecting unnatural or implausible motions. O'Brien has explored one possible solution to this problem: using physically based methods to automatically generate motion for animated objects through the numerical simulation of their physical counterparts. In particular, he has developed a series of techniques for modeling the behavior of passive systems such as water, cloth, and breaking objects, as well as techniques for coupling multiple, heterogeneous systems together. In this talk, O'Brien will emphasize recent research on modeling fracture propagation in a dynamically restructured finite element mesh in order to animate objects that can crack or tear. Because his goal is realistic motion, he will also discuss evaluation techniques such as user testing and side by side comparison with high-speed video footage.