ABSTRACT
Most of this book has been dedicated to describing the dynamics of systemsthat are fundamentally unconstrained. By that, we mean that a particle, or rigid body is free to move in any direction, subject only to internal and external
forces acting on it. However, there have been some exceptions. Particles cannot
penetrate the walls of objects or fall through a floor-so they are constrained to
move only in the region free of solid boundaries. The same is true for rigid bodies-
they can tumble and fall freely, but cannot penetrate another solid. These are
examples of problems that are usually solved as if there were no constraints, and
then the constraints are handled as special cases, such as in the collision detection
and response modules of a particle or rigid body simulation.
