Summary: From 1987 to the present, I have been involved with Al Barr's
research group's physically-based modeling projects. These generally
involved physics-based motion for 3-d computer animation. This
requires a modeling system that can handle heterogeneous collections
of objects and constraints between them, which can then be
automatically collected into a form where the continuous equations of
motion can be integrated.

Some of the specific modeling tasks I've been involved with include:

* rigid body dynamics
* dynamic constraints on rigid bodies
* flexible bodies using Lagrange multipliers
* flexible bodies using finite elements
* collision detection between rigid bodies
* contact mechanics between rigid bodies
* articulated bodies
* combining contiuous motions with discrete events such as collisions
* space-time constraints

The system we've developed, in which most of this was implemented, is
based on the "Structured Modeling" approach described by Ronen Barzel
in his 1992 book _Physically-based Modeling for Computer Graphics: a
Structured Approach_. This system allows one to desribe physical
objects in a natural, conceptual way, and have the system construct an
appropriate state vector for the system of objects, which can then be
passed to a numerical solver. By separating the conceptual level from
the solver, the system can be made robust and extensible by reusing
the proven mathematical and numerical foundations for different
simulations.