Differential Geometry of Rigid Bodies Collisions and Non-standard Billiards

Abstract: The configuration manifold $M$ of a mechanical system consisting of two unconstrained rigid bodies in $\mathbb{R}^n$, $n\geq 1$, is a manifold with boundary (typically with singularities.) A complete description of the system requires boundary conditions that specify how orbits should be continued after collisions. A boundary condition is the assignment of a collision map at each tangent space on the boundary of $M$ that gives the post-collision state of the system as a function of the pre-collision state. Our main result is a complete description of the space of linear collision maps satisfying energy and (linear and angular) momentum conservation, time reversibility, and the natural requirement that impulse forces only act at the point of contact of the colliding bodies. These assumptions can be stated in geometric language by making explicit a family of vector subbundles of the tangent bundle to the boundary of $M$: the diagonal, non-slipping, and impulse subbundles. Collision maps at a boundary configuration are shown to be the isometric involutions that restrict to the identity on the non-slipping subspace. The space of such maps is naturally identified with the union of Grassmannians of $k$-dimensional subspaces of $\mathbb{R}^{n-1}$, $0\leq k\leq n-1$, each subspace specifying the directions of contact roughness. We then consider non-standard billiard systems, defined by fixing the position of one of the bodies and allowing boundary conditions different from specular reflection. We also make a few observations of a dynamical nature for simple examples of non-standard billiards and provide a sufficient condition for the billiard map on the space of boundary states to preserve the canonical (Liouville) measure on constant energy hypersurfaces.