  We prove that to each initial datum in a set of positive measure in phase
space, there exist uncountably-many associated weak solutions of Newton's
equations of motion which govern the dynamics of two non-spherical sets with
real-analytic boundaries subject to the conservation of linear momentum,
angular momentum and kinetic energy. We prove this result by first exhibiting
non-uniqueness of classical solution to a constrained Monge-Amp\`ere equation
posed on Euclidean space, and then applying the deep existence theory of
Ballard for hard particle dynamics. In the final section of the article, we
discuss the relevance of this observation to the kinetic theory of hard
particle systems.
