The Hamiltonian is composed of three parts : H _ \ text { h . p . }, representing the energy of the free heavy particles, H _ \ text { l . p . }, representing the energy of the free light particles, and a part giving the interaction H _ \ text { int . }.
42.
With respect to the center of mass both velocities are reversed by the collision : in the case of particles of different mass, a heavy particle moves slowly toward the center of mass, and bounces back with the same low speed, and a light particle moves fast toward the center of mass, and bounces back with the same high speed.
