An α-particle with a kinetic energy of 2.1 eV makes a head on collision with a hydrogen atom moving towards it with a kinetic energy of 8.4 eV. The collision

Pi=m.4.E−4mE=0For completely inelastic collision both come to rest after collision and net energy of 4E + E = 10.5eV is lost. But electron in ground state of H-atom can accept only an energy of 10.2 eV. Hence the collision may be inelastic but it can never be perfectly inelastic.