In an atomic physics experiment, two atoms collide with each other in such a way that the initial total momentum of the atoms is zero. The atoms are chosen so that M2 = 4.5 M1.
In the collision, the atom M1 has a final velocity, V1f = - 0.25 V1i. What is the ratio of the final kinetic energy to the initial kinetic energy of the system (the two atoms)? In other words, what is KEfinal / KEinitial in this collision?