In a nuclear reactor, neutrons released by nuclear fission must be slowed down before they can trigger additional reactions in other nuclei. To see what sort of material is most effective in slowing (or moderating) a neutron, calculate the ratio of a neutron's final kinetic energy to its initial kinetic energy, Kf / Ki for a head-on, elastic collision with each of the following stationary target particles. (Note: The mass of a neutron is m = 1.009 u, where the atomic mass unit, u, is defined as follows: 1 u = 1.66 10-27 kg.) (a) An electron (M = 5.49 10-4 u). I'm not quite sure how to tackle this problem. I know that kinetic energy is 1/2 mv^2, but I don't know how I can calculate the ratio of the final and kinetic energies without an initial velocity of the neutron.