1. The problem statement, all variables and given/known data Moderating a Neutron 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. An electron (M = 5.49*10-4 u). A proton (M = 1.007 u). The nucleus of a lead atom (M = 207.2 u). 2. Relevant equations Pf=Pi m1u1 = m1v1,f2 + m2v2,f2 Kf=Ki (1/2)m1u2 = (1/2)m1v1,f2 + (1/2)m2v2,f2 3. The attempt at a solution Honestly, I do not even know how to begin this problem set due to lacking an initial velocity. I have been searching for a few hours of how to properly derive an equation and have not come up with anything ground breaking. Any tips are greatly appreciated, I wish I had more information to give.