1. The problem statement, all variables and given/known data A Helium 5 at rest decays into Helium 4 and a neutron. Mass of Helium 4 = 6.648 x 10^-27 Mass of Neutron = 1.67493 x 10^-27 Helium 4 has a momentum of 1.903 x10^-20 and Ek 2.723x10^-14 2. Relevant equations E=mc^2 v= square root of (E2/m) 3. The attempt at a solution I have found the mass defect for both Helium 4 and the neutron. and Helium 5. I think i can use E=mc^2, and then use E in v= square root of (E2/m). I am stuck in trying to figure out what I should use for m? The mass defect on the before or after the decay? Any help appreciated.