1. The problem statement, all variables and given/known data A uranium atom of mass 238 u decays by emitting an alpha particle (the nucleus of a helium atom) of mass 4 u at a speed of 2 x 107 m/s. (Note that "u" is the symbol for atomic mass units and 1 u = 1.67 x 10-27 kg.) What is the recoil speed of the resulting nucleus immediately after the decay? 2. Relevant equations m1v1i + m2v2i = m1v1f + m2v2f 3. The attempt at a solution The first thing I did was realize that the total momentum before the uranium emittes the alpha particle is zero so the momentum after the alpha particle is released is zero. So m1v1 + m2v2 = 0. I plugged in the masses of the two particles and the speed of the smaller particle and solved for the velocity of the larger particle to get the recoil speed.