1. The problem statement, all variables and given/known data The decay of a neutron into a proton, an electron, and a neutrino is an example of a three-particle decay process. Use the vector nature of momentum to show that if the neutron is initially at rest, the velocity vectors of the three must be coplanar (that is, all in the same plane). The result is not true for numbers greater than three. 2. Relevant equations 3. The attempt at a solution I knew that 0 would be equal to the net momentum of the three particles in all 3 directions, but don't really know where to go from here. I don't even know how to start this problem.