1. The problem statement, all variables and given/known data http://s359.photobucket.com/albums/oo40/jsmith613/?action=view¤t=MomentumConservation.png 2. Relevant equations 3. The attempt at a solution Working out how momentum is consered in this situation,... I know that the mass of an alpha particle is 4*mass of proton....however, I am still slighly concerned because the alpha particles move in opposite vertical directions hence their momenta cancel...how is momentum conserved here (I know that horizontal momentum is conserved but how is vertical momentum conserved)??