1. The problem statement, all variables and given/known data So a kaon moving at some speed in the +x direction spontaneously decays into one pion and one anti-pion. The anti-pion moves away with velocity of 0.8c, and the pion moves away with velocity of 0.9c. Mass of kaon = 498 MeV/c^2 Mass of pion/anti-pion = 140 MeV/c^2 2. Relevant equations I understand that momentum of the kaon qualms the momentums of the two pions. p(kaon) = p(pion) + p(anti-pion). I can then square both sides and use the principle of invariance. 3. The attempt at a solution What I'm having issues with is how to calculate the term of +2 p(pion)•p(anti-pion). I don't understand how to multiply the vector parts. p(pion) = ( E(pion)/c , vector p(pion) ) p(anti-pion) = ( E(anti-pion)/c , vector p(anti-pion) ) I'll get a cos(theta) term out of this dot product on the vector side but how do I use the velocities I have to get a dot product of those two momentum vectors? They're both moving so I am guessing I have to substitute in (gamma)(mass of the particle)(velocity vector) but I just don't understand how to do the math after that. Help please!