1. The problem statement, all variables and given/known data A car of mass 600kg traveling at 0.8c hits another, stationary car of 800kg. The two cars stick together. What is the mass and speed of the coupled cars? 2. Relevant equations 3. The attempt at a solution I found the momentum of the first car as p=gamma*mu = (600kg*0.8c)/sqrroot(1-0.8^2) = 2.4*10^11 kg.m/s The momentum of car 2 is zero. Then according to p=mu, i thought p(inital)=p(final)=m(final)*u(final) This would mean i would find the final velocity, u = p(initial)/m(final) = 2.4*10^11/(800+600) However i don't think this is right? If anyone could help me it would be great, thanks.