1. The problem statement, all variables and given/known data A proton strikes a stationary alpha particle (4He nucleus) head-on. Assuming the collision is completely elastic, what fraction of the proton’s kinetic energy is transferred to the alpha particle? 2. Relevant equations Pi = Pf Ki = Kf 3. The attempt at a solution For Pi = Pf mv = m1v1 + 4m2v2 cancel the m's out v = v1 + 4v2 For Ki = Kf 1/2mv^2 = 1/2m1v1^2 + 1/2(4)m2v2^2 Cancel out the 1/2 and the m's v^2 = v1^2 + 2v2^2 Now I'm lost.. can't substitute it into the equation, I feel like I got nowhere. Help please.