1. The problem statement, all variables and given/known data Two nuclei make a head-on elastic collision. One nucleus (mass m) is initially stationary. The other nucleus has an initial velocity (v) and a final velocity of (-v/5). What is the mass of this nucleus? 2. Relevant equations conservation of momentum (m1v1+m2v2=m1v1'+m2v2') conservation of energy (1/2m1v1^2 + 1/2m2v2^2 = 1/2m1v1'^2 + 1/2m2v2'^2) 3. The attempt at a solution I've substituted in the knowns and came up with (m2v = mv1' + m2*-v/5) I isolated for v1' though I'm not really sure why and came up with v1' = (6m2v1)/5m This is probably totally wrong and I'm not sure why I even did it. The examples given to me for elastic collision all involve a bunch of substitutions and isolated for variables without any explanation. This question has too many variables for me to keep it straight in my head. I'd love a thorough explanation if anyone has the time. Thanks.