1. The problem statement, all variables and given/known data One hockey puck of mass 0.2 kg strikes an identical stationary puck on a frictionless ice rink. If the first puck had a velocity V0 = 3.8 m/s before the collision and V1 = 1 m/s after in the same direction, what fraction of the energy was lost? 2. Relevant equations M1V1 + M2V2 = M1V1' + M2V2' 3. The attempt at a solution So, I think I am supposed to use the above equation but I am not completely sure. Also, I am a bit confused as to the question "what fraction of energy was lost". Not sure what to do as my efforts in trying to use the above equation has failed. Any assistance or suggestions are greatly appreciated. Thanks.