1. The problem statement, all variables and given/known data A ball of mass 2m is moving with a velocity Vo collides elastically and head-on with another ball of mass m at rest. Find the minimum kinetic energy of the system. 2. Relevant equations K = 1/2 mv2 3. The attempt at a solution I personally do not understand what is expected of me here. The answer expected is (2mVo2)/3. But collision is elastic. And I have to find the Kinetic energy of the system. Won't the Kinetic energy of the system remain constant (mVo2) throughout?