1. The problem statement, all variables and given/known data A ball moving translationally collides elastically with another, stationary, ball of the same mass. At the moment of impact the angle between the straight line passing through the centres of the balls and the direction of the initial motion of the striking ball is equal to α = 45° Assuming the balls to be smooth, find the fraction of the kinetic energy of the striking ball that turned into potential energy at the moment of the maximum deformation. 2. Relevant equations Conservation of momentum Energy Conservation Principle 3. The attempt at a solution I don't understand what will happen at the time of max deformation. I mean to say that what will be the velocities of the particle at that instant. I can apply conservation of momentum but that will lead me to nowhere. Also applying conservation of energy I can know what will be the final velocities but not the velocities at the time of max deformation. So what should I use?