1. The problem statement, all variables and given/known data 2. Relevant equations 3. The attempt at a solution I don't know how to make the equations here. When the balls are penetrating each other, a cavity is being formed due to overlap of the negative and positive charges. The size of this cavity keeps on increasing. The trouble is I cannot figure out the instant of maximum velocity and the equations to write down. I need a few hints to begin with. Any help is appreciated. Thanks!