1. The problem statement, all variables and given/known data The problem states that two protons are aimed at each other with a certain velocity v. It asks what is the maximum electrical force they will exert on each other. The velocity is given at 1000km/s. And that is everything given. 2. Relevant equations KE = 1/2mv^2 F = q*q/(4*pi*e*r^2) 3. The attempt at a solution I am stuck on the approach. I have managed to find the kinetic energy of the protons but that is it. I don't know where to go from here . I think I should use Coulomb's Law, but I need to find the radius for that. Any suggestions?