I was thinking about this problem today: You have two nearly identical charged particles, except that one has a charge of +1 C and the other has a charge of -1 C. If you place them at rest one meter apart, how long will it take for them to collide, and what will be their velocities when they do collide? Using Coulomb's Law, we get F=k/(r^2)=ma. The problem is that r is changing, as well as the acceleration. So, I know there also has to be a jerk term that appears in there somewhere. Other than that, I'm not sure how to approach this problem.