Ok here's the question: One particle has a mas of 3.00 x10^-3 kg and a charge of +8.00uC. A second particle has a mass of 6.00x10^-3 kg and the same charge. The two particles are initally held in place and then released. The particles fly apart, and when the separation between them is 0.100m, the speed of the 3.00x10^-3kg particle is 125m/s. Find the inital separation between the particles. Ok, I think I can start by using conservation of momentum to solve for V2. So m1V1 + m2V2= m1V1(final)+m2V2(Final) Since my inital velocities are 0, I'm left with the two final terms. I want V2, so V2=(-m1/m2)V1. Ok so V2=-62.5m/s. Now I'm stuck. Should I be using the conservation of energy next? Thanks for the input!