Halliday, Resnick & Walker — Electric Potential: Problem 13

13 A particle of charge \(q\) is fixed at point \(P\), and a second particle of mass \(m\) and the same charge \(q\) is initially held a distance \(r_1\) from \(P\). The second particle is then released. Determine its momentum magnitude when it is a distance \(r_2\) from \(P\). Let \(q = 3.1 \mu\text{C}\), \(m = 20 \text{ mg}\), \(r_1 = 0.90 \text{ mm}\), and \(r_2 = 1.5 \text{ mm}\).