Sorry about my english , i'm still learning. I did it by energy, but i want to solve by integration QUESTION : A charged and massive particle runs with 66 m/s, 3^10-6 coulomb and 6^10-3 Kg towards a fixed particle with 4,5^10-6 coulomb, separated by 4,3 meters . What's the distance between them which the initial velocity is zero ? ANSWER : 0,00761 meters . I started that way : 1) a = F/m ; F= kQq/r² ; 2) v²=vo² - 2.a.d; and i substituted the acceleration gives by 1 in equation 2 . So i isolated d and integrated 4,2 and (4,2-d) interval. It results in a cubic equation which doesn't make sense .