A charged particle q is shot towards another charged particle Q which is fixed, with a speed v . It approaches Q upto a closest distance r and then returns. If q were given a speed 2v, the closest distances of approach would be

Charge q will momentarily come to rest at a distance r from charge Q when all its kinetic energy is converted to potential energy i.e.Therefore the distance of closest approach is given byHence if v is doubled, r becomes one fourth.