Two metal spheres, each of mass 20 g and initially at rest, are dropped from a height of 10 m in an evacuated chamber. One sphere has a charge of +90 ?C, and the other has a charge -90 ?C. Find the difference in final speeds of the two spheres (the speeds just before each one hits the ground). Assume that there is a downward electric field of about 150 V/m near the Earth's surface.