not sure why i keep getting it wrong.............maybe caz i misunderstood some concept or something. anhyoo.......here is it: A cork ball of mass 5.20 g is placed between two very large horizontal plates. The bottom plate has a uniform charge density of +0.22E-06 C/m2, whereas the upper plate has a uniform charge density of -0.06E-06 C/m2. The cork ball, which carries an unknown charge, is placed between the plates and is observed to float motionlessly. What is the charge on the ball? how i plan to solve it: since it give a mass, so i can find the force its acting downward due to gravity, which is "mg". n since the ball is not moving up or down, there must be a force equal in magnitude but opposite in direction pointing upward). thus the charge of the ball should be positive. i used E = (surface charge density) / permittivity of free space (8.85x10^-12) then use F = qE ----> q = F/E i ended up with 2.01E-16 C .............but its wrong. anyone know why?