I'm having a problem with one of my physics problems. A plastic ball of radius 5 cm is hanging from a string 2m away from a charged glass rod. The ball has been charged to -5.13 microCoulombs. If the electric force on the ball is 0.6672 milliNewtons in the horizontal direction, what is the magnitude of the electric field at the balls position? I've worked at it by using E = F/Qo, but it just doesnt seem to be working out ><. I'm not looking for an answer, i'm looking to see if i am going about this problem in the right way.