A small glass bead has been charged to +19 nC. A metal ball bearing 1.0 cm above the bead feels a 0.030 N downward electric force. What is the charge on the ball bearing?

Answer 1

It makes no difference what the materials are. Use Coulomb's law. The force depends only upon the charge and the separation. Since the electrical force is attractive, the charge on the ball bearing is negative (opposite to the glass charge.)

Note that the gravity force has already been subtracted.