1. The problem statement, all variables and given/known data What is the force on an infinite conducting plane due to two point charges: a +2q located at a height 4h above the plane and a -q charge located at a height 2h above the plane, directly beneath the other point charge. 2. Relevant equations 3. The attempt at a solution I know that due to Newton's 3rd Law, I can just calculate the force on a single charge (due to an image charge symmetrically below the plane) and the negative of this force is the force on the plane. Does this method extend to 2 charges? Then I would think that I place a -2q charge at -4h, and a +q charge at -2h, and then calculate the two forces on the real charges due to the 3 other charges present... then the opposite of that force is the force ON the plane? Can you add the forces like that?