posted by Sarah .
In the rectangle in the drawing, a charge is to be placed at the empty corner to make the net force on the charge at corner A point along the vertical direction. What charge (magnitude and algebraic sign) must be placed at the empty corner?
(the rectangle has +3 uC charges in 3 corners and one empty corner. The long sides are 4d and the short sides are d. A is the bottom left corner. The empty corner is the top right corner)