No idea on how to do this problem

[dowz0r]

Q: 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 picture shows a normal rectangle. The upper left and lower left corners have a sphere with a charge of +3μC, and the lower right corner has a sphere with a charge of +3μC. The upper right corner is the blank one and point A is located at the lower left sphere. The upper horizontal side/line shows a length of 4d, and the right vertical side/line shows d. I know you have to use f = k(q1)(q2) / r^2 , but I don't know where to go from there.

Help is greatly appreciated!

 

[tiny-tim]

welcome to pf!

hi dowz0r! welcome to pf!

… 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.

if the net force is vertical, then you need the horizontal components of the forces to add to zero