Five unknown charges are shown on the screen along with vectors representing the net forces on each. The length of the vector is proportional to the magnitude of the force. You can click-drag on any charge to change its position.
Which charges have like signs? How did you arrive at your answer?
Move charges a and b to the far left of the window and the other three charges to the far right. What can you say about the force on charge a as compared to the force on charge b (is Newton's third law obeyed)? Now move one of the other three charges closer to the pair on the left. Why are the forces on a and b no longer equal and opposite?