Put a charge exactly halfway between two fixed positive charges and it sits still — but only just. Nudge it sideways and it comes back, over and over, in simple harmonic motion. Nudge it along the line joining them and it runs away instead. Same starting point, opposite outcomes, and this lets you try both.
Choose whether the free charge is positive or negative. Pick a motion mode: perpendicular locks it to sideways motion, axial locks it to the line between the fixed charges, and Free lets it go anywhere. Then drag the charge somewhere and let go. Reset returns it to the centre and clears the trail.
Two fixed positive charges sit either side, marked Q. The panel shows the free charge's position, the two components of the force on it, and a line telling you what kind of motion you have set up — oscillating, running away, or being pulled in.
The two fixed charges are always positive and cannot be moved or changed. The numbers here are model values with no units — the shape of the motion is the point, not the size of the force.
You should be able to say why the same position is stable one way and unstable the other way.