You have a lightweight spring whose unstretched length is 4.0cm .
You’re curious to see if you can use this spring to measure charge.
First, you attach one end of the spring to the ceiling and hang a
3.0g mass from it. This stretches the spring to a length of 5.5cm .
You then attach two small plastic beads to the opposite ends of the
spring, lay the spring on a frictionless table, and give each
plastic bead the same charge. This stretches the spring to a length
of 4.5cm .

What is the magnitude of the charge (in nC ) on each bead?