MHB Analyzing Electromagnetic Forces in a Two-Sphere System with Insulating Fiber

[Drain Brain]

the two small plastic spheres are arranged so that they can slide freely along the insulating fiber. Each has a mass of 0.1g. The spheres are given identical positive charges. a. If the lower sphere is resting against the lower knot while the upper sphere moves to a location 6 cm distant from that knot, find the charge on each. b. if the charges remain the same, what angle should the fiber make with the horizontal to double the separation?

can you help on this problem. I'm having a hard time analyzing it. There's no given force(repulsive) I can't seem to use coulombs law here. please help!

 

[jacobi1]

Each of the plastic spheres repels the other, since they are both positively charged-this is why one of the spheres moves 6 cm up the fiber. Therefore, the third-law pair here is electric force with electric force, and you can use Coulomb's Law.
We also need another force, one that keeps the higher bead from moving away further than 6 cm. So, what do you think you should do now?