# 2 suspended balls with charge q. find q

1. Sep 18, 2009

### Kastner86

1. The problem statement, all variables and given/known data
Two Styrofoam balls with the same mass m = 7E-8 kg and the same positive charge q are suspended from the same point by insulating threads of length L = 0.92 m. The separation of the balls is d = 0.024 m. What is the charge q?

I am unable to add a graphic at this time. But i understand this is a common electrophysics problem, i think the problem describes it pretty well.

2. Relevant equations

F = (k*q1*q2)/r^2

3. The attempt at a solution

One thing I will do first is find the value "h", the vertical distance between 1/2d and the ceiling (essentially completing the triangle)

Well I wasn't exactly sure where to start, and that may be part of the problem, so here is the thought process, I hope you can follow.

I know Fnet = 0 Fy = 0 Fx = 0

I know to get q i need to find F from each charge.

(note: FTy represents the Tension Force from the string in the y direction.)

So Fy = -mg + FTy = 0 so mg = FTy

How do find FTy? I understand it is a component of FT. would it be FTy = FT x (h/L) ??

So Fy = -mg + FT x (h/L) and from this i could get FT???? is that correct?

If this is correct, I can also find FTx.

FTx = FT x (1/2d)/L

And Fx should = (FT x (1/2d)/L) - (k*q1*q2)/d^2

Since Fx = 0 i can set the above equation to 0 and solve for q??

This is what I tried to do anyways, does this make sense?

Any input is appreciated!

2. Sep 18, 2009