http://imgur.com/a/wEUgn question #70 in the attached image.
Three charged spheres are at rest in a plane as shown in the figure. Spheres A and B are fixed, but sphere C is attached to the ceiling by a thread. The tension in the string is .240 N. Spheres A and B have charge Qa = 28 x 10^-9 and Qb = -28 x 10^-9.
What charge is carried by sphere C?
Fe = (kQq)/r^2
The Attempt at a Solution
This is a pretty hard question so please work with me guys.
I decided to start by listing all of the forces on sphere C
You have a tension force with N = .240, with the horizontal component being .240sin(30deg) left and vertical component being .240cos(30deg) up.
Since everything is at rest that means the horizontal and vertical tension forces would be canceled out right?
That would mean you have a force with magnitude .240sin(30deg) right and .240cos(30deg) down.
But that makes no sense. Because C has to be either + or -, which means it can only be either
1) .240sin(30deg) right and .240cos(30deg) up
2) .240sin(30deg) left and .240cos(30deg) down
I'm guessing I just don't understand the concept somewhere... can anyone please help me out?