by BuBbLeS01
 P: 605 1. The problem statement, all variables and given/known data Two 3.52 g point charges on 7.49-m-long threads repel each other after being equally charged. What is the charge q? (θ=31°.) 2. Relevant equations F = KQQ/R^2 3. The attempt at a solution How do I find Q without having a distance r???
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P: 40,257
 Quote by BuBbLeS01 How do I find Q without having a distance r???
You are given the thread length and the angle--use that to figure out the distance.
 P: 605 Oh yea...so the length from the center to one of the masses is 3.858 m. So I can do... F = K*Q*Q/r^2 F = K*Q^2/ r^2 An wouldn't F = 0 since they cancel each other out?
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Since the masses are in equilibrium, the net force on each is zero. What forces act on each mass?
 P: 605 Tension
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P: 40,257
 Quote by BuBbLeS01 Tension
That's one force. List them all. (And draw yourself a free body diagram showing how the force act on each mass.)
 P: 605 Tension and weight are the forces
 P: 605 So do I include the electric force? Like... T - W + Fel = 0
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P: 40,257
 Quote by BuBbLeS01 Tension and weight are the forces
Don't forget the electrostatic force!
 P: 605 Isn't that Fel?
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P: 40,257
 Quote by BuBbLeS01 So do I include the electric force? Like... T - W + Fel = 0
Yes, but realize that forces are vectors--direction counts. Set up two equations: One for the horizontal components, one for the vertical components. Combine these to solve for the charge.
 P: 605 X.) TSINθ + qE = 0 y.) TCOSθ - mg = 0 T = mg/COSθ mg/COSθ * SINθ + qE q = -mg*TANθ/E
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P: 40,257
 Quote by BuBbLeS01 X.) TSINθ + qE = 0 y.) TCOSθ - mg = 0
I'd write that first equation as:
Tsinθ - qE = 0 (since the force components are in opposite directions)

Realize that E is also a function of q, so rewrite that in terms of k, q, and r (which you figured out).

Otherwise, you are on the right track.
 P: 605 Okay so I end up with... -q = -(mg*tanθ) / (kq/r^2) q = (mg*tanθ) / (kq/r^2) and the q's cancel? = (mg*tanθ) / (k/r^2)
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P: 40,257
 Quote by BuBbLeS01 Okay so I end up with... -q = -(mg*tanθ) / (kq/r^2) q = (mg*tanθ) / (kq/r^2) and the q's cancel?
The q's don't cancel. And when you divide by a fraction, simplify the result. (Invert and multiply.)

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