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Homework Statement
Object A has mass 90.0 g and hangs from an insulated thread. When object B, which has a charge of +130 nC, is held nearby, A is attracted to it. In equilibrium, A hangs at an angle θ = 7.20° with respect to the vertical and is 5.00 cm to the left of B.(a) What is the charge on A?
(b) What is the tension in the thread?
http://i694.photobucket.com/albums/vv301/redraven45/physics1_zps6759b55e.png
Homework Equations
F = ma
F = kq1q2/r^2
The Attempt at a Solution
Im pretty clueless on this one. I know it's some type of equilibrium question, and it looks like the forces on the x direction cancel out. To find the charge on A, I need the force between the two charges but I don't have that and I'm not sure if I'm doing this right.
The only force on the string in the y direction is from gravity so
Fy = 0.09*9.8
=0.882N
The only force pulling the string in the x direction is the force between charge A and charge B
tantheta = Fx/Fy
Fx = tantheta*Fy
= tan7.2*0.882
= 1.15N
This equals to the force between the two charges, so
1.15 = k(qA)(qB)/(r^2)
(qA) = 1.15 *0.05^2/ (8.85e-12)(130e-9)
(qA) = 2.49e15
this value doesn't make much sense. not sure what to do.
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