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Two small plastic balls hang from threads of negligible mass. Each ball has a mass of 0.110g and a charge of magnitude q. The balls are attracted to each other, and the threads attached to the balls make an angle of 20.0 deg with the vertical, as shown in the figure

How do I compute the magnitude of electric force acting on each ball?

I tried using coulomb's law: $F = k \frac{Q_1\cdot Q_2}{r^2} = 2.248 \times 10^{13} q^2$ but seems like the answer is wrong. The hint was that the answer did not depend on $q$ I guess I have to use other info? But how?

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closed as off-topic by jinawee, centralcharge, tpg2114, John Rennie, Brandon Enright Feb 21 '14 at 18:08

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up vote 1 down vote accepted

I think you may have to include the weight ($mg$) of the ball using the force vector along Y-axis ($\vec{F_y}$) along with the force vector along X-axis (which is the electrostatic attraction, which here is $\vec{F_x}$). The magnitude of the resultant of the two forces may end up with the answer.

There should be a reason why they've given mass and inclined angle. Both are used for projecting the force along the respective axes. Dot product should help for projecting $F_n$ as $Fcos \theta$.

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Try drawing a free-body diagram of each charge. Since the balls are stationary, the net vector sum of forces must be 0. Then the answer you are looking for is the force that balances out the electric force.

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