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Problem 25.10: Find unknown charge on pendulum

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A charged ball with a mass of 30 grams is suspended from a string as shown. A second ball with a negative charge of 3 mC is placed on a wood table. Play the animation and move the ball on the table toward and away from the pendulum (position is given in meters and time is given in seconds). Restart.

  1. Find the equilibrium point of the pendulum for two different positions of the ball on the table.
  2. Write an expression for the change in potential energy (both gravitational and electrostatic) from one equilibrium position to the other.
  3. What is the charge on the pendulum? Assume that the charge on the two balls is uniformly distributed.
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