91. (a) Suppose the pendulum is at the angle θwith the vertical. The force diagram is shown below. GTis the tension in the thread, mgis the magnitude of the force of gravity, and qEis the magnitude of the electric force. The field points upward and the charge is positive, so the force is upward. Taking the angle shown to be positive, then the torque on the sphere about the point where the thread is attached to the upper plate is ()sinmgqE Lτθ= −−. If mg> qEthen the torque is a restoring torque; it tends to pull the pendulum back to its equilibrium position. If the amplitude of the oscillation is small, sinθcan be replaced by θin radians and the torque is
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