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 Lτ=−−. 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, sincan be replaced by in radians and the torque is
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This note was uploaded on 01/25/2011 for the course PHYSICS 17029 taught by Professor Rebello,nobels during the Fall '10 term at Kansas State University.