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Homework Help: Pendulum and spinning mass -- Coupled

  1. Dec 10, 2014 #1
    1. The problem statement, all variables and given/known data
    Imagine two masses connected to each other by a string. One mass sits atop a table, while the other mass dangles below the table, as the string is fed through a hole in the center. The configuration looks like this: http://tuhsphysics.ttsd.k12.or.us/Tutorial/NewIBPS/PS5_3/5-32.JPG. Now push the swinging pass so that it start oscillating as a pendulum, and simultaneously push the top mass so that it starts moving in a circular motion. Assume the length of the pendulum arm is constant. Write the lagrangian and find the equations of motion.

    2. Relevant equations

    3. The attempt at a solution

    My confusion is this: If we assume the length of the pendulum arm to be constant, it seems that the two masses are no longer coupled. Thus the problem immediately reduces to a simple pendulum and a mass spinning with constant angular velocity. This doesn't seem correct. How am I supposed to interpret this problem so that the string never slides anywhere, yet the two masses are still coupled in some way?
  2. jcsd
  3. Dec 10, 2014 #2


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    You have nailed it.
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