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Homework Help: Angular Momentum of Glass on a String

  1. Oct 20, 2016 #1
    Hi Folks,

    Problem Statement

    How would one use the conservation of angular momentum to explain the attached picture?
    The rod is held fixed horizontally..the person holds on to the cork and then lets go...apparently the glass is saved due to this conservation...

    Relevant Equations

    Momentum P=m*v

    Attempted Answer

    Letting go of the cork..I suspect the cork to fall under gravity while the glass is pulling it horizontally due to gravity.
    The cork will then spin around the horizontal bar.
    Not sure how one would use conservation of momentum to describe what is happening quantitatively?


    Attached Files:

  2. jcsd
  3. Oct 20, 2016 #2
    If the glass was held stationary, what would the motion of the cork be when it was released? I would expect it to swing downward in a circular motion, then back upward on the opposite side, almost to the height from where it was initially released. (Friction and air resistance would prevent it from reaching the original height.)

    So why would you expect it to wrap around the rod in the case where the glass was released? Would it? If so, what would cause that to happen? It would have to reach a height higher than its original height. What could cause that - if it did indeed happen?
  4. Oct 20, 2016 #3


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    In the system of glass, cork, and string, what unknown external force is there? How can you take moments in such a way that a given external force is irrelevant?
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