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I Pendulum on a rotating arm

  1. Jan 11, 2017 #1

    There is a pendulum suspended on an arm that rotates a certain angle in the horizontal plane. The arm suddenly stops, how far will the mass of the pendulum be displaced in the horizontaly?

    Thought so far:

    The momentum of the arm will be L=IW and when it stops I think the mass at the end of the rope will have this amount of momentum. I know that momentum is conservative, is it valid to equate angular momentum to linear momentum. If so, knowing the momentum of the suspended mass how could I work out how this would translate to horizonal displacement?

    Is my thinking ok or am I off. Any help or advice would be much appreciated.
  2. jcsd
  3. Jan 11, 2017 #2


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    Yes and yes, in that order.
    If you don't tell us what L, I or W is your account is hard to follow. I suspect dimensional errors in your thinking:
    No, they have different dimensions.

    The problem statement is, anyway, incomplete: you do not mention any initial conditions at all.

    [edit] This question fits very well in the context of your other threads. My advice would be to try and re-write your post #1 into a complete example problem statement using the template:

    1. The problem statement, all variables and given/known data
    include initial conditions. These can be general (difficult) or very simple -- so simple this whole problem amounts to having initial conditions for a spherical pendulum and you can forget about the boom altogether
    2. Relevant equations

    3. The attempt at a solution

    and then work it out with the Euler-Lagrange formalism
    Last edited: Jan 11, 2017
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