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Angular Velocity: spinning in chair, dropping weights with arms out

  1. Feb 22, 2014 #1
    Would someone be willing to explain this? Here: https://www.physicsforums.com/showthread.php?t=487058

    I read the entire thread and don't understand. I need one person to explain it step by step all in one place. In my case it's not a homework question. Please don't use the Socratic method! Just explain in plain English. Thank you!

    (As an aside, I've never been able to learn from the Socratic method. It confuses me. With the Socratic method, I always remember the incorrect theories and never the correct ones. This question is a real doozy: http://forums.studentdoctor.net/threads/moment-of-inertia-problem.809233/ http://forums.studentdoctor.net/threads/tbr-physics-ch-4-passage-vii-45.975274/)
  2. jcsd
  3. Feb 23, 2014 #2


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    Gold Member

    It might help for you to explain what specific part of the the thread or statement within is bothering you.
    Otherwise it could become an exercise of futility, discussing that which you already do understand, and then not discussing that which you do not. That would not be much help to you.

    The other two links, by the way, seem to be an execise in how to promote confusion.
  4. Feb 23, 2014 #3
    I have the same question as the OP. I quote, "can someone explain to me why "moment of inertia would not change for the system when the student drops the weight" since Inertia is proportional to mass x r^2 wouldn't a decrease in mass after the weight drop decrease the moment of inertia?"
  5. Feb 23, 2014 #4


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    Science Advisor

    Angular momentum is conserved if you are considering a closed system with no external torques. If you have 5 kg weights first considered as being inside the system and then considered as being outside, it's obviously no longer a closed system. Angular momentum need not be conserved. Removing those weights obviously results in a decrease in angular momentum.

    So don't analyze the non-closed system consisting of the student plus weights. Analyze the closed system consisting of the student alone. When the student opens his or her hands, releasing the weights, does this involve an external torque on the student? Nope. So the angular momentum of the student alone does not change as a result. When the student opens his or her hands does this involve a change in moment of inertia of the student alone? Nope. So the angular velocity of the student alone does not change either.
  6. Feb 23, 2014 #5
    Yes, the moment of inertia changes. But the angular velocity doesn't. The weights take some angular momentum with them, so you can't assume that angular momentum is conserved.
  7. May 25, 2014 #6
    Read below.
    Last edited: May 25, 2014
  8. May 25, 2014 #7
    How do the released weights take any of the angular momentum?? The instant they are dropped, they begin travelling tangentially to the axis of rotation (.I.e. zero angular velocity, all translational velocity). Considering this, the spinning guy with a now reduce moment of inertia must speed up since the angular momentum of the system has stayed constant but its now "concentrated" on just the guy, not the weights. Where am I going wrong??
  9. May 25, 2014 #8


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    the weights do take some angular momentum with them. Yes, they are moving tangentially to the axis of rotation, but this is not zero angular velocity. For example, if a car goes past me in a straight line, it will have angular momentum with respect to me.
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