Explanation for this interesting rotational effect?

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Discussion Overview

The discussion revolves around an observed rotational effect in a video, particularly focusing on the conservation of angular momentum in a non-isolated system. Participants explore the mechanics behind the motion of balls and the role of external interactions, such as with a surface or table.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation

Main Points Raised

  • One participant suggests that the un-twist of the elastic band and the rotation of the ball maintain the initial zero angular momentum, but expresses uncertainty about this interpretation.
  • Another participant questions the conservation of angular momentum in a non-isolated system, prompting further discussion on the topic.
  • There is a repeated inquiry regarding whether the reverse of the rotations is due to the interaction of the balls with the table, indicating a need for clarification on the mechanics involved.
  • A participant notes that for the system to be considered closed, the surface and its connection to the Earth must be included, as the balls exert torque on the surface.
  • One participant acknowledges the reasoning presented by another, indicating some level of agreement on the mechanics discussed.

Areas of Agreement / Disagreement

Participants express differing views on the conservation of angular momentum in non-isolated systems, and the discussion remains unresolved with multiple competing perspectives on the mechanics of the observed effect.

Contextual Notes

The discussion highlights assumptions regarding system isolation and the definitions of closed systems, as well as the dependence on external interactions that may not be fully explored.

andresB
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You can see the effect around minute 1:20 in this video. It seems to me that the un-twist of the elastic band and the rotation of the ball about the line that joins them is what keep the constant the initial zero angular momentum, though I can't tell for sure.

The inversion of in the direction of rotation somewhat reminds me of Dzhanibekov effect, though most likely not related at all.
 
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andresB said:
An effect that seemingly goes against the law of conservation of angular momentum.
Why should angular momentum be conserved in a non-isolated system?
 
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A.T. said:
Why should angular momentum be conserved in a non-isolated system?

Are you saying that the reverse of the rotations comes from the interaction of the balls with the table?
 
andresB said:
Are you saying that the reverse of the rotations comes from the interaction of the balls with the table?
Why just the reverse? How do you think the balls start moving in the first place, after being released?
 
A.T. said:
Why should angular momentum be conserved in a non-isolated system?
To clarify A. T.'s post, the balls exert a torque onto the surface, so to be a closed system, you'd have to include the surface and whatever the surface is attached to (like the earth).
 
A.T. said:
Why just the reverse? How do you think the balls start moving in the first place, after being released?
That makes sense.
 

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