Understanding Angular Momentum and Its Effects on Spinning Objects

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Discussion Overview

The discussion centers on the concept of angular momentum, particularly in relation to spinning objects and the behavior of pieces that break off from them. Participants explore the conditions under which these pieces maintain their rotational motion and the implications for angular momentum conservation. The conversation includes theoretical considerations and hypothetical scenarios.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants question whether the statement about pieces breaking off from a spinning object retaining the same spin direction is accurate without additional context, particularly regarding the forces involved at the moment of separation.
  • Others propose that if the pieces were already rotating with the object before separation, they would continue to do so after breaking off, assuming no external forces act on them.
  • A participant introduces a hypothetical scenario involving a ball on a turntable to illustrate the complexities of angular momentum conservation, raising questions about the role of internal torque during separation.
  • One participant provides a mathematical explanation of angular momentum conservation, suggesting that a decrease in moment of inertia due to a piece breaking off would lead to an increase in angular velocity to maintain constant angular momentum.

Areas of Agreement / Disagreement

Participants express differing views on the clarity and accuracy of the initial statement regarding angular momentum and the behavior of spinning pieces. There is no consensus on the conditions necessary for the pieces to retain their spin direction after separation, and the discussion remains unresolved.

Contextual Notes

Participants highlight the importance of understanding the forces at play during the separation of spinning objects and the implications for angular momentum conservation. There are unresolved questions regarding the nature of torque and the conditions under which angular momentum is conserved.

Dreamdweller
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I'm trying to find out if this is angular momentum or something else. It says this, "If an object spins in a certain direction any pieces that break off that object must also spin in the same direction."

Is that angular momentum or something else?
 
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That statement is unclear to me. Can you provide more context?. A piece breaking off has no reason to spin unless there was some shearing force (or torque about its center of mass) at the time of separation.
 
brainpushups said:
That statement is unclear to me. Can you provide more context?. A piece breaking off has no reason to spin unless there was some shearing force (or torque about its center of mass) at the time of separation.
All the pieces of the rotating whole were themselves rotating prior to separation. They have no reason to stop spinning just because they are no longer attached to anything.
 
I'm not sure if this is a good explanation or not but I guess imagine a ball. Take 2 more balls and attach them to opposite sides of the first ball and set that first ball spinning. Have the 2 balls break off from the first. Will those 2 balls spin in the same direction that the first ball was spinning?
 
Dreamdweller said:
I'm not sure if this is a good explanation or not but I guess imagine a ball. Take 2 more balls and attach them to opposite sides of the first ball and set that first ball spinning. Have the 2 balls break off from the first. Will those 2 balls spin in the same direction that the first ball was spinning?
Prior to those two balls breaking off, do you agree that they were already spinning in the same direction as the first ball?
 
To me, the sentence seems to be correct and clear.
The explanation might be so -
If an object A, say, is spinning clockwise then it has some angular momentum about the axis of rotation. It will be given by,
L= Iw, where I is moment of inertia and w is the angular velocity.
If a piece breaks off of the object A then I decreases, and since there is no external torque or force acting on it, angular momentum will be conserved. Hence, if I goes down, w has to increase to keep L constant.
Talking about the particle which breaks off, no external torque is still applied, so it will have a spin in the same direction viz. clockwise, as that of object A.
I hope you understood :-)
 
I guess I never really thought about a question like this before in relation to the rotation of the object flying off. So suppose a ball is attached to a rotating turntable by a string. The string is cut and the ball flies off at a tangent. The translation of the ball accounts for part of its initial angular momentum about the center of the turntable. Are you saying that the ball must also rotate about its center to account for the fact that angular momentum is conserved of does the turntable just reduce its angular velocity accordingly? If the ball rotates about its center what provides the (internal) torque at the separation?
 

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