Conceptual Q: Angular Momentum

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Discussion Overview

The discussion centers on the concept of angular momentum, particularly its significance when acting along the z axis. Participants explore its relationship to translational momentum and the implications for rotating objects, using a dreidel as a specific example to illustrate their questions and thoughts.

Discussion Character

  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant states the formula for angular momentum as L = r x p and questions its significance when acting along the z axis, comparing it to translational momentum.
  • Another participant explains that angular momentum along the z axis indicates rotation about that axis, noting that the velocity of particles is perpendicular to the axis of rotation.
  • A participant expresses understanding of the explanation and mentions learning about precessional motion, suggesting it may clarify their understanding further.
  • A different participant reiterates the explanation of angular momentum along the z axis and poses two questions regarding the behavior of a spinning dreidel under different conditions (no gravity and slight angle under gravity).

Areas of Agreement / Disagreement

Participants generally agree on the definition and implications of angular momentum along the z axis, but there are unresolved questions regarding the behavior of the dreidel in specific scenarios, indicating a lack of consensus on those points.

Contextual Notes

The discussion includes assumptions about the conditions affecting the dreidel's motion, such as the absence of gravity and the angle of placement, which are not fully explored or resolved.

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L = r x p

What's the significance of angular momentum, L, acting along the z axis? Angular momentum is being compared to translational momentum, p = mv, but translational momentum is acting the same direction of the velocity, right?

Thanks for any help!

edit: this is a nice gif:

http://en.wikipedia.org/wiki/File:Torque_animation.gif
 
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Angular momentum along the z axis means that the object is rotating about the z axis. The velocity of the particles is perpendicular to the axis of rotation, hence L=rxp.
 
DaleSpam said:
Angular momentum along the z axis means that the object is rotating about the z axis. The velocity of the particles is perpendicular to the axis of rotation, hence L=rxp.

I think I understand what you're saying. Just hit a section about precessional motion. I think this will fill in the blanks when I get through it.

Closing my book for tonight I think~

Thank you very much for your reply
 
DaleSpam said:
Angular momentum along the z axis means that the object is rotating about the z axis. The velocity of the particles is perpendicular to the axis of rotation, hence L=rxp.

Alright, not sure I completely understand this concept yet. Could you answers these questions for me?

A dreidel is spinning away, no resistance from air or friction from surface (or anywhere else). It mas mass, angular speed, etc. Nothing special about it.

1) The dreidel is placed perfectly perpendicular on top of a horizontal surface. If there is no gravity, will the dreidel move in any way or eventually come to a stop? Of course not, right? The graphs of L acting on z were confusing me, though.

2) The dreidel is placed at a slight angle away from perpendicular on top of a horizontal surface, under gravity. Will the dreidel eventually come to a stop?
 

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