Conservation of Angular Momentum is Dumbfounding

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Discussion Overview

The discussion revolves around the concept of conservation of angular momentum, particularly in scenarios involving internal forces and systems at rest. Participants explore examples that challenge their understanding of how angular momentum behaves in isolated systems, comparing it to linear momentum and energy conservation.

Discussion Character

  • Exploratory
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • One participant presents a scenario with a stick and marbles, questioning how angular momentum can be non-zero when no external forces are applied.
  • Another participant corrects the misunderstanding regarding the angular momentum of the marbles, asserting they have a combined non-zero angular momentum when fired in the same direction.
  • A participant expresses realization of their error and explores the implications of a single marble's angular momentum when the origin of the coordinate system is at the marble's position.
  • Discussion includes the idea that the stick's angular momentum can be zero when considering both its rotation and linear motion, leading to a cancellation of angular momentum components.
  • One participant expresses confusion about how a collapsing gas cloud could lead to rotation, suggesting it seems counterintuitive.
  • Another participant argues that a collapsing gas cloud would not inherently start spinning, leading to a central star that does not rotate.
  • Further discussion raises questions about the initial conditions of gas clouds and their parameters that could lead to rotation.
  • One participant reflects on the implications of angular momentum in the universe, questioning how the solar system's angular momentum relates to the rest of the universe.

Areas of Agreement / Disagreement

Participants exhibit a mix of agreement and disagreement, particularly regarding the behavior of angular momentum in different scenarios. Some participants correct earlier claims while others express ongoing confusion about the implications of these concepts, indicating that the discussion remains unresolved in certain areas.

Contextual Notes

Participants acknowledge the complexity of angular momentum calculations and the dependence on initial conditions, but do not resolve the nuances of these discussions.

  • #31
russ_watters said:
This is not correct. Both marbles are being fired in the same angular direction, so they have a combined non-zero angular momentum

Could you clarify this. Aren't the marbles going in opposite directions: i.e. 180 degrees apart? Why are you saying they have the same angular direction?

Are you saying that the momentum of the marbles is not zero because their trajectories draw parallel lines and not a single line (if the trajectory lines are extended in both directions)? Sorry for my thick brain on this one.
 
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  • #32
Buckethead said:
Could you clarify this. Aren't the marbles going in opposite directions: i.e. 180 degrees apart? Why are you saying they have the same angular direction?
If the stick is vertical and you fire a marble to the right from the top end and to the left from the bottom end, both are being fired clockwise (albeit on tangents), pushing the stick counterclockwise.
 
  • #33
russ_watters said:
If the stick is vertical and you fire a marble to the right from the top end and to the left from the bottom end, both are being fired clockwise (albeit on tangents), pushing the stick counterclockwise.

I see. Interesting. So does this mean that if you have two marbles passing each other in space (parallel lines, not the same line) their total momentum is not zero?
 
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  • #34
Buckethead said:
I see. Interesting. So does this mean that if you have two marbles passing each other in space (parallel lines, not the same line) their total momentum is not zero?
Total linear momentum can be zero. If it is zero (e.g. if you use a center-of-momentum reference frame) then the total angular momentum is guaranteed to be non-zero.
 
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  • #35
jbriggs444 said:
Total linear momentum can be zero. If it is zero (e.g. if you use a center-of-momentum reference frame) then the total angular momentum is guaranteed to be non-zero.

Got it! Thank you. I can see this as well if a line is drawn between two passing masses, the angle of the line changes as the distance between the masses increases.
 
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  • #36
Regarding (a), "if I applied no external forces or torques to my system, and now the thing is spinning...", of the original post; sometime before T=1s, the experimenter compressed each of the springs with a required force, thus clearly, an external vector force (torque about the fulcrum) was indeed applied. Then at T=1s, the energy stored in the previously compressed springs were dissipated producing an equal and opposite torque about said fulcrum.
 
  • #37
Brendan Graham said:
sometime before T=1s, the experimenter compressed each of the springs with a required force, thus clearly, an external vector force (torque about the fulcrum) was indeed applied.
Nonsense. One can compress a spring by pushing on both ends equally for zero net torque and zero net linear momentum imparted. In any case, the initial conditions for the exercise are what they are and include zero total angular momentum.
 

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