How Does Angular Impulse Affect a Rotating Cylinder's Momentum?

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Discussion Overview

The discussion centers on the effects of angular impulse on the momentum of a rotating cylinder, specifically examining how an angular impulse added perpendicularly to the existing angular momentum alters the system's overall angular momentum. The conversation includes theoretical implications, mathematical representations, and conceptual clarifications regarding angular momentum and precession.

Discussion Character

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

Main Points Raised

  • One participant proposes that the resulting angular momentum after adding an angular impulse will have the same magnitude as the original angular momentum but will shift direction, with a specific angle derived from trigonometric principles.
  • Another participant asserts that the vectors of angular momentum add, resulting in a larger magnitude of angular momentum, and states that the cylinder will precess rather than simply follow the new direction of angular momentum.
  • A different participant mentions that if the angular momentum vectors are perpendicular, they will only change the direction of the final angular momentum, referencing the concept of precession and its relation to continuous torque.
  • One participant requests a mathematical expression for the final angular momentum in terms of the original angular momentum and the angular impulse, suggesting that the final angular momentum can be expressed as a vector sum.
  • Another participant provides a mathematical representation of the final angular momentum's magnitude, indicating the use of the Pythagorean theorem for perpendicular components.

Areas of Agreement / Disagreement

Participants express differing views on the effects of angular impulse on angular momentum, particularly regarding the nature of the resulting angular momentum and the concept of precession. There is no consensus on the implications of these effects or the conditions under which they apply.

Contextual Notes

Some participants reference the relationship between torque and angular momentum, indicating that the discussion may depend on specific definitions and assumptions about the system's conditions, such as the nature of the angular impulse and the continuity of its application.

24forChromium
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True or false.jpg

Hopefully the image is self-explanatory, if not:

A cylinder is rotating around its central axis with angular momentum L1; an angular impulse, ΔL is then added to the cylinder, perpendicularly with respect to L1.

The hypothetical result is: the cylinder has one angular mometum at the end, L-result. L-result is equivalent in magnitude to L1, and its direction is shifted counter-clockwise by a certain amount so that a line parallel to L1 draw from the tip of L-result will meet the tip of ΔL, mathemetically, the angle has increased in the counter-clockwise direction by (90-arccos(ΔL / L1)) degrees. (<-- that stuff is got from basic trignometry)

Is this true? If it is false, what is the right way to do it?

Additional question:
will the cylinder "follow" the angular momentum arrow and also point in the new direction, or will it remain in place and then rotate around the new arrow in a weird fashion?
 
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The vectors add, so the resulting angular momentum will have a larger magnitude.

24forChromium said:
will the cylinder "follow" the angular momentum arrow and also point in the new direction, or will it remain in place and then rotate around the new arrow in a weird fashion?
Neither. It will precess.
 
mfb said:
The vectors add, so the resulting angular momentum will have a larger magnitude.

Neither. It will precess.
I heard from various sources that if the two angular momentum vectors are perpendicular to one another, they will only change the direction of the final angular mometum. I understand, albeit to a shallow extent, precession, and I think precession only occurs when a torque is continuously exerted on a wheel. Also, it would be the most helpful of all if you can give the final angular momentum expressed in terms of L1 and ΔL.
 
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24forChromium said:
Also, it would be the most helpful of all if you can give the final angular momentum expressed in terms of L1 and ΔL.
As vector: L = L1 + ΔL
This is conservation of angular momentum.
As magnitude: ##|L|=\sqrt{L1^2 + ΔL^2}## - using the right angle between the two components.

See the section "Torque-free" in the wikipedia article.
 

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