Does small angle mean small angular velocity?

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Discussion Overview

The discussion revolves around the relationship between small angles and small angular velocities, particularly in the context of the small-angle approximation and its implications for terms in equations of motion. Participants explore the mathematical implications of approximating terms involving angular displacement and velocity.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant questions why the term ##(\sin\theta\,\dot{\theta})^2## is considered fourth order in small ##\theta##, suggesting that for this to hold, ##\theta## must equal ##\dot{\theta}##.
  • Another participant challenges the assertion that quantities of different dimensions must be equal, indicating that this is not a requirement.
  • A later reply emphasizes the importance of understanding one's own reasoning regarding the correctness of the claims made.
  • One participant asserts that ##(\sin\theta\,\dot{\theta})^2## is actually second order in ##\theta##, which raises concerns about its neglect in certain approximations.
  • Another participant clarifies that the degree of the term ##\theta^2\dot{\theta}^2## is indeed four, explaining that both ##\theta## and ##\dot{\theta}## are assumed to be small.
  • It is noted that while ##\theta## is small, the timescale for ##\dot{\theta}## is not necessarily small, leading to further discussion about the relationship between these quantities.

Areas of Agreement / Disagreement

Participants express differing views on the order of the term ##(\sin\theta\,\dot{\theta})^2## and the conditions under which certain approximations hold. There is no consensus on whether the equality of ##\theta## and ##\dot{\theta}## is valid or necessary.

Contextual Notes

Participants highlight the potential confusion arising from the dimensionality of the quantities involved and the assumptions made in the small-angle approximation. The discussion remains focused on the mathematical relationships without resolving the underlying assumptions.

Happiness
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Why is the term ##(\sin\theta\,\dot{\theta})^2## fourth order in the small ##\theta##, as claimed by the sentence below (5.108)?

By small-angle approximation, ##(\sin\theta\,\dot{\theta})^2\approx\theta^2\,\dot{\theta}^2##.

For this to be fourth order, it seems like we must have ##\theta=\dot{\theta}##. Why is this true? What are the conditions for this to be true?

Screen Shot 2016-08-04 at 5.14.16 pm.png
 
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Happiness said:
For this to be fourth order, it seems like we must have θ=˙θ\theta=\dot{\theta}. Why is this true?
why quantities of different dimension must be equal? They must not. David Morin wrote a good book but definitely not for high school
 
wrobel said:
why quantities of different dimension must be equal? They must not. David Morin wrote a good book but definitely not for high school

So is he right or wrong?
 
Happiness said:
So is he right or wrong?
My advice is that you should first think about your own understanding whether it is right or wrong.
 
wrobel said:
My advice is that you should first think about your own understanding whether it is right or wrong.

My understanding is that ##(\sin\theta\,\dot{\theta})^2## is second order in ##\theta##. But that would mean that it cannot be ignored, since (5.109) contains second-order terms.
 
The degree of the term ## \theta^2\dot \theta^2## is equal to 2+2=4. It is a degree of polynomial ##P(\theta,\dot \theta)= \theta^2\dot \theta^2##. Both quantities ## \theta,\dot \theta## are assumed to be small. The approximation of the Lagrangian up to the second order terms corresponds to linearization of the Lagrange equations in the vicinity of equilibrium
 
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Here it is important that θ is small, but the timescale is not. ##\dot \theta## is "something with θ divided by time", which is small of θ is small.
 

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