Locus if velocity component is zero

Click For Summary

Discussion Overview

The discussion revolves around the behavior of the angles of a symmetrical top on a unit sphere, particularly focusing on the conditions under which the velocity components of the angles are zero at specific points on the sphere. Participants explore the implications of these conditions on the locus of the figure axis.

Discussion Character

  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant asserts that at the upper bounding circle, both angular velocities ##\dot{\theta}## and ##\dot{\psi}## should be zero, suggesting that ##\theta## remains unchanged.
  • Another participant questions the assertion that ##\dot{\psi}## is zero at both circles, clarifying that it is only valid at the upper circle and noting the presence of cusps.
  • A participant acknowledges a mistake in their earlier statement regarding the conditions for ##\dot{\psi}## and reiterates that it is zero only at the upper circle.
  • One participant emphasizes that both circles are defined by ##\dot{\theta}=0##, while ##\dot{\psi}=0## applies only to the upper circle, as indicated in the figure.
  • Another participant expresses confusion about the implications of the fixed angles and requests clarification on their misunderstanding.
  • A later reply indicates that at the upper circle, both ##\dot{\psi}## and ##\dot{\theta}## are zero, while at the lower circle, ##\dot{\vartheta}=0## and ##\dot{\psi} \neq 0##, suggesting that this is contingent on initial conditions.

Areas of Agreement / Disagreement

Participants exhibit disagreement regarding the conditions under which ##\dot{\psi}## is zero, with some asserting it applies only to the upper circle while others challenge this interpretation. The discussion remains unresolved as participants seek clarification on the implications of these conditions.

Contextual Notes

There are indications of confusion regarding the definitions and implications of the angular velocities, particularly how they relate to the positions on the sphere. The discussion highlights the need for careful interpretation of the figure and the conditions presented.

Kashmir
Messages
466
Reaction score
74
IMG_20211212_114148.JPG


Figure shows a locus of the figure axis of a symmetrical top on a unit sphere such that
##\dot{\theta}=\dot{\psi}=0## at the upper bounding circle. Where
##{\theta}## is the polar angle and ##{\psi}## is the azimuthal angle.



Suppose the figure axis is at the upper circle, since ##\dot{\theta}=0## at the upper bounding circle, we should expect ##{\theta}## to remain unchanged hence fixed on the upper circle. However the figure shows a change in ##{\theta}##. Why is that so?
 
Last edited:
Physics news on Phys.org
Why should ##\dot{\psi}=0## at these circles? From the figure it's only valid at the upper circle, which is why you have cusps there. On the lower circle the tangent of the curve is horizontal and thus also there ##\dot{\theta}=0##.
 
  • Like
Likes   Reactions: Kashmir
vanhees71 said:
Why should ##\dot{\psi}=0## at these circles? From the figure it's only valid at the upper circle, which is why you have cusps there. On the lower circle the tangent of the curve is horizontal and thus also there ##\dot{\theta}=0##.
Sorry, I wasn't able to write it properly,the Latex was a bit tough to write so I made a mistake . Yes ##\dot{\psi}=0## is zero at the upper bounding circle only.
 
  • Like
Likes   Reactions: vanhees71
vanhees71 said:
Why should ##\dot{\psi}=0## at these circles? From the figure it's only valid at the upper circle, which is why you have cusps there. On the lower circle the tangent of the curve is horizontal and thus also there ##\dot{\theta}=0##.
I'm still confused. Can you please point out my mistake.
 
At both circles ##\dot{\theta}=0## (that's what defines the circles). ##\dot{\psi}=0## only on the upper but not on the lower circle, as you can clearly read off the figure.
 
vanhees71 said:
At both circles ##\dot{\theta}=0## (that's what defines the circles). ##\dot{\psi}=0## only on the upper but not on the lower circle, as you can clearly read off the figure.
When the axis is at the upper circle both angles ##\theta## and ##\phi## will remain fixed as their rates of change is zero.
So the axis should remain fixed there? But that's not what the figure shows.

Where am I going wrong. Can you please tell me.
 
At the upper circle ##\dot{\psi}=\dot{\theta}=0##, on the lower ##\dot{\vartheta}=0## and ##\dot{\psi} \neq 0##. It's just one possible case due to the initial conditions.
 

Similar threads

  • · Replies 1 ·
Replies
1
Views
1K
  • · Replies 12 ·
Replies
12
Views
4K
  • · Replies 16 ·
Replies
16
Views
3K
  • · Replies 2 ·
Replies
2
Views
2K
  • · Replies 3 ·
Replies
3
Views
2K
  • · Replies 11 ·
Replies
11
Views
1K
  • · Replies 5 ·
Replies
5
Views
2K
  • · Replies 8 ·
Replies
8
Views
3K
  • · Replies 9 ·
Replies
9
Views
3K
  • · Replies 7 ·
Replies
7
Views
5K