A differential geometry question

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Discussion Overview

The discussion revolves around the relationship between the normal vector function of a unit-speed curve and the determination of curvature and torsion, particularly in the context of differential geometry. Participants explore how knowledge of the normal vector impacts the computation of these geometric properties.

Discussion Character

  • Technical explanation
  • Homework-related
  • Debate/contested

Main Points Raised

  • One participant states that the normal vector function n = n(s) of a unit-speed curve with non-zero torsion determines the curvature and torsion, but expresses uncertainty about how to prove this.
  • Another participant suggests that curvature and torsion should be computed from the normal vector function, indicating that curvature is defined as ||r''||=||t'|| and torsion as -n.b', with both being functions of the tangent vector t.
  • A participant questions their understanding of torsion, noting that while n and b are functions of t, they are unsure how to proceed with the problem using n alone.
  • One participant expresses confusion about what is meant by "the knowledge of n" and highlights that knowing t, n, and b would allow for the determination of curvature and torsion, but questions how this applies when only n is known.
  • A later reply suggests considering the Frenet-Serre frames as a potential approach to the problem.

Areas of Agreement / Disagreement

Participants do not appear to reach a consensus on how to approach the problem or the implications of knowing only the normal vector. Multiple competing views and uncertainties remain regarding the relationship between the normal vector and the curvature and torsion.

Contextual Notes

Participants express uncertainty about the definitions and relationships between curvature, torsion, and the normal vector, indicating potential limitations in their understanding or the assumptions underlying their definitions.

binglee
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Show that the knowledge of the vector function n = n(s) (normal vector) of a unit-speed
curve
, with non-zero torsion everywhere, determines the curvature and the torsion
don't have any clues about what i am supposed to prove!~
 
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You are supposed to compute the curvature and torsion of a curve from its normal vector function. You probably defined curvature and torsion as a function of the tangent vector.
 
ya, we defined curvature = ||r''||=||t'||. It is a function of t, while we defined torsion = -n.b' it is not a function of t. I don't know if my understanding about torsion is right. Since we defined n as a function of t and b as a function of t so is t. Then in this question i just need to go through the same process wrt n??right? thank you for your helping
 
bump can anybody help me?!~~~~
I thought about it for a while. what does it mean by the knowledge of n?
if we know t n b, we can determine the curvature and the torsion, but here we only know n??
please!~ help me!~~
 
binglee:

Have you tried working with Frenet-Serre frames.?
 

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