Finding Curvature of Vector Function

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Homework Help Overview

The discussion revolves around finding the curvature \( K \) of a vector function defined by the curve \(\vec{r}(t) = \langle 2 \cos t , 2 \sin t, t \rangle\), with a focus on the arc length parameter \( s \) and its implications for curvature calculation.

Discussion Character

  • Exploratory, Assumption checking

Approaches and Questions Raised

  • Participants discuss the necessity of determining the arc length function to find curvature, with specific attention to the choice of the lower limit \( a \) in the integral for arc length. Questions arise about the arbitrariness of this choice and its implications.

Discussion Status

Some participants suggest using the Frenet-Serret formulas and the chain rule, indicating that integration may not be necessary. There is an ongoing exploration of the implications of choosing different values for \( a \), with one participant noting that the choice could be arbitrary as it may cancel out in the final expression.

Contextual Notes

Participants are navigating the constraints of the problem, particularly regarding the definition of arc length and its relationship to the trajectory of an object, as well as the implications of starting the arc length measurement from different points.

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Homework Statement


Find the curvature K of the curve, where s is the arc length parameter:

[itex]\vec{r}(t) = \langle 2 \cos t , 2 \sin t, t \rangle[/itex]

Homework Equations



[itex]s(t) = \int_a ^t ||\vec{r}'(u)||du[/itex]

The Attempt at a Solution



I know I need to find the arc length function, in order to find the curvature function; however, I am unsure as to what I should choose a to be for the lower limit of the integral.
 
Last edited:
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You need to use the Frenet-Serret formulae and the chain rule.

You should not need to perform any integrations.
 
Last edited:
Bashyboy said:

Homework Statement


Find the curvature K of the curve, where s is the arc length parameter:

[itex]\vec{r}(t) = \langle 2 \cos t , 2 \sin t, t \rangle[/itex]




Homework Equations



[itex]s(t) = \int_a ^t ||\vec{r}'(u)||du[/itex]

The Attempt at a Solution



I know I need to find the arc length function, in order to find the curvature function; however, I am unsure as to what I should choose a to be for the lower limit of the integral.

Choose it so that s(0)=0.
 
So, the choice is arbitrary? If so, why?
 
Bashyboy said:
So, the choice is arbitrary? If so, why?

Why not? Just as you can start time at any instant, you can measure an arc length from any point on a line. Usually the arc length is connected to the trajectory of an object if time is involved, that is why I suggested s(0)=0. But you also can keep "a" in the expression s(t). Solve the problem and you will see that"a" cancels.

ehild
 

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