What Is the Role of ξ in the Arc Length Formula?

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Discussion Overview

The discussion centers around the role of the variable ##\xi## in the arc length formula, specifically in the context of vector calculus and the Frenet frame. Participants explore the meaning and implications of using ##\xi## in place of other variables typically involved, such as ##t## and ##s##.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant states that ##\xi## is a variable and that ##R'## appears to be a function of ##\xi##.
  • Another participant mentions that ##\xi## is a "mute variable" and cannot be ##t## since ##t## is used as the boundary of the integral.
  • Some participants express confusion about the introduction of ##\xi##, noting that they are accustomed to seeing ##t## and ##s## as the primary variables.
  • A participant suggests that ##\xi## could be interpreted as ##t## because the integral runs from ##t_0## to ##t##, but acknowledges the confusion caused by the change in notation.
  • Another participant clarifies that ##s## is defined as an integral that represents the cumulative area under the curve of ##|R'|## up to time ##t##, and explains the necessity of using a different symbol for the variable of integration to avoid poor notation.

Areas of Agreement / Disagreement

Participants express varying interpretations of the role of ##\xi##, with some viewing it as a standard variable while others find the notation confusing. There is no consensus on a single interpretation of ##\xi##.

Contextual Notes

Some participants highlight the importance of distinguishing between the variable of integration and the limits of integration, indicating a potential misunderstanding of notation in the context of calculus.

Kuhan
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[itex]s(t)=\int_{t_0}^{t}\left | R'(\xi) \right | d\xi[/itex]
What is [itex]\xi[/itex] ?

In the above arc length formula with ##\xi##, what is ##\xi##?
 
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A variable. R' seems to be a function of [itex]\xi[/itex].
 


I found this equation while studying vector calculus/ multivariable calculus. I was studying about binormal, tangent, normal, frenet frame, etc.

I just don't understand ##\xi## because the variables usually involved were t and s
 


[itex]\xi[/itex] is just a mute variable. You can't use t inside the integral because t is used on the boundary of the interval. So [itex]\xi[/itex] is just a normal variable.
 


Kuhan said:
I found this equation while studying vector calculus/ multivariable calculus. I was studying about binormal, tangent, normal, frenet frame, etc.

I just don't understand ##\xi## because the variables usually involved were t and s

t and s are just letters. You can denote a variable by whatever symbol you want.
 


I guess ##\xi## is just t, because the integral is from ##t_0## to ##t## so it just has to be t in the sense that it represents time...
I just found it confusing why he suddenly put ##\xi## when all along he was talking about t, s, velocity, binormal, and Frenet frames of vectors
Thanks now I understand !
 
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R and s are both functions of time, but s is defined in terms of an integral, and represents the cumulative area under |R'|, up to time t. So, what makes s a function of t is the fact that the upper limit of the integral is a variable (t), not a constant. So, t is the variable corresponding to the time value up to which you integrate. It would be poor notation (and just wrong) to use the same symbol (t) for the other time variable that corresponds to all the particular times at which R is evaluated. So the time values with respect to with R varies are instead given another symbol, xi. In this context, xi is an example of what is termed a dummy variable of integration:

http://mathworld.wolfram.com/DummyVariable.html

EDIT: Thread title fixed :smile:
 
Last edited:

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