Finding Arc Length of a Curve: Using ##dx## and ##dy##

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Discussion Overview

The discussion revolves around the calculation of arc length for a curve defined by the function ##y = f(x)##, specifically focusing on the mathematical treatment of differentials ##dx## and ##dy## in the context of arc length formulas. The scope includes mathematical reasoning and conceptual clarification regarding the implications of directionality in integration.

Discussion Character

  • Mathematical reasoning, Conceptual clarification

Main Points Raised

  • One participant presents the formula for arc length and questions whether ##\sqrt{dx^2}## should be represented as ##|dx|## instead of ##dx##.
  • Another participant agrees that ##\sqrt{dx^2}## is indeed equal to ##|dx|##, but notes that in the context of moving in the positive x-direction, this distinction may not matter.
  • A third participant expresses confusion about the concept of "going in the positive direction" while plotting a function and requests further explanation.
  • A fourth participant clarifies that when calculating arc length between specific points, such as from x=2 to x=4, the integration should be performed from the lower to the upper limit, reinforcing the importance of direction in integration.

Areas of Agreement / Disagreement

Participants exhibit some agreement on the mathematical representation of differentials, but there is uncertainty regarding the implications of directionality in integration and its relevance to the arc length calculation.

Contextual Notes

The discussion highlights potential limitations in understanding the treatment of differentials and the implications of integrating over specific intervals, which may depend on the context of the problem.

PFuser1232
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The arc length of any curve defined by ##y = f(x)## is found as follows:
$$ds = \sqrt{dx^2 + dy^2}$$
$$ds = \sqrt{dx^2(1 + {\frac{dy}{dx}}^2)}$$
$$ds = \sqrt{dx^2} \sqrt{1 + [f'(x)]^2}$$
$$ds = \sqrt{1 + [f'(x)]^2} dx$$
Isn't ##\sqrt{dx^2}## equal to ##|dx|##, and not ##dx##?
 
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In principle it is, but if you go in positive x-direction only it does not matter. That way of dealing with differentials is not very mathematical anyway.
 
mfb said:
In principle it is, but if you go in positive x-direction only it does not matter. That way of dealing with differentials is not very mathematical anyway.

I'm not very familiar with the notion of "going in the positive direction" while plotting a function, I had no idea it makes a difference. Could you please elaborate?
 
"dx>0"

If you want to know the arc length between x=2 and x=4 for example, you integrate x from 2 to 4 and not from 4 to 2.
 
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