Convention when changing integral limits

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Discussion Overview

The discussion revolves around the validity of changing the limits of integration in the context of kinetic energy and work done, specifically when transitioning from position to velocity in integral expressions. Participants explore whether it is appropriate to use limits of integration that correspond to velocity while maintaining the integrand in terms of position.

Discussion Character

  • Debate/contested

Main Points Raised

  • One participant questions the validity of writing the integral $$\int_{0}^{v} F dx = KE_{2}$$ without changing the variable, suggesting that it is necessary to express the integrand in terms of velocity, such as $$\int_{0}^{v} F \frac{dx}{dv} dv$$.
  • Another participant asserts that the original formulation is incorrect and emphasizes the need to change variables to ##v(x)## to properly use the limits of integration.
  • A later reply supports the assertion that the initial equations presented in the referenced material are incorrect, indicating that the author later corrects this by changing variables.

Areas of Agreement / Disagreement

Participants generally agree that the original formulation is incorrect and that changing variables is necessary. However, there is no consensus on the implications of the author's approach in the referenced material.

Contextual Notes

There is a lack of clarity regarding the assumptions made about the relationship between position and velocity, as well as the implications of using different variables in the integral. The discussion highlights potential confusion stemming from the author's presentation in the referenced equations.

etotheipi
Sorry for the silly question! If we start of with the relationship $$\int_{x_{1}}^{x_{2}} F dx = KE_{2} - KE_{1}$$ and then state that at position x1 the velocity (and hence also kinetic energy) of the particle is 0, and at x2 its velocity is v, is it sloppy or valid to write the integral representing the work done to increase the velocity from 0 to v as $$\int_{0}^{v} F dx = KE_{2}$$ or is it necessary to change the integrand to something like the following $$\int_{0}^{v} F \frac{dx}{dv} dv = KE_{2}$$
 
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It is not sloppy or valid. It is just wrong. You need to change variables to ##v(x)## to use those limits.
 
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Orodruin said:
It is not sloppy or valid. It is just wrong. You need to change variables to ##v(x)## to use those limits.

Thank you, that's what I'd hoped was the case. I got confused since on this page in equation 2.1.13 the author uses limits 0 and v whilst everything still being in terms of x, which seemed a little off.
 
Yeah, that's definitely wrong. The author fixes things by changing variables at (2.1.16) so that the limits now make sense but the equations before that point are nonsense. The ##v## is just serving the role of placeholder in (2.1.13-15), "this equation is incorrect and something else goes into this position but I'm not going to bother figuring out what"
 
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