How Do You Integrate (dx/dt) dx in Physics Problems?

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Discussion Overview

The discussion revolves around the integration of the expression ∫ b (dx/dt) ⋅ dx, particularly in the context of physics problems involving work done by forces proportional to velocity, such as drag. Participants explore different approaches to rewriting the integral and the implications of variable dependencies.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Homework-related

Main Points Raised

  • One participant asks how to integrate ∫ b (dx/dt) ⋅ dx, mentioning difficulty in substituting dx with v dt.
  • Another participant suggests expressing v as dx/dt, leading to the substitution dx = v dt, which transforms the integral into ∫ b v² dt.
  • A different viewpoint emphasizes that it may be easier to express dx/dt as a function of x instead of t, indicating that specific integrals are necessary for further progress.
  • One participant reiterates the substitution of dx = v dt and the resulting integral form, thanking the previous contributor for the clarification.

Areas of Agreement / Disagreement

Participants present multiple approaches to the problem, with no consensus on a single method or solution. The discussion remains unresolved regarding the best way to proceed with the integration.

Contextual Notes

Participants note the importance of having a specific integral to work with and suggest that the problem should be posted in the homework forums with a complete statement and attempts.

danunicamp
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Good Night,

Can someone please tell me how to do: ∫ b (dx/dt) ⋅ dx ?

Like in the work done by a force which is proportional to the velocity (like drag). I tried to change dx to v dt but couldn´t go much further.

Thank you in advance.
 
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If you write ##v=\frac{dx}{dt}## then you have ##dx=v\,dt## and you can write your equation as ##\int b\,v^2 dt##.
 
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Note that it is sometimes easier to express ##dx/dt## as a function of ##x## rather than ##t##. In these cases, you can simply integrate that function. You really cannot get any further without having a specific integral to do. Note that such a problem should be posted in the homework forums with the template properly filled in, including your own attempt and the full problem statement.
 
andrewkirk said:
If you write ##v=\frac{dx}{dt}## then you have ##dx=v\,dt## and you can write your equation as ##\int b\,v^2 dt##.
Thank you very much
 

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