Why is dx/dt*(\delta x*dx/dt) equal to 1/2\delta(dx/dt)^2?

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Discussion Overview

The discussion revolves around a transformation presented in a paper regarding the relationship between the expression dx/dt*(δx*dx/dt) and the result 1/2δ(dx/dt)^2. Participants are exploring the mathematical reasoning behind this transformation, which involves derivatives and differentials.

Discussion Character

  • Exploratory, Technical explanation, Conceptual clarification

Main Points Raised

  • One participant expresses confusion about a specific transformation in the paper, questioning the equivalence of the two expressions.
  • Another participant corrects the initial misrepresentation of the equation, clarifying that it should be written as dx/dt*(δ(dx/dt)), and suggests that this transformation can be interpreted as v δv to δ(v^2)/2.
  • A later reply acknowledges the mistake in writing the equation and admits difficulty in understanding the transformation.
  • Another participant prompts further exploration by asking what the result would be if one differentiates v^2.
  • A final response indicates a realization and gratitude for the clarification provided.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the transformation's clarity initially, but there is a progression towards understanding as corrections and clarifications are made. The discussion remains somewhat unresolved regarding the initial confusion.

Contextual Notes

The discussion highlights a potential misunderstanding of notation and the underlying mathematical principles involved in the transformation, which may depend on specific definitions and assumptions about differentials.

Gloyn
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Hello.

I don't understand one transformation that is made on page 25 of this paper:

http://www.atm.ox.ac.uk/user/read/mechanics/LA-notes.pdf

It is the second equation from the top, ont the one marked as '2', but just the second one.

dx/dt*(\delta x*dx/dt)=1/2\delta(dx/dt)^2

Why is that?
 
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Gloyn said:
Hello.

I don't understand one transformation that is made on page 25 of this paper:

http://www.atm.ox.ac.uk/user/read/mechanics/LA-notes.pdf

It is the second equation from the top, ont the one marked as '2', but just the second one.

dx/dt*(\delta x*dx/dt)=1/2\delta(dx/dt)^2

Why is that?
You've slightly misrepresented what the paper says. It's dx/dt*(δ dx/dt), not dx/dt*(δx dx/dt). To make it easier to read, let's write v for dx/dt. The transformation is then from v δv to δ(v2)/2. Is that clearer?
 
Right, I've made a mistake during writing the equation. Unfortunately, the transformation I don't get is the one you wrote. It's probably sth. very basic and I can't figure it out :s.
 
Well, what do you get if you differentiate v2?
 
Oh. My. God. I'm dumb, thanks.
 

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