Is the derivative in my textbook correct here?

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Discussion Overview

The discussion revolves around the correctness of a derivative presented in a textbook, specifically in the context of differentiating kinetic and potential energy with respect to time. Participants explore the implications of differentiating functions of position and time in their calculations.

Discussion Character

  • Homework-related
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant attempts to differentiate kinetic energy and potential energy but expresses confusion over the correct application of derivatives, suggesting a misunderstanding of the relationship between position and velocity.
  • Another participant clarifies that the variable x is a function of time, indicating that derivatives should be taken with respect to time, which is implied by the notation x'.
  • A further reply points out that the differentiation in the textbook is indeed with respect to time, contrasting the initial participant's approach which mistakenly uses differentiation with respect to position.
  • A later response acknowledges the oversight and expresses gratitude for the clarification, indicating a realization of the basic concept involved.
  • Another participant introduces an equation involving both kinetic and potential energy, suggesting a cancellation of a common factor in the context of the discussion.

Areas of Agreement / Disagreement

Participants generally agree on the need to differentiate with respect to time, but there is no consensus on the initial misunderstanding or the implications of the introduced equation.

Contextual Notes

The discussion highlights potential confusion regarding the notation and the context of differentiation, particularly the distinction between derivatives with respect to position versus time.

whitejac
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Homework Statement


Principle of Cons Ener.JPG


Homework Equations


d/dx

The Attempt at a Solution


d/dx (T) = d/dx(1/2mx'2) = mx''
d/dx(U) = d/dx(1/2kx2) = kx' ≠ kx

It's probably me who made an error because I know that that equation (2.3) is the one I should be getting, but I don't understand how they did it because potential energy relies on position, so the derivative has to be a velocity vector.
 
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Here x is a function of t and the derivatives should be with respect to t. That's what x' implies.
 
(2.6) shows differentiation with respect to time. Your working is erroneously using differentiation with respect to x.
 
Oh right, it's x(t)... I guess the caffeine hasn't kicked in because that was really basic. Thank you!
 
No worries. Enjoy your coffee!
 
I think (2.3) starts out as ##m \ddot x \dot x + k x \dot x = 0## and the common factor ##\dot x## can be canceled from both sides.
 

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