Question from Lanczos book - genearlized momentum

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Discussion Overview

The discussion revolves around a specific transition in equations from Cornelius Lanczos' book on Variational Principles of Mechanics, focusing on the relationship between generalized momentum and the derivatives of the Lagrangian with respect to velocities and positions. The scope includes theoretical mechanics and mathematical reasoning.

Discussion Character

  • Technical explanation
  • Mathematical reasoning

Main Points Raised

  • One participant seeks clarification on the transition from equation 53.4 to 53.5 in Lanczos' book, questioning how the generalized momentum defined in 53.4 relates to the derivatives of the Lagrangian in 53.5.
  • Another participant requests the specific equations mentioned to facilitate understanding, indicating that not all readers may have access to the book.
  • A third participant cites the Euler-Lagrange equations to explain the relationship, stating that setting the generalized momentum equal to the derivative of the Lagrangian with respect to velocities leads to the desired result.
  • A later reply expresses gratitude for the clarification, indicating that the explanation was helpful.

Areas of Agreement / Disagreement

The discussion does not appear to reach a consensus on the clarity of the transition between the equations, as one participant expresses confusion while another provides a mathematical explanation. The understanding of the equations remains a point of contention.

Contextual Notes

There is an assumption that participants are familiar with the Euler-Lagrange equations and the specific equations from Lanczos' book, which may not be universally shared. The discussion also highlights a potential gap in the availability of the book among participants.

jackiefrost
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If you have the 4th ed. of Cornelius Lanczos' book on Variational Principles of Mechanics, could you explain to me how he got from eq. 53.4 to 53.5 (page 121). Eq. 53.4 defines the generalized momentum as the derivative of the Lagrangian, L, with respect to the velocities. Then, in eq. 53.5 he refers to the derivatives of the generalized momenta as equaling the derivative of L with respect to the positions. Is this right? This seems so simple yet for some reason (or lack thereof :blushing:) I'm not seeing the math from 53.4 to 53.5.

Thanks
 
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I don't think everyone has this book, so could you please write down the equations that he mentions?
 
It follows from the Euler-Lagrange equations:

[tex]\frac{d}{dt} \frac{\partial{\mathcal{L}}}{\partial v_{\alpha}} = \frac{\partial \mathcal{L}}{\partial x_{\alpha}} .[/tex]​

Setting [tex]p_{\alpha} = {\partial{\mathcal{L}}}/{\partial v_{\alpha}}[/tex] in this, you get
[tex]\frac{d p_{\alpha} }{dt} = \frac{\partial \mathcal{L}}{\partial x_{\alpha}} .[/tex]​
 
Last edited:
Thanks dx! Don't know why I didn't seen that.
 

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