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The Variational Principles of Mechanics (Dover Books on Physics and Chemistry)
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The Variational Principles of Mechanics (Dover Books on Physics and Chemistry) Customer Reviews
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♥♥♥♥♥
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a lot of unfamiliar variational tricks, sometimes lacks proofs or underexplains
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I've read this gem and done most of the evercises in about 3 months. Before that legendary book I'd had the usual crappy course in Classical Mechanics based on Goldstein. The bottom line is the book will show you a lot of advanced material and unfamiliar manipulations. On the other hand there are sometimes statements lacking proof or more detailed lucid explanation. The book is appropriate for readers that already know what action is, totall beginners will be too shocked by the new concepts and won't be able to pick up the important nuances revealed by Lanczos.
Lanczos work clarified some of the concepts in which my CM course failed:
- the important difference in treating holonomic and nonholonomic constraints
- exact constraints are mathematical idealization of infinitely rigid constraint forces
- Lagrange multipliers for functionals (actions) not only functions
- the logical thread virtual work -> d'Alembert -> Hamilton's principle
- the connection between the action in configuration space and in phase space
The book introduced me to topics not covered by the course, which was my initial goal:
- elimination of ignorable variables in L or H formulation
- canonical transformations, definition and importance
- generating function of canonical transformation
- test for canonicity of transformation using Poisson brackets
- integral invariants of canonical transformations
- Hamilton's principal function
- Hamilton-Jackobi equation and analogy with optical wave surfaces
- separation of variables in H-J equation
- action-angle variables for separable periodic systems
- evolution of the system as a sequence of canonical transformation
- introducing geometry and geodesics in phase space
The reading definitely increased my freedom in manipulating the variational problem into equivalent variational problem. Examples of the two most weird for me manipulations are in the appendices. In the first appendix the Hamiltonian formulation is derived from the Lagrangian by introducing new variables, constraints and corresponding Lagrange multipliers, and then eliminating the variables. In appendix II, the most popular cases of Noether's theorem are derived by introducing new field variables in the action - I had no idea that was allowed. Very interesting was the idea that the world line of the system in configuration space can be parametrized with arbitrary parameter and the time becomes a function of that parameter that is varied together with the other generalized coordinates. Such variation is normal for GR but I've never seen it done in non-relativistic mechanics.
Some of the other reviews described the book as 'lucid'. I find that eggagerated - although the book shows lots of unfamiliar manipulations, sometimes proofs of validity or the necessary more detailed conceptual or calculational explanations are lacking. An example is the inclusion, all of a sudden, of the time as variable to be varied - where is the proof one is allowed to do that? In another case, the book tells you that by nullifying the boundary term when varying the action, one gets 'natural' boundary conditions for the Euler-Lagrange diff. equations. I failed to see how the physics of the problem would demand exactly those boundary conditions. Where the analogy between mechanics and optics was discussed, the book creates the impression it derived the Fermat's principle but in reality it simply proved that the path following the gradient of of constant surfaces is shortest between two points. So there is a certain gegree of fuzziness on calculational level (lacking proofs of validity) or conceptual level (underexplained concepts and relations).
I liked the the abundance of historical notes. You will learn that there are several formulations of the least action principle - Euler and Lagrange version, Jackobi version and Hamilton version. Each subsection has a small summary and there are a few problems per section to illustrate the main ideas but not enough for exercises.
There are two chapters that I think appeared in later editions and are too sketchy compared to the book core:
Chapter 9 discusses special relativity where you can see that guessing the relativistic Lagrangian on general grounds of Lorentz invariance gives almost effortlessly the relativistic dynamics without the usual gedanken experiments. At the end, Lanczos dives a little into GR using the Schwartzchild metric to derive orbits, bending of light rays and gravitational redshift around spherical body.
Chapter 11 gives a short presentation of fluid mechanics (a little unclear derivation, in Lagrange and Euler coordinates), elasticity, and electromagnetism. Noether's principle is used to derive the canonical and the symmetric energy momentum tensor. I haven't seen a crystal clear derivation of Noether anywhere and Lancsoz is not an exception. The problem is as usual ommiting what exactly is being transformed and why is that allowed.
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