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A Nonholonomic mechanics in Landau and Lifshitz

  1. May 26, 2016 #1

    vanhees71

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    <<Mentor's note: this is spin-off this thread>>

    One error I'm aware of in LL vol. I is the claim on integrability. But what's wrong with LL's treatment of anholonomous constraints (in sectin 38 in my German edition)? It just leads to the usual equations with Lagrange parameters you also get from d'Alembert's principle ("virtual displacements"). There are wrong statements about this in the literature, e.g., in Goldstein, where he uses (38.2) instead of (38.4), leading to the socalle "vaconomic mechanics", which is considered wrong. On this issue, see

    Zampieri, Gaetano. "Nonholonomic versus vakonomic dynamics." Journal of Differential Equations 163.2 (2000): 335-347.
    https://www.researchgate.net/profile/Gaetano_Zampieri/publication/224039915_Nonholonomic_versus_Vakonomic_Dynamics/links/544fb9a50cf201441e934bcd.pdf [Broken]
     
    Last edited by a moderator: May 7, 2017
  2. jcsd
  3. May 26, 2016 #2
    They write correct equations but they obtain these equations from significantly incorrect argument.

    Furthermore Their definition of canonical transformation is also not equivalent to the standard one. In the sequel pages this generates another error. etc
     
    Last edited: May 26, 2016
  4. May 26, 2016 #3

    vanhees71

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    Hm, you must have read another book ;-)).

    BTW also in the by you recommended book [Nonholonomic Mechanics and Control (Interdisciplinary Applied Mathematics), Anthony Bloch, et al] the make come to the same conclusion as Landau/Lifhitz and other standard textbooks, i.e., that the usual equations derived from d'Alembert's principle of virtual displacement leads to the right equations of motion, i.e., you have to take the contraints (holonomic or anholonomic) as constraints on the variations with ##\delta t=0##. Then you can use the Lagrange-multiplier method also in the action principle, i.e., you make
    $$A[q]=\int_{t_1}^{t_2} \mathrm{d} t L(q,\dot{q},t)$$
    stationary under the constraints ##\delta q(t_1)=\delta q(t_2)=0## and under the (perhaps anholonomous) contraints
    $$c_{\alpha,j} \delta q^j=0$$
    Then you get
    $$\delta A=\int_{t_1}^{t_2} \mathrm{d} t \delta q^j \left [\frac{\partial L}{\partial q^j} -\frac{\mathrm{d}}{\mathrm{d} t}\frac{\partial L}{\partial \dot{q}^j}-\lambda^{\alpha} c_{\alpha,j} \right ] \stackrel{!}{=} 0,$$
    leading to the same equation as with the d'Alembert principle
    $$\frac{\partial L}{\partial q^j} -\frac{\mathrm{d}}{\mathrm{d} t}\frac{\partial L}{\partial \dot{q}^j}=\lambda^{\alpha} c_{\alpha,j},$$
    Together with the contraints these is a set of differential equations for the ##q^j## and ##\lambda^{\alpha}##.

    I don't know to what section you are referring to when claiming that there are errors concerning canonical transformations. What's in Sect. 45 (again in my German edition of LL vol. 1) is very much standard and a very economic way to derive canonical transformations via generating functions. Also the Poisson brackets are right (note that LL uses a convention where the sign of the brackets differs from the more usual one, but that you cannot claim to be an error, it's just a different convention, which may be unfortunate but it's not wrong).
     
  5. May 26, 2016 #4
    this is exactly vaconomic variational principle and it leads to vaconomic equations, you can not deduce formula:
    from this principle
     
  6. May 26, 2016 #5

    vanhees71

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    No! I just have derived it for you in the previsou posting (indeed that's also what LL does in his book, and I guess you find it in any textbook on analytical mechanics, except in Goldstein who derives the errorensou vakonomic equations without a clear discussion of its (in)validity; of course you find also in this textbook the right equations, derived from d'Alembert's principles).

    The vakonomic equations follow if you take
    $$c_{\alpha j} \dot{q}^j=0 \qquad (*)$$
    as constraints, which is NOT what I did in my previous posting. To get the vakonomic equations, you use the Lagrange-multiplier method for these constraints, but these are different from the constraints on the variations that I (and any reasonable textbook does). It's also all the same in chapter 1 of the textbook by Bloch et al! It's only amazing that he discusses vakonomic dynamics at length in a later chapter, but I have to check what's his motivation for this in more detail to understand why he is doing that.

    The point is that the right way of anholonomic constraints have to be applied to the variations themselves. Then you derive the equations of motion for the generalized coordinates and Lagrange multiplyers and then, i.e., "on shell" you can use the constraints in terms of (*), and that's how it is done in LL using Hamilton's principle. LL doesn't discuss the vakonomic dynamics at all, but also doesn't put a warning against their use. Admittedly, I've only recently seen the vakonomic dynamics via a discussion in this forum.
     
  7. May 26, 2016 #6
    o, now I have read your posting carefully. But then another question arises. What is the statement of variational problem you are solving? You have the functional ##q(\cdot)\mapsto A[q(\cdot)]##, ok. In which class of functions ##q(t)## you differentiate this functional ##A##?
     
  8. May 26, 2016 #7
    upd in which class of functions you are looking for extremums of the functional?
     
  9. May 26, 2016 #8

    vanhees71

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    Well, I use the usual physicist's sloppyness (which is also applied in d'Alembert's principle and all other principles of mechanics). I just assume the the functions are "sufficiently smooth". Then you apply the usual (sloppy) way to vary the action but take into account the nonholonomous constraints on this variation using Lagrange multipliers.

    Here's a paragraph on the difference between the correct d'Alembert and vaskonomic problem from Bloch's book (which is marvelous, as far as I can say from the little I read in it):

     
  10. May 26, 2016 #9
    I did not mean to nitpick. I will also hold the sloppiness. General statement of the conditional extremums finding problem is as follows. Let ##f:X\to\mathbb{R}## stand for a functional of a vector space ##X##. The space ##X## may be infinite-dimensional. And there is a manifold ##M\subset X##. We shall say that a point ##x_0\in M## is a conditional extremum of ##f## iff ##d g(x_0)=0##, where ##g=f\mid_M:M\to\mathbb{R}.##

    If you apply this concept to our case: $$f(x)=\int_{t_0}^{t_1}L(x,\dot x)dt,\quad M=\{x(t)\mid \langle c(x),\dot x\rangle=0,\quad x(t_i) --\mathrm{fixed}\}\qquad (**)$$
    then you will have the vaconomic statement.

    The statement
    $$\int_{t_0}^{t_1}\Big(\frac{d}{dt}\frac{\partial L}{\partial \dot x^k}-\frac{\partial L}{\partial x^k}\Big)\delta x^kdt=0,\quad \langle c(x),\delta x\rangle=0\qquad (*)$$ gives the equations of classical mechanics, but this is not a problem on finding conditional extremums, it is just d'Alembert's principle in the integral form. So it is called in Bloch's book and it is correct. In general the solution ##x(t)## to problem (*) is not an exteremum of problem (**). It is an extremum to problem (**) if only the constraint ##\langle c(x),\dot x\rangle=0## is holonomic. LL sound like that they still apply the Hamilton variational principle just with several additional constraints that is incorreect: equation (*) is the D'Alambert principle it does not reduce to Hamilton's one.

    To summarize this, roughly speaking, (**) does not imply (*)
     
    Last edited: May 26, 2016
  11. May 27, 2016 #10

    vanhees71

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    Ok, then it's semantics. You can call it d'Alembert's principle in integral form. The only thing I wonder about is, how do we know, in the case of nonholonomic constraints, which principle to apply. Both principles are equivalent for holonomic constraints, and obviously every source claims that d'Alembert's principle is the correct description in the case of nonholonomic constraints. In the paper of Zampieri it's shown that the vakonomic treatment of the problem of a skater on an inclined plane leads to quite strange solutions, which is not the case for d'Alembert's principle and thus it is argued that the d'Alembert principle of virtual displacement is the correct treatment. On the other hand, in the book of Bloch there are whole chapters about the vakonomic dynamics. So I wonder, why one should consider them at all, if it's known to be the wrong description? I guess, one should only warn students against the use of vakonomic mechanics and then stay with good old d'Alembert.
     
  12. May 27, 2016 #11
    moreover, vaconomic problems do not have uniqueness of solutions in some sense. I am not an expert in vaconomic, I have just heard that vaconomic problems appear in the control theory.
    Sure students should be aware about these stumbling blocks. But I believe that these things a very beautiful and fundamental and it is good to discuss them in lectures. It is also good to discuss the Frobenius' theorem in this context I guess.
    Variational principles are useful for example to prove that the double pendulum has very many periodic trajectories. Anothe interesting thing is that the problems (*), (**) help to define a weak solution to the dynamical equations, this leads to pretty things in the theory of collisions
     
    Last edited: May 27, 2016
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