The Helmholtz theorem of classical mechanics reads as follows:
Let [math]\displaystyle{ H(x,p;V) = K(p) + \varphi(x;V) }[/math] be the Hamiltonian of a one-dimensional system, where [math]\displaystyle{ K = \frac{p^2}{2m} }[/math] is the kinetic energy and [math]\displaystyle{ \varphi(x;V) }[/math] is a "U-shaped" potential energy profile which depends on a parameter [math]\displaystyle{ V }[/math]. Let [math]\displaystyle{ \left\langle \cdot \right\rangle _{t} }[/math] denote the time average. Let
Then [math]\displaystyle{ dS = \frac{dE+PdV}{T}. }[/math]
The thesis of this theorem of classical mechanics reads exactly as the heat theorem of thermodynamics. This fact shows that thermodynamic-like relations exist between certain mechanical quantities. This in turn allows to define the "thermodynamic state" of a one-dimensional mechanical system. In particular the temperature [math]\displaystyle{ T }[/math] is given by time average of the kinetic energy, and the entropy [math]\displaystyle{ S }[/math] by the logarithm of the action (i.e., [math]\displaystyle{ \oint dx \sqrt{2m\left( E - \varphi \left( x, V\right) \right) } }[/math]).
The importance of this theorem has been recognized by Ludwig Boltzmann who saw how to apply it to macroscopic systems (i.e. multidimensional systems), in order to provide a mechanical foundation of equilibrium thermodynamics. This research activity was strictly related to his formulation of the ergodic hypothesis.
A multidimensional version of the Helmholtz theorem, based on the ergodic theorem of George David Birkhoff is known as generalized Helmholtz theorem.
Original source: https://en.wikipedia.org/wiki/Helmholtz theorem (classical mechanics).
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