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''principle of stationary action''
 
''principle of stationary action''
  
An integral variational principle in mechanics that was established by C.G.J. Jacobi [[#References|[1]]] for holonomic conservative systems. According to the Jacobi principle, if the initial position <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/j/j054/j054110/j0541101.png" /> and the final position <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/j/j054/j054110/j0541102.png" /> of a holonomic conservative system are given, then for the actual motion the Jacobi action
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An integral variational principle in mechanics that was established by C.G.J. Jacobi [[#References|[1]]] for holonomic conservative systems. According to the Jacobi principle, if the initial position $P_0$ and the final position $P_1$ of a holonomic conservative system are given, then for the actual motion the Jacobi action
  
<table class="eq" style="width:100%;"> <tr><td valign="top" style="width:94%;text-align:center;"><img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/j/j054/j054110/j0541103.png" /></td> </tr></table>
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$$S=\int\limits_{P_0}^{P_1}\sqrt{2(U+h)}ds,\quad ds^2=\sum_{i,j=1}^na_{ij}dq_idq_j$$
  
has a stationary value in comparison with all other infinitely-near motions between <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/j/j054/j054110/j0541104.png" /> and <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/j/j054/j054110/j0541105.png" /> with the same constant value <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/j/j054/j054110/j0541106.png" /> of the energy as in the actual motion. Here <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/j/j054/j054110/j0541107.png" /> is the force function of the active forces on the system, and <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/j/j054/j054110/j0541108.png" /> are the generalized Lagrange coordinates of the system, whose kinetic energy is
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has a stationary value in comparison with all other infinitely-near motions between $P_0$ and $P_1$ with the same constant value $h$ of the energy as in the actual motion. Here $U(q_1,\ldots,q_n)$ is the force function of the active forces on the system, and $q_i$ are the generalized Lagrange coordinates of the system, whose kinetic energy is
  
<table class="eq" style="width:100%;"> <tr><td valign="top" style="width:94%;text-align:center;"><img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/j/j054/j054110/j0541109.png" /></td> </tr></table>
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$$T=\frac12\sum_{i,j=1}^na_{ij}\dot q_i\dot q_j,\quad\dot q_i\equiv\frac{dq_i}{dt}.$$
  
Jacobi proved (see [[#References|[1]]]) that if <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/j/j054/j054110/j05411010.png" /> and <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/j/j054/j054110/j05411011.png" /> are sufficiently near to one another, then for the actual motion the action <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/j/j054/j054110/j05411012.png" /> has a minimum. The Jacobi principle reduces the problem of determining the actual trajectory of a holonomic conservative system to the geometrical problem of finding, in a Riemannian space with the metric
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Jacobi proved (see [[#References|[1]]]) that if $P_0$ and $P_1$ are sufficiently near to one another, then for the actual motion the action $S$ has a minimum. The Jacobi principle reduces the problem of determining the actual trajectory of a holonomic conservative system to the geometrical problem of finding, in a Riemannian space with the metric
  
<table class="eq" style="width:100%;"> <tr><td valign="top" style="width:94%;text-align:center;"><img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/j/j054/j054110/j05411013.png" /></td> </tr></table>
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$$ds^2=\sum_{i,j=1}^na_{ij}dq_idq_j,$$
  
 
an extremal of the variational problem.
 
an extremal of the variational problem.

Latest revision as of 09:49, 5 August 2014

principle of stationary action

An integral variational principle in mechanics that was established by C.G.J. Jacobi [1] for holonomic conservative systems. According to the Jacobi principle, if the initial position $P_0$ and the final position $P_1$ of a holonomic conservative system are given, then for the actual motion the Jacobi action

$$S=\int\limits_{P_0}^{P_1}\sqrt{2(U+h)}ds,\quad ds^2=\sum_{i,j=1}^na_{ij}dq_idq_j$$

has a stationary value in comparison with all other infinitely-near motions between $P_0$ and $P_1$ with the same constant value $h$ of the energy as in the actual motion. Here $U(q_1,\ldots,q_n)$ is the force function of the active forces on the system, and $q_i$ are the generalized Lagrange coordinates of the system, whose kinetic energy is

$$T=\frac12\sum_{i,j=1}^na_{ij}\dot q_i\dot q_j,\quad\dot q_i\equiv\frac{dq_i}{dt}.$$

Jacobi proved (see [1]) that if $P_0$ and $P_1$ are sufficiently near to one another, then for the actual motion the action $S$ has a minimum. The Jacobi principle reduces the problem of determining the actual trajectory of a holonomic conservative system to the geometrical problem of finding, in a Riemannian space with the metric

$$ds^2=\sum_{i,j=1}^na_{ij}dq_idq_j,$$

an extremal of the variational problem.

See also Variational principles of classical mechanics.

References

[1] C.G.J. Jacobi, "Vorlesungen über Dynamik" , G. Reimer (1884)


Comments

References

[a1] V.I. Arnol'd, "Mathematical methods of classical mechanics" , Springer (1978) (Translated from Russian)
How to Cite This Entry:
Jacobi principle. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Jacobi_principle&oldid=32721
This article was adapted from an original article by V.V. Rumyantsev (originator), which appeared in Encyclopedia of Mathematics - ISBN 1402006098. See original article