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| The '''principle of least constraint''' is another formulation of [[classical mechanics]] enunciated by [[Carl Friedrich Gauss]] in 1829.
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| The principle of least constraint is a [[least squares]] principle stating that the true motion of a mechanical system of <math>N</math> masses is the minimum of the quantity
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| :<math>
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| Z \ \stackrel{\mathrm{def}}{=}\ \sum_{k=1}^{N} m_{k} \left| \frac{d^{2} \mathbf{r}_{k}}{dt^{2}} - \frac{\mathbf{F}_{k}}{m_{k}} \right|^{2}
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| </math>
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| for all trajectories satisfying any imposed constraints, where <math>m_{k}</math>, <math>\mathbf{r}_{k}</math> and <math>\mathbf{F}_{k}</math> represent the mass, position and applied forces of the <math>\mathrm{k^{th}}</math> mass.
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| Gauss' principle is equivalent to [[D'Alembert's principle]].
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| The principle of least constraint is qualitatively similar to [[Hamilton's principle]], which states that the true path taken by a mechanical system is an extremum of the [[action (physics)|action]]. However, Gauss' principle is a true (local) ''minimal'' principle, whereas the other is an ''extremal'' principle.
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| ==Hertz's principle of least curvature==
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| Hertz's principle of least curvature is a special case of Gauss' principle, restricted by the two conditions that there be no applied forces and that all masses are identical. (Without loss of generality, the masses may be set equal to one.) Under these conditions, Gauss' minimized quantity can be written
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| :<math>
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| Z = \sum_{k=1}^{N} \left| \frac{d^{2} \mathbf{r}_{k}}{dt^{2}}\right|^{2}
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| </math>
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| The kinetic energy <math>T</math> is also conserved under these conditions
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| :<math>
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| T \ \stackrel{\mathrm{def}}{=}\ \frac{1}{2} \sum_{k=1}^{N} \left| \frac{d\mathbf{r}_{k}}{dt}\right|^{2}
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| </math>
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| Since the line element <math>ds^{2}</math> in the <math>3N</math>-dimensional space of the coordinates is defined | |
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| :<math>
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| ds^{2} \ \stackrel{\mathrm{def}}{=}\ \sum_{k=1}^{N} \left| d\mathbf{r}_{k} \right|^{2}
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| </math>
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| the conservation of energy may also be written
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| :<math>
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| \left( \frac{ds}{dt} \right)^{2} = 2T
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| </math>
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| Dividing <math>Z</math> by <math>2T</math> yields another minimal quantity
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| :<math>
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| K \ \stackrel{\mathrm{def}}{=}\ \sum_{k=1}^{N} \left| \frac{d^{2} \mathbf{r}_{k}}{ds^{2}}\right|^{2}
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| </math>
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| Since <math>\sqrt{K}</math> is the local [[curvature]] of the trajectory in the <math>3N</math>-dimensional space of the coordinates, minimization of <math>K</math> is equivalent to finding the trajectory of least curvature (a [[geodesic]]) that is consistent with the constraints. Hertz's principle is also a special case of [[Carl Gustav Jakob Jacobi|Jacobi]]'s formulation of [[Maupertuis' principle|the least-action principle]].
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| ==See also==
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| * [[Appell's equation of motion]]
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| ==References==
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| * Gauss CF. (1829) ''Crelle's Journal f. Math., '''4''', 232.
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| * Gauss CF. ''Werke'', '''5''', 23.
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| * Hertz H. (1896) ''Principles of Mechanics'', in ''Miscellaneous Papers'', vol. III, Macmillan.
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| ==External links==
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| *[http://eom.springer.de/g/g043500.htm] Gauss' principle of least constraint
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| *[http://eom.springer.de/H/h047140.htm] Hertz's principle of least curvature
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| {{classicalmechanics-stub}}
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| [[Category:Classical mechanics]]
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