The principle of least constraint is one variational formulation of classical mechanics enunciated by Carl Friedrich Gauss in 1829, equivalent to all other formulations of analytical mechanics. Intuitively, it says that the acceleration of a constrained physical system will be as similar as possible to that of the corresponding unconstrained system.^[1]

Carl Friederich Gauss

Heinrich Hertz

Gauss and Hertz devised variational principles of mechanics

The principle of least constraint is a least squares principle stating that the true accelerations of a mechanical system of ${\displaystyle n}$ masses is the minimum of the quantity

${\displaystyle Z\,{\stackrel {\mathrm {def} }{=}}\sum _{j=1}^{n}m_{j}\cdot \left|\,{\ddot {\mathbf {r} }}_{j}-{\frac {\mathbf {F} _{j}}{m_{j}}}\right|^{2}}$

where the jth particle has mass ${\displaystyle m_{j}}$, position vector ${\displaystyle \mathbf {r} _{j}}$, and applied non-constraint force ${\displaystyle \mathbf {F} _{j}}$ acting on the mass.

The notation ${\displaystyle {\dot {\mathbf {r} }}}$ indicates time derivative of a vector function ${\displaystyle \mathbf {r} (t)}$, i.e. position. The corresponding accelerations ${\displaystyle {\ddot {\mathbf {r} }}_{j}}$ satisfy the imposed constraints, which in general depends on the current state of the system, ${\displaystyle \{\mathbf {r} _{j}(t),{\dot {\mathbf {r} }}_{j}(t)\}}$.

It is recalled the fact that due to active ${\displaystyle \mathbf {F} _{j}}$ and reactive (constraint) ${\displaystyle \mathbf {F_{c}} _{j}}$ forces being applied, with resultant ${\textstyle \mathbf {R} =\sum _{j=1}^{n}\mathbf {F} _{j}+\mathbf {F_{c}} _{j}}$, a system will experience an acceleration ${\textstyle {\ddot {\mathbf {r} }}=\sum _{j=1}^{n}{\frac {\mathbf {F} _{j}}{m_{j}}}+{\frac {\mathbf {F_{c}} _{j}}{m_{j}}}=\sum _{j=1}^{n}\mathbf {a} _{j}+\mathbf {a_{c}} _{j}}$.

Hertz's principle of least curvature is a special case of Gauss's principle, restricted by the three conditions that there are no externally applied forces, no interactions (which can usually be expressed as a potential energy), and all masses are equal. Without loss of generality, the masses may be set equal to one. Under these conditions, Gauss's minimized quantity can be written

${\displaystyle Z=\sum _{j=1}^{n}\left|{\ddot {\mathbf {r} }}_{j}\right|^{2}}$

The kinetic energy ${\displaystyle T}$ is also conserved under these conditions

${\displaystyle T\ {\stackrel {\mathrm {def} }{=}}\ {\frac {1}{2}}\sum _{j=1}^{n}\left|{\dot {\mathbf {r} }}_{j}\right|^{2}}$

Since the line element ${\displaystyle ds^{2}}$ in the ${\displaystyle 3N}$-dimensional space of the coordinates is defined

${\displaystyle ds^{2}\ {\stackrel {\mathrm {def} }{=}}\ \sum _{j=1}^{n}\left|d\mathbf {r} _{j}\right|^{2}}$

the conservation of energy may also be written

${\displaystyle \left({\frac {ds}{dt}}\right)^{2}=2T}$

Dividing ${\displaystyle Z}$ by ${\displaystyle 2T}$ yields another minimal quantity

${\displaystyle K\ {\stackrel {\mathrm {def} }{=}}\ \sum _{j=1}^{n}\left|{\frac {d^{2}\mathbf {r} _{j}}{ds^{2}}}\right|^{2}}$

Since ${\displaystyle {\sqrt {K}}}$ is the local curvature of the trajectory in the ${\displaystyle 3n}$-dimensional space of the coordinates, minimization of ${\displaystyle K}$ 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 Jacobi's formulation of the least-action principle.

• Appell's equation of motion

• Gauss, C. F. (1829). "Über ein neues allgemeines Grundgesetz der Mechanik". Crelle's Journal. 1829 (4): 232–235. doi:10.1515/crll.1829.4.232. S2CID 199545985.
• Gauss, Carl Friedrich. Werke [Collected Works]. Vol. 5. p. 23–28.
• Hertz, Heinrich (1896). Principles of Mechanics. Miscellaneous Papers. Vol. III. Macmillan.
• Lanczos, Cornelius (1986). "IV §8 Gauss's principle of least constraint". The variational principles of mechanics (Reprint of University of Toronto 1970 4th ed.). Courier Dover. pp. 106–110. ISBN 978-0-486-65067-8.
• Papastavridis, John G. (2014). "6.6 The Principle of Gauss (extensive treatment)". Analytical mechanics: A comprehensive treatise on the dynamics of constrained systems (Reprint ed.). Singapore, Hackensack NJ, London: World Scientific Publishing Co. Pte. Ltd. pp. 911–930. ISBN 978-981-4338-71-4.