# Hamilton's principle

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## Hamilton's principle

A variational principle from which can be derived the equations of motion of a classical dynamical system in which friction or other forms of dissipation of energy do not occur. In the original formulation of Newton's laws of motion, the position of each particle of the system of interest is specified by the cartesian coordinates of that particle. In many cases, these coordinates are not all independent of each other or do not reflect the structure of the system in a convenient way. It is then advantageous to introduce a system of generalized coordinates which are independent of each other and do reflect any special features of the system such as its symmetry about some center. The number of degrees of freedom of the system, *f*, is the number of such coordinates required to specify the configuration of the system at any time. *See* Degree of freedom (mechanics)

The problem of determining how a system moves may be formulated in the following way: If the configuration of the system at time *t*_{1} is specified by the generalized coordinates *q*_{1}(*t*_{1}), …, *q*_{f}(*t*_{1}) and at the time *t*_{2} by *q*_{1}(*t*_{2}), …, *q*_{f}(*t*_{2}), then it is required to find the trajectory along which the system travels from the initial to the final configuration. Hamilton's principle addresses this problem similarly to the way that a geometer addresses the problem of finding the shortest path lying in a curved surface between two given points on the surface. The geometer specifies the distance *ds* between any two close-lying points in terms of the coordinates *q*_{i} of the two points and their differences, the coordinate differentials *dq** _{i}*, as in Eq. (1).

*D*between the two specified points, given by the integral in Eq. (2), is then required to be a minimum. Hamilton defined a characteristic function &PHgr;, analogous to

*D*, by Eq. (3),

*L*(

*q*, …,

*t*) of the system in a way analogous to the geometer's

*g*. Hamilton's principle states that the system follows the trajectory that makes the integral in Eq. (3) have a minimum value, provided the time interval between times

*t*

_{1}and

*t*

_{2}is not too great. It can be shown that this principle implies Lagrange's equations of motion for the system, and that it follows from Lagrange's equations.

*See*Lagrange's equations, Lagrangian function, Least-action principle, Minimal principles, Variational methods (physics)