Editing Analytical mechanics (section)

{{short description|Overview of mechanics based on the least action principle}}
{{Classical mechanics|cTopic=Formulations}}

In [[theoretical physics]] and [[mathematical physics]], '''analytical mechanics''', or '''theoretical mechanics''' is a collection of closely related formulations of [[classical mechanics]]. Analytical mechanics uses ''[[Scalar (physics)|scalar]]'' properties of motion representing the system as a whole—usually its [[kinetic energy]] and [[potential energy]]. The [[equations of motion]] are derived from the scalar quantity by some underlying principle about the scalar's [[calculus of variations|variation]]. 

Analytical mechanics was developed by many scientists and mathematicians during the 18th century and onward, after [[Newtonian mechanics]]. Newtonian mechanics considers [[Euclidean vector|vector]] quantities of motion, particularly [[acceleration]]s, [[Momentum|momenta]], [[force]]s, of the constituents of the system; it can also be called ''vectorial mechanics''.<ref name=Lanczos>{{cite book |title=The variational principles of mechanics |last=Lanczos |first=Cornelius |page=Introduction, pp. xxi–xxix |edition=4th |publisher=Dover Publications Inc. |location= New York |isbn=0-486-65067-7 |year=1970 |url=https://books.google.com/books?id=ZWoYYr8wk2IC&pg=PR4 |no-pp=true}}</ref> A scalar is a quantity, whereas a vector is represented by quantity and direction. The results of these two different approaches are equivalent, but the analytical mechanics approach has many advantages for complex problems.

Analytical mechanics takes advantage of a system's ''constraints'' to solve problems. The constraints limit the [[Degrees of freedom (physics and chemistry)|degrees of freedom]] the system can have, and can be used to reduce the number of coordinates needed to solve for the motion. The formalism is well suited to arbitrary choices of coordinates, known in the context as [[generalized coordinates]]. The kinetic and potential energies of the system are expressed using these generalized coordinates or momenta, and the equations of motion can be readily set up, thus analytical mechanics allows numerous mechanical problems to be solved with greater efficiency than fully vectorial methods. It does not always work for non-[[conservative force]]s or dissipative forces like [[friction]], in which case one may revert to Newtonian mechanics.

Two dominant branches of analytical mechanics are [[Lagrangian mechanics]] (using generalized coordinates and corresponding generalized velocities in [[Configuration space (physics)|configuration space]]) and [[Hamiltonian mechanics]] (using coordinates and corresponding momenta in [[phase space]]). Both formulations are equivalent by a [[Legendre transformation#Hamilton–Lagrange mechanics|Legendre transformation]] on the generalized coordinates, velocities and momenta; therefore, both contain the same information for describing the dynamics of a system. There are other formulations such as [[Hamilton–Jacobi theory]], [[Routhian mechanics]], and [[Appell's equation of motion]]. All equations of motion for particles and fields, in any formalism, can be derived from the widely applicable result called the [[principle of least action]]. One result is [[Noether's theorem]], a statement which connects [[conservation law]]s to their associated [[Symmetry (physics)|symmetries]].

Analytical mechanics does not introduce new physics and is not more general than Newtonian mechanics. Rather it is a collection of equivalent formalisms which have broad application. In fact the same principles and formalisms can be used in [[relativistic mechanics]] and [[general relativity]], and with some modifications, [[quantum mechanics]] and [[quantum field theory]].

Analytical mechanics is used widely, from fundamental physics to [[applied mathematics]], particularly [[chaos theory]].

The methods of analytical mechanics apply to discrete particles, each with a finite number of degrees of freedom. They can be modified to describe continuous fields or fluids, which have infinite degrees of freedom. The definitions and equations have a close analogy with those of mechanics.