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The Lagrangian of a given system is not unique. A Lagrangian L can be multiplied by a nonzero constant a and shifted by an arbitrary constant b, and the new Lagrangian L′ = aL + b will describe the same motion as L.
A subset of the constants of motion are the integrals of motion, or first integrals, defined as any functions of only the phase-space coordinates that are constant along an orbit. Every integral of motion is a constant of motion, but the converse is not true because a constant of motion may depend on time. [ 2 ]
is conserved throughout the motion (on the physical path). Thus, the absence of the ignorable coordinate q k from the Lagrangian implies that the Lagrangian is unaffected by changes or transformations of q k; the Lagrangian is invariant, and is said to exhibit a symmetry under such transformations. This is the seed idea generalized in Noether's ...
The relativistic Lagrangian can be derived in relativistic mechanics to be of the form: = (˙) (, ˙,). Although, unlike non-relativistic mechanics, the relativistic Lagrangian is not expressed as difference of kinetic energy with potential energy, the relativistic Hamiltonian corresponds to total energy in a similar manner but without including rest energy.
Lagrangian field theory is a formalism in classical field theory. It is the field-theoretic analogue of Lagrangian mechanics. Lagrangian mechanics is used to analyze the motion of a system of discrete particles each with a finite number of degrees of freedom.
The azimuth , being absent from the Lagrangian, is a cyclic coordinate, which implies that its conjugate momentum is a constant of motion. The conical pendulum refers to the special solutions where θ ˙ = 0 {\displaystyle {\dot {\theta }}=0} and ϕ ˙ {\displaystyle {\dot {\phi }}} is a constant not depending on time.
Action principles are "integral" approaches rather than the "differential" approach of Newtonian mechanics.[2]: 162 The core ideas are based on energy, paths, an energy function called the Lagrangian along paths, and selection of a path according to the "action", a continuous sum or integral of the Lagrangian along the path.
The solution can be related to the system Lagrangian by an indefinite integral of the form used in the principle of least action: [5]: 431 = + Geometrical surfaces of constant action are perpendicular to system trajectories, creating a wavefront-like view of the system dynamics. This property of the Hamilton–Jacobi equation connects ...