As widely accepted, justified by the historical developments of physics, the background for standard formulation of postulates of physical theories leading to equations of motion, or even the form of equations of motion themselves, come from empirical experience. Equations of motion are then a starting point for obtaining specific conservation laws, as, for example, the well-known conservation laws of momenta and mechanical energy in mechanics. On the other hand, there are numerous examples of physical laws or equations of motion which can be obtained from a certain variational principle as Euler-Lagrange equations and their solutions, meaning that the \true trajectories" of the physical systems represent stationary points of the corresponding functionals.

It turns out that equations of motion in most of the fundamental theories of physics (as e.g. classical mechanics, mechanics of continuous media or fluids, electrodynamics, quantum mechanics, string theory, etc.), are Euler-Lagrange equations of an appropriately formulated variational principle. There are several well established geometrical theories providing a general description of variational problems of different kinds. One of the most universal and comprehensive is the calculus of variations on fibred manifolds and their jet prolongations. Among others, it includes a complete general solution of the so-called strong inverse variational problem allowing one not only to decide whether a concrete equation of motion can be obtained from a variational principle, but also to construct a corresponding variational functional. Moreover, conservation laws can be derived from symmetries of the Lagrangian defining this functional, or directly from symmetries of the equations.

In this paper we apply the variational theory on jet bundles to tackle some fundamental problems of physics, namely the questions on existence of a Lagrangian and the problem of conservation laws. The aim is to demonstrate that the methods are universal, and easily applicable to distinct physical disciplines: from classical mechanics, through special relativity, waves, classical electrodynamics, to quantum mechanics.

[1] I. M. Anderson: The Variational Bicomplex. (1989). Book preprint, technical report of the Utah State University

[2] M. de León,P. R. Rodrigues: Generalized Classical Mechanics and Field Theory. North-Holland, Amsterdam (1985).

[3] G. Giachetta, L. Mangiarotti, G. Sardanashvily: New Lagrangian and Hamiltonian Methods in Field Theory. World Scientific, Singapore (1997).

[4] I. S. Krasilschik, V. V. Lychagin, A. M. Vinogradov: Geometry of Jet Spaces and Differential Equations. Gordon and Breach (1986).

[5] M. Krbek, J. Musilová: Representation of the variational sequence by differential forms. Acta Appl. Math. 88 (2) (2005) 177-199.

[6] D. Krupka: Introduction to Global Variational Geometry. Atlantis Press (2015).

[7] D. Krupka, J. Musilová: Trivial lagrangians in field theory. Differential Geometry and its Applications 9 (1998) 293-305.

[8] O. Krupková: The Geometry of Ordinary Differential Equations. Lecture Notes in Mathematics 1678 (1997).

[9] L. D. Landau, E. M. Lifshitz: Quantum Mechanics: Non-Relativistic Theory. Pergamon Press (1977). 3rd ed.

[10] L. D. Landau, E. M. Lifshitz: he Classical Theory of Fields. Pergamon Press (1975). 3rd ed.

[11] M. Palese, O. Rossi, E. Winterroth, J. Musilová: Variational Sequences, Representation Sequences and Applications in Physics. SIGMA 12 (2016) 1-44.

[12] O. Rossi, J. Musilová: The relativistic mechanics in a nonholonomic setting: a unified approach to particles with non-zero mass and massless particles. J. Phys. A: Math. Theor. 45 (2012). 255202, 27 pp.

[13] G. Sardanashvily: Noether's Theorems, Applications in Mechanics and Field Theory. Atlantis Studies in Variational Geometry, Atlantis Press (2016).

[14] D. J. Saunders: The Geometry of Jet Bundles. Cambridge University Press, Cambridge (1989).