ABSTRACT
According to the correspondence principle, the basics of quantum mechanics and quantum-mechanical equations cannot be properly formulated and fully understood without classical mechanics. Cartesian coordinates are not always the best choice. In many cases, it is convenient to use a different coordinate system. The kinetic energy in generalized coordinates is no longer a function of only velocities but also depends on coordinates. From the principle of least action it also immediately follows that the Lagrangian function is determined up to the total derivative with respect to time of any function of coordinates and time. There are general symmetries namely the symmetry of space and time. It is assumed that space and time are homogeneous and isotropic. One of the main idealizations used in physics is the concept of a material point within which the real size of a physical body is neglected.
