Abstract We study the analog of Snell's law for particles moving across the interface of two regions with two different potential energies, from two different points of view. First, a simple demonstration involving marbles and a potential step is shown. Then, from a theoretical point of view, this law describing mechanical refraction is derived from the Maupertuis and Jacobi variational principles, in close analogy with the well known derivation of Snell's law for refraction from Fermat's least time principle. Finally, the relativistic version of mechanical refraction is obtained by the Maupertuis principle, by trading the Newtonian dispersion relation with the relativistic one. The pedagogical significance of this treatment is discussed.

Examinando em detalhes o mecanismo que produz o movimento das portas corrediças, observa-se que o movimento desses objetos usualmente usados podem ser entendidos resolvendo um problema modelado por blocos e polias. O sistema é per si interessante para ser modelado como um problema na mecânica newtoniana e a solução da equação do movimento pode nos levar a estimar o tempo de abertura e fechamento de uma porta corrediça.

Looking closely to the mechanism which make sliding doors move, one sees that the motion of the commonly used objects can be understood by solving a problem related to the motion of blocks and pulleys. The system is by itself interesting to be modelled as a problem in Newtonian mechanics, and the solution of the equation of the motion can lead us to estimate the time it takes for a sliding door to open or close.