Is it allowed, in the context of the Lagrange multiplier formalism, to assume
that nonholonomic constraints are already in effect while setting up Lagrange's
function? This procedure is successfully applied in a recent book [L. N. Hand
and J. D Finch, {\it Analytical Mechanics}] to the problem of the rolling
penny, but it does not work in general, as we show by means of a
counterexample. It turns out that in many cases the use of nonholonomic
constraints in the process of construction of the Lagrangian is allowed, but
the correct equations of motion are the little known Voronec's equations.Comment: Translation of the paper "Vinculos Nao-Holonomos e Equacoes de
Voronec", to be published in Portuguese in Revista Brasileira de Ensino de
Fisic
