The practice of employing time dependent mechanical parameters in Lagragian dynamics is dicussed. While time dependent 'force constants' are easy to justify, the fact that variable masses may be used derives from the generally unnoticed fact that in the Lagrange equation for the kinetic energy and generalized force the masses are not required to be constants. The corresponding adiabatic invariants are obtained according to method suggested in recently published book, for various kinds of force constants.
Lagrangians; time dependent parameters; adiabatic invariants
