  The existence of radial solutions of a nonlinear Dirichlet problem in a ball
is translated to the language of Mechanics, i.e. to requirements on the time of
motion of a particle in an external potential and under the action of a
viscosity force. This approach reproduces existing theorems and, in principle,
provides a method for the analysis of the general case. Examples of new
theorems are given, which prove the usefulness of this qualitative method.
