  We present a unified geometric framework for describing both the Lagrangian
and Hamiltonian formalisms of contact autonomous mechanical systems, which is
based on the approach of the pionnering work of R. Skinner and R. Rusk. This
framework permits to skip the second order differential equation problem, which
is obtained as a part of the constraint algorithm (for singular or regular
Lagrangians), and is specially useful to describe singular Lagrangian systems.
Some examples are also discussed to illustrate the method.
