  We present two methods for describing, from a pedagogical point of view, the
solutions of Einstein's equations applied to a homogeneous and isotropic
universe. In the first method, we define an effective gravitational potential
of the universe, and the second method makes use of the fact that we can define
a dynamical system of equations. The methods are applied to different
cosmological models whose properties are discussed in turn. The ultimate
intention is to provide simple examples to be revised in a first Cosmology
course for undergraduates.
