  Using the Hamilton-Jacobi formalism, we study extra force and extra mass in a
recently introduced noncompact Kaluza-Klein cosmological model. We examine the
inertial 4D mass $m_0$ of the inflaton field on a 4D FRW bulk in two examples.
We find that $m_0$ has a geometrical origin and antigravitational effects on a
non inertial 4D bulk should be a consequence of the motion of the fifth
coordinate with respect to the 4D bulk.
