  We use covariant techniques to examine the implications of the dynamical
equivalence between geodesic motions and adiabatic hydrodynamic flows. Assuming
that the metrics of a geodesically and a non-geodesically moving fluid are
conformally related, we calculate and compare their mass densities. The density
difference is then expressed in terms of the fundamental physical quantities of
the fluid, such as its energy and isotropic pressure. Both the relativistic and
the non-relativistic case are examined and their differences identified. Our
analysis suggests that observational determinations of astrophysical masses
based on purely Keplerian motions could underestimate the available amount of
matter.
