  In a gravitational virialized bound system built up of two components, one of
which is embedded in the other, the Clausius' virial energy of one subcomponent
is not, in general, equal to its total potential energy, as occurs in a single
system without external forces. This is the main reason for the presence, in
the case of two non-coinciding concentric spheroidal subsystems, of a minimum
(in absolute value) in the Clausius' virial of the inner component B, when it
assumes a special configuration characterized by a value of its semi-major axis
we have named "tidal radius". The physical meaning, connected with its
appearance, is to introduce a scale length on the gravity field of the inner
subsystem, which is induced from the outer one. Its relevance in the galaxy
dynamics has been stressed by demonstrating that some of the main features of
the Fundamental Plane may follow as consequence of its existence. More physical
insight into the dynamics of a two component system may be got by looking at
the location of this scale length inside the plots of the potential energies of
each subsystem and of the whole system and by also taking into account the
trend of the anti-symmetric residual-energy, that is the difference between the
tidal and the interaction-energy of each component. Some thermodynamical
arguments related to the inner component are also added to prove as special is
the "tidal radius configuration". Moreover the role of the divergency at the
center of the two subsystems in obtaining this scale length is considered. For
the sake of simplicity the analysis has been performed in the case of a frozen
external component even if this constraint does not appear to be too relevant
in order to preserve the main results.
