  We present a time-dependent calculation of the velocity-dependent forces
which act on an excited atomic dipole in relative motion with respect to ground
state atoms of a different kind. Both, its interaction with a single atom and
with a dilute atomic plate are evaluated. In either case, the total force
consists of a conservative van der Waals component and a non-conservative
Rontgen component. On physical grounds, the former corresponds to the
velocity-dependent recoil experienced by the excited atom in the processes of
absorption and emission of the photons that it exchanges with the ground state
atoms on a periodic basis. The latter corresponds to the time-variation of the
Rontgen momentum, which is also mediated by the periodic exchange of
quasi-resonant photons. We find that, at leading order, all these interactions
are linear in the velocity. In the non-retarded regime the van der Waals force
dominates, being antiparallel to the velocity. On the contrary, in the retarded
regime the velocity-dependent forces oscillate in space, van der Waals and
Rontgen forces are of the same order in the atom-atom interaction, and the
Rontgen component dominates in the atom-surface interaction.
