  Previous work established a universal form for the equation of motion of
small bodies in theories of a metric and other tensor fields that have
second-order field equations following from a covariant Lagrangian in four
spacetime dimensions. Differences in the motion of the "same" body in two
different theories are entirely accounted for by differences in the body's
effective mass and charges in those different theories. Previously the process
of computing the mass and charges for a particular body was left implicit, to
be determined in each particular theory as the need arises. I now obtain
explicit expressions for the mass and charges of a body as surface integrals of
the fields it generates, where the integrand is constructed from the symplectic
current for the theory. This allows the entire prescription for computing the
motion of a small body to be written down in a few lines, in a manner universal
across bodies and theories. For simplicity I restrict to scalar and vector
fields (in addition to the metric), but there is no obstacle to treating
higher-rank tensor fields. I explicitly apply the prescription to work out
specific equations for various body types in Einstein gravity, generalized
Brans-Dicke theory (in both Jordan and Einstein frames), Einstein-Maxwell
theory and the Will-Nordvedt vector-tensor theory. In the scalar-tensor case,
this clarifies the origin and meaning of the "sensitivities" defined by Eardley
and others, and provides explicit formulae for their evaluation.
