  We derive a formalism, within the theory of linear response, for the analysis
of the interaction of a satellite (the perturber) with a spherical galaxy whose
equilibrium is described by a one-particle distribution function. We compute
the formal expression of the force on the satellite including the self-gravity
of the stars and the shift of the stellar center of mass. We apply the
perturbative technique to the case of a satellite moving at high speed across a
stellar system and find a natural decomposition of the force into a global
component resulting from the tidal interaction and a component that is related
to dynamical friction. When the satellite orbits outside the galaxy, we derive
the force in the impulse approximation. In penetrating shortlived encounters,
the wake excited in the density field is responsible for most of the
deceleration. We find that dynamical friction rises from a memory effect
involving only those stars perturbed along the path. The force can be written
in terms of an effective Coulomb logarithm which now depends on the dynamical
history. It is derived for two simple equilibrium density distributions. In the
case of a homogeneous cloud, we compute the total energy loss: Tides excited by
the satellite in the galaxy reduce the value of the energy loss by friction.
