  We study the interaction between gas and dust particles in a protoplanetary
disk, comparing analytical and numerical results. We first calculate
analytically the trajectories of individual particles undergoing gas drag in
the disk, in the asymptotic cases of very small particles (Epstein regime) and
very large particles (Stokes regime). Using a Boltzmann averaging method, we
then infer their collective behavior. We compare the results of this analytical
formulation against numerical computations of a large number of particles.
Using successive moments of the Boltzmann equation, we derive the equivalent
fluid equations for the average motion of the particles; these are
intrinsically different in the Epstein and Stokes regimes. We are also able to
study analytically the temporal evolution of a collection of particles with a
given initial size-distribution provided collisions are ignored.
