  We discuss the role of diffuse, Mie and cooperative scattering on the
radiation pressure force acting on the center of mass of a cloud of cold atoms.
Even though a mean-field Ansatz (the `timed Dicke state'), previously derived
from a cooperative scattering approach, has been shown to agree satisfactorily
with experiments, diffuse scattering also describes very well most features of
the radiation pressure force on large atomic clouds. We compare in detail an
incoherent, random walk model for photons and a diffraction approach to the
more complete description based on coherently coupled dipoles. We show that a
cooperative scattering approach, although it provides a quite complete
description of the scattering process, is not necessary to explain the previous
experiments on the radiation pressure force.
