  We study the origin of buoyancy forces acting on a larger particle moving in
a granular medium subject to horizontal shaking and its corrections before
fluidization. In the fluid limit Archimedes' law is verified; before the limit
memory effects counteract buoyancy, as also found experimentally. The origin of
the friction is an excluded volume effect between active particles, which we
study more exactly for a random walker in a random environment. The same
excluded volume effect is also responsible for the mutual attraction between
bodies moving in the granular medium. Our theoretical modeling proceeds via an
asymmetric exclusion process, i.e., via a dissipative lattice gas dynamics
simulating the position degrees of freedom of a low density granular sea.
