  The penetration by a gravity driven impact of a solid sphere into a granular
medium is studied by two-dimensional simulations. The scaling laws observed
experimentally for both the final penetration depth and the stopping time with
the relevant physical parameters are here recovered numerically without the
consideration of any solid friction. Collisional processes are thus found as
essential in explaining the physics of the qualitatively observed phenomena
whereas frictional processes can only be considered as secondary effects in the
granular penetration by impact.
