  Impact dynamics is measured for spherical and cylindrical projectiles of many
different densities dropped onto a variety non-cohesive granular media. The
results are analyzed in terms of the material-dependent scaling of the inertial
and frictional drag contributions to the total stopping force. The inertial
drag force scales similar to that in fluids, except that it depends on the
internal friction coefficient. The frictional drag force scales as the
square-root of the density of granular medium and projectile, and hence cannot
be explained by the combination of granular hydrostatic pressure and Coulomb
friction law. The combined results provide an explanation for the
previously-observed penetration depth scaling.
