  Predicting the force exerted on an object as it penetrates a granular medium
is of interest in engineering, locomotive, and geotechnical applications.
Current models of granular drag, however, vary widely in applicability and
parameterization, and the physical origin of the granular resistive force
itself is a subject of debate. Here we perform constant speed penetration
experiments, combined with calibrated, large-scale molecular dynamics
simulations, at velocities up to 2 m/s to test the effect of impact velocity on
the depth dependent `hydrostatic' drag force. We discover that the evolution of
the granular flow field around an intruder regulates the presence of depth
dependent drag forces. In addition, we find that the observed linear depth
dependence is commensurate with the mass of flowing grains. These results
suggest that, as the impact speed increases beyond the quasistatic regime, the
depth dependent drag term becomes intertwined with inertial effects.
