  The drag force exerted on an object intruding into granular media can depend
on the object's velocity as well as the depth penetrated. We report on
intrusion experiments at constant speed over four orders in magnitude together
with systematic molecular dynamics simulations well beyond the quasi-static
regime. We find that velocity dependence crosses over to depth dependence at a
characteristic time after initial impact. This crossover time scale, which
depends on penetration speed, depth, gravity and intruder geometry, challenges
current models that assume additive contributions to the drag.
