Ratio of a collision where the inertial drag force h(z)v² is superior to the quasi-static friction force f(z) for a 1 kg, 10 cm diameter spherical projectile impacting glass beads at different Froude numbers. z_h is the depth where the f(z) = h(z)v² and z_stop is the final penetration depth of the projectile. The vertical dashed line indicates Fr_t, or the transition from collision regime I to regime II, and the horizontal dashed line shows where the inertial drag term exceeds the friction term for more than 50% of the collision.