  This work summarizes a series of studies on two-dimensional granular impact,
where an intruding object strikes a granular material at high speed. Many
previous studies on granular impact have used a macroscopic force law, which is
dominated by an inertial drag term proportional to the intruder velocity
squared. The primary focus here is on the microscopic force response of the
granular material, and how the grain-scale effects give rise to this inertial
drag term. We show that the inertial drag arises from intermittent collisions
with force-chain-like structures. We construct a simple collisional model to
explain the inertial drag, as well as off-axis instability and rotations.
Finally, we show how the granular response changes when the intruder speed
approaches $d/t_c$, leading to a failure of the inertial drag description in
this regime. Here, $d$ is the mean particle diameter and $t_c$ the
characteristic momentum-transfer time between two grains.
