  We study the dynamic process occurring when a granular assembly is displaced
by a solid impactor. The momentum transfer from the impactor to the target is
shown to occur through sporadic, normal collisions of high force carrying
grains at the intruder surface. We therefore describe the stopping force of the
impact through a collisional based model. To verify the model in impact
experiments, we determine the forces acting on an intruder decelerating through
a dense granular medium using high-speed imaging of its trajectory. By varying
the intruder shape and granular target, intruder-grain interactions are
inferred from the consequent path. As a result, we connect the drag to the
effect of intruder shape and grain density based on a proposed collisional
model.
