  We experimentally and computationally study the early-stage forces during
intruder impacts with granular beds in the regime where the impact velocity
approaches the granular force propagation speed. Experiments use 2D assemblies
of photoelastic disks of varying stiffness, and complimentary discrete-element
simulations are performed in 2D and 3D. The peak force during the initial
stages of impact and the time at which it occurs depend only on the impact
speed, the intruder diameter, the mass density of the grains, and the elastic
modulus of the grains according to power-law scaling forms that are not
consistent with Poncelet models, granular shock theory, or added-mass models.
The insensitivity of our results to many system details suggest that they may
also apply to impacts into similar materials like foams and emulsions.
