I had a similar issue in trying to determine impact force of a speeding train upon an object placed on the tracks. The assumptions I was forced to make regarding friction, duration of impulse, elasticity, etc. in this complicated system made the resulting answer correct only to ROM at best.
We ended up doing actual impact tests to proof the design, using the plastic deformation from the test on the initial design to allow us to judge relative strength increases in the improved versions, which allowed us to sidestep the need to estimate the actual impact force accurately.
Is it feasible to slap an accelerometer on a prototype and measure the acceleration upon impact directly? I've seen those "G force" stickers that you can just stick on a box to see if it was roughly handled during shipment, which you can purchase and do yourself, although they may not be ideal or practical for this instance. I would think this would give you much more realistic numbers than anything you could calculate. Contact your local testing labs; they may be able to do for you right for a few hundred bucks or so. Heck, you could build your own test setup for future use if you wanted.
Mike
