Regarding the head-on vehicle collision. There are far too many variables to even begin to answer that question without seeing the damage to the vehicles (distortion) and movement of the vehicles after contact and making assumptions of occupant movement inside the vehicle. Even the insurance companies do not often try to make these calculations with highly trained accident reconstruction specialists.

When G forces are measured, there are 3 key factors. Speed before collision, speed after collision and the amount of time for that change in speed. The change in speed over time is either acceleration or deceleration and can be converted to G forces. If there is a complete stop, then deflection can be used in place of the time for that change in speed.

That tennis ball is traveling at 55555 mm/sec. It would likely deflect by 12 mms or more. But, if the skull it hits moves back 6 inches (150 mms) during the contact, that movement is part of the deflection. So, the deflection would be 162 mms. The formula for G force is V1 squared minus V2 squared divided by 2 times distortion divided by acceleration of gravity. So, 55555 squared minus 0 squared divided by 324 divided by 9800. That means the tennis ball experienced 972 G's. But the head did not experience the same G force since it started at 0 velocity and ended at X velocity and moved 6 inches (150 mms) during contact with the ball. That calculation is far more complex. This is where the mass of each object would be needed to calculate the G force to determine velocity X. Even so, assumptions would need to be made.

So, hopefully, you see that it is extremely difficult to calculate these forces.