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It is the natural frame to work in for this situation. We are looking at the impact speed of the hail, which is most naturally compared when the car velocity is zero and the hail moves at differing speed. If you need more examples, how about meteoroids hitting the space station, or for that matter any highly assymetrical collision? I think secretly you agree with me, but are just embarassed that you stuck your foot in it and are now trying to cover-up. Keep studying, you'll do fine as you gain experience.
- John

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