Ahhhhh, OK. I see what you mean now. I'm probably thinking of them as combining because my book has given so many examples of them combining entirely.
And as far as an existential crisis, you might be right, haha. Seeing as physics is my weakest area and I'm taking the MCAT in two weeks. :|
I see how the masses are different, but if a perfectly inelastic collision is when the two stick together, wouldn't their masses combine into one? I see how if you look at them with separate masses even after they collide they would have different momenta. But, even though they've stuck...