I realize that when someone refers to a vague scenario about a "friend" who did something, people often jump to conclusions and assume they are sharing an embarrassing personal anecdote. However that does not apply at all here. Recently, I was in a little fender bender with one of my friends (his/her identity remaining undisclosed) and it was unfortunately a rear end collision. I'm not sure if I could've been in a scenario that screamed momentum any more that this one.
If we treat this like an inelsatic collision (energy is not conserved since there is definitely energy lost to heat/friction) we know that M1V1 + M2V2 = (M1 + M2)V' assuming that they stick together for a short amount of time before braking. If we assume that the mass of our car was 2 tons (1814.37 kg) and we were traveling at about 15 mph (6.7 m/s) and that the other car weighed about 1.2 tons (1088.62 kg) and was at rest, then as a system the two would have a final velocity of about 4.2 m/s (9.4 mph).
When all was said and done the experience of an accident obviously was not fun, however it was a pleasure to blog about.
(p.s. the collision below was not even close to what happened but I thought it looked pretty dumb)