I hear this from people sometimes. Upon hearing about a bad head on collision, I will hear a person promptly add the two speeds together and claim the wreck for the drivers is as bad as slamming into a brick wall with that new speed (S = s1+s2). Is this really a good analogy? I'm not so sure it is. When two cars collide head on mv = F[tex]\Delta[/tex]t applies, and the time it takes for the two cars to stop when they hit might be longer than slamming into a brick wall, so the force (which is what I think people mean by "is the same as running into...") could be smaller. I realize I'm not being extremely specific about the crash so there are a lot of things to nitpick over, but lets say we're talking about an inelastic accordion-like wreck on a frictioney (made up word) surface. Am I wrong or does this analogy to a brick wall fall apart?