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Archive October 2007
Wednesday 31 October 2007
This post is about physics. You may want to not bother.

So, I've been arguing in the comments of someone else's livejournal. The dispute is me saying that a car at 110mph colliding head-on with a stationary car is an equivalent impact to a car at 55mph colliding head-on with another car going the opposite way at 55mph. The other person is arguing that a car going at 55mph has 313kJ of kinetic energy, and a car going 110mph has 1252kJ of kinetic energy, so the collision at 110mph must be more severe. (1252 > 313+313)

I was frankly surprised that he would believe this because it's self-evident to me that he is wrong, from two or three distinct physics approaches that all agree - but he's not a stupid guy, and I couldn't immediately explain why he is wrong, only why I am right. His math is right, and kinetic energy is proportional to velocity squared. After a little research I found the answer, and after a bit of argument also found why he was so fixated on the wrong answer - because it has been argued a lot with a slightly different situation, in which he would be right. A 55mph car colliding head-on with another 55mph car is not equivalent to a 110mph car hitting a wall. It is, in fact, equivalent (for the car's occupants) to a 55mph car hitting a wall. But it is also equivalent to a 110mph car hitting a stationary identical car.

But how and why is he wrong about the kinetic energy? I'll explain it in comments later if anyone is interested and nobody else has explained it. [07:17] [4 comments]