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Comments on 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. [16:17]

Evan
I'll bite, care to explain the physics behind it?

RavenBlack
Explained in the LJ copy's comments here: http://ravenblack.livejournal.com/235443.html

Jade
i dont have a live journal and my post comes 10 months too late, but i am confused.
you keep calculating the 55mph and 110mph cars energies as a numeric value for kinetic energy which you simply double with the doubled speed. certainly this is is inaccurate since it takes more energy (and thus builds more kinetic energy) to get from 55mph to 110mph than it does to get from 0mph to 55mph.
that said, the amount of kinetic energy in a car traveling at 110mph is not a multiplicative of the energy in the 55mph car, but rather it is a factor of the energy used to get the car from 0mph to 1mph eg: "110 factor (*kinetic energy used to accelerate* + 1)" or something similar.

RavenBlack
You don't need a livejournal to read the comments, it's a public entry.
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