Tuesday, December 11, 2012

Speed doesn't kill, Δv does

People always like to say "Speed Kills" but really, it isn't the speed that kills, it is the acceleration when you hit something.  If speed killed, we would probably all be dead.  The earth's rotational speed at the equator is over 1,000 mph.  Our rotation around the sun is at 67,000 mph.  Our galaxy is moving at a blistering 600 km/s relative to the universe (600 km/s is about 1.3 MILLION miles per hour)[1].  Going 85 mph is not going to suddenly kill me.  Hitting a wall at 85 mph might do it though, but that is caused by acceleration.

This is why crumple zones exist.  The idea of crumple zones is to make a crash last as long as possible.  That may sound counter intuitive, but velocity = acceleration x time.  If you are going from 85 mph -> 0 mph, the idea is to extend the time to reduce the amount of acceleration and thus reduce the amount of force exerted on your body (because force = mass x acceleration). These are the things you learn in physics class.  I could do more math to prove the point, but why bother?

I will say there is some point where speed can kill when that speed is sufficiently different than the surrounding particles (such as air).  For instance, having something travel at 9/10s of the speed of light at ground level is bad!  For comparison, that is 604 MILLION miles per hour.  I don't know where that threshold is where things go from ok, to painful, to deadly, but people aren't reffering to such extreme speeds when they say "Speed Kills".

The point is, next time someone tells you speed kills, let them know they are wrong in the practical sense.  Well, unless it is a police officer who pulled you over, they might not find it amusing.

While having this discussion with a co-worker, she posed an interesting question which I have told her to submit to what-if.xkcd.com, so I won't spoil that by posting it here, but I hope it is picked because I would love to read the response to that one.

[1]: http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/971028e.html

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