January 18th, 2011

planet

mythbusters

I think I just saw my favorite Mythbusters episode ever. It's episode #143 and the myth is "Two cars crashing into each other at 50 mph will result in the same damage (for each car) as a single car hitting a wall at 50 mph."

In the episode they do three things:
  1. Crash two cars into each other head-on, each traveling at 50 mph
  2. Crash a car into a wall at 50 mph
  3. Crash a car into a wall at 100 mph
You might think that another car coming at you would do more damage than a wall, but that's not the way momentum works. It reminds me of one my dad's favorite physics concepts: Newton's Third Law of motion. For every action, there is an equal and opposite reaction. We'd be driving along, and a bug would hit the windshield, and dad would talk about how it seems like that bug went from hovering above the earth to zooming (dead) along the freeway at 60 mph, but the change wasn't actually instant, our car just slowed down a teeny tiny bit and the bug accelerated up to 60 mph. No comment from the bug.

Side note: Dad also liked to bring up the Pythagorean theorum whenever possible, taught me to count in binary when I was eight, and convinced me it was very important to memorize at least 10 digits of pi. 3.141592654. Until I was well into high school, I assumed everyone had a parent who did stuff like this.

So anyway, the car that hits the wall at 100 mph is smashed to bits, it's obviously very different from the cars that hit a wall or oncoming car at 50 mph. Reason? Because it takes a certain "equal opposite reaction" to stop a car going that fast... and it takes a very different reaction to stop a car going 100 mph. It doesn't matter where the reaction comes from, it's going to be the same force. The other car is going 50 mph but it gives a lot more than the wall, which is immovable and able to exert a different force just by being where it's at. fun.