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Old May 15 2012, 05:37 PM   #97
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Re: Heads up! MYTHBUSTERS season premiere tonight at 9 Eastern!

Bouncing bullets: I'm a little surprised it even came as close to working as it did, in terms of the ricochet off the asphalt. I guess asphalt's a harder material than I figured. But I hadn't realized there was so much sturdy, bullet-resistant stuff underneath the floor of a car.

I was surprised to hear them talking about the "firewall" as part of the construction of a car. I figured that "firewall" as a computing term had to be named for something more physical, but I never would've thought it referred to a piece of a car. Although I just looked it up and apparently it's mainly used in construction to refer to a fire-resistant wall of a building. The firewall in a car is what separates the passenger compartment from the engine; is it called that because it blocks the heat from the engine, or because it provides temporary protection to the passengers if the engine catches fire?

The rig they built to get high-speed footage of bullet ricochets while the truck was in motion was pretty ingenious.

Shock wave riding: Not too surprised by the result, since I'm not sure I'd expect a shock wave to have a "pushing" effect at all. I mean, it isn't really a substance moving forward; it's just an outwardly propagating density/pressure increase in the air. It's like a wave in the ocean. If you're on a boat in the ocean and a big wave comes by, it doesn't so much push you forward as lift you up and then lower you again. The water in a wave doesn't actually move forward, just rises up and down, so an object in that water would be affected the same way. I know surfers can ride waves, but I think they already have to have built up a certain amount of speed. From the footage I've seen, if you're sitting still on a surfboard when a wave crest reaches you, you just bob up and down. So maybe surfing isn't so much about being pushed forward by a wave as it is about propelling yourself forward until you can match velocity with the wave. Ah, and the Wikipedia article on surfing bears this out. Surfers do have to match velocity with the waves before they hit, and they need to be towed by a boat to match velocity with really fast waves. So the waves aren't actually propelling the surfers at all.

So really, there's no reason this should have worked, and the Mythbusters shouldn't have been surprised by its failure. The notion that a shock wave even could push someone forward is a misunderstanding of the physics of the phenomenon. Sure, the actual expanding gases from the explosion push stuff outward -- that's how an explosion works in the first place -- but the shock wave that hit Buster isn't made of those gases; it's just a compression in the surrounding air as a consequence of the explosion. So all Buster would've felt was a temporary, sharp increase in air pressure, not an acceleration.
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