Tuesday, May 3, 2011

Boom!

Be as a tower firmly set; Shakes not its top for any blast that blows.
-Dante
Gizmodo.com published a clever story on whether people can actually survive the kinds of explosions seen in movies. The short answer is - No.

Jumping behind a car or wall may save you from flying shrapnel but the main force of an explosion is the blast wave of air that flows over and around objects (or blasts them out of its way). Normal air pressure is 14 pounds per square inch (psi) and amazingly the human body can survive up to 400 psi if the pressure is increased very slowly. But a quick change in pressure of 20-40 psi is fatal. Buildings can crumble under pressure anywhere between 5 and 20 psi. That may not sound like much but keep in mind that a 100 mph wind blowing on a buliding will only extert .2 psi.

The damage of a blast is a combination of the force and the time over which the force is applied. Rapid changes in pressure can rupture the lungs and the bowels.
A wave of pressure that lasts less than .3 milliseconds leaves the eardrum no time to adjust to changes in pressure, and simply tears it. This can happen with pressure change as small as 5 psi.
I was suprised to read that a pressure change of just 5 psi can create 160 mile-per-hour winds - serious hurricane force! And while the body might physically survive a change of 20 psi, that pressure can create winds of 470 miles per hour which will lift people in the air and toss them on to something with fatal impact.

Bottom Line

The big fire balls in movies represent
"military-grade explosives which unleash millions of pounds per square inch of pressure. Anything near it is getting destroyed. For more modest explosives, the best defense is distance. Since force is applied over area, it decreases by the square of the distance it travels. Run like hell."

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