Sunday, September 17, 2006

Swords and Thrown Spears

Yesterday I watched a Mythbusters Mega Movie Myths special episode. Mythbusters looks at those questions you've always wondered about. Could a medieval Chinese would-be astronaut have ascended to heaven in a rocket propelled chair? Could methane buildup trigger a port-a-john explosion? And, if not, can we find an excuse to blow something up anyway? "Well, Jamie, it looks from our controlled experiment that it's plausible that someone could have used a stick of dynamite to try and clean the hardened cement out of their cement truck. Looks like a bunch of the cement got chipped off. Now let's see what happens when we use 850 pounds of mining explosive. Folks, don't try this at home." Great fun.

Happily, one of the movie myths they looked at was "the scene where one guy slices through the other guy's sword". Step one demonstrated that none of them were particularly competent at swinging swords. They then brought in an experienced test cutter, filmed him slicing through tatami mats and deeply into ballistic gel, and calculated the speed of the sword's point of percussion at impact. This turned out to be 48 mph. More on this later.

They then built a swinging arm to give the same velocity, which replicated the ballistic gel performance. Cutting to the chase, unless it's a low quality sword, you can't cut it, but you can break it.

Now, we also have data on how fast a modern javelin can be thrown: nearly 70 mph for an 800 gram (1.76 lbs)javelin. The runup is a significant contributor to the difference in velocity. Since kinetic energy increases with the square of velocity, a mass moving at 70 mph has more than twice the kinetic energy of the same mass at 48 mph.

For comparison, Hardy's Longbow claims that a 70 lb bow can shoot a 57.4 gram arrow with a long bodkin point at 97.5 MPH. The javelin has seven times as much kinetic energy.

Which helps explain the persistence of thrown spears in 15th c. deeds of arms. I should caution that we don't know how their technique compares with a modern athlete's, and throwing a spear in armor would have reduced performance to some degree.

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