The physics of breaking stuff with your fists


iO9 recently ran a story on how martial artists are able to break boards and cement blocks, using their hands rather than mystical powers. I thought it was pretty neat, but then I read an interesting counter-analysis by science journalist (and, significantly, martial arts practitioner) John Rennie.

iO9 is right about the lack of magic powers, he says. But they got the physics wrong. Key slip-up: Assuming martial artists strike like a cobra—fast punch, with a quick pull back at the end—when they have their smashing fun times. iO9's theory was that that movement caused the boards to bend and snap. But that's not how it works, Rennie says. In fact, martial artists are taught to follow through with their punches, aiming not at the board-to-be-broken, but at a point beyond it.

So how's the breaking really done? Rennie quotes an episode of the awesome old PBS show Newton's Apple:

One key to understanding brick breaking is a basic principle of motion: The more momentum an object has, the more force it can generate. When it hit the brick, [karateka Ron] McNair's hand had reached a speed of 11 meters per second (24 miles per hour). At this speed, his hand exerted a whopping force of 3,000 Newton's -or 675 pounds-on the concrete. A slab of concrete could likely support the weight of a few people weighing a total of 675 pounds (306 kilograms). But apply that amount of force concentrated into an area as small as a fist and the concrete slab will break.

The fact that martial artists also pick their materials very carefully doesn't hurt, either.

When breaking wooden boards, you use pine (not oak, not mahogany) that isn't marred by dense knots, cut ¾ inch thick and about 12 inches on the diagonal; you hit them to break along the wood's natural grain. (It's not playing by Hoyle but some breakers have been known to bake their boards in ovens before demonstrations to make them more brittle.) One good board, if held securely so that it won't move on impact, is so easy to break that even those with no training at all can be taught to do it in under five minutes.

P.S.: Rennie's blog, The Gleaming Retort, is part of a new family of science blogs, hosted by the Public Library of Science—a non-profit that publishes open-access science journals. I highly recommend checking out the entire PLoS Blogosphere.