A Karate Chop Offers Proof of the Power of Physics

Martial artists use great force and precision in their attacks—but no trickery.

This content of this article was originally written in an article by Curtis Rist for Discover Magazine and was published on August 5, 2008. We felt there was good information in the article and wanted to share the content with our readers.

A karate chop, or shuto strike, is one of the few things in life that offers visceral proof of the power of physics. If you are untrained in the martial arts and you karate chop a brick, you may break a finger or worse. But if you karate chop that brick with the proper force, momentum, and positioning, you’ll break the brick instead. “Amazingly, there are no tricks involved,” says Michael Feld, a physicist at MIT. “What you have here is one of the most efficient human movements ever conceived.”

Michael Feld earned his Brown Belt in karate in the late 1970s. His instructor, *Ronald McNair, also happened to be Feld’s physics student. Both men agreed that the speed and focus of the strike or kick is what makes the strike or kick deadly. The first thing they looked at was the speed of a karate chop, punch and kick. Working with an undergraduate, Stephen Wilk, they set up a strobe light that flashed either 60 or 120 times per second. They then photographed McNair and others throwing karate chops, kicks and punches and counted how many times the strobe flashed until the foot or fist hit its target.

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Michael Feld and Ronald McNair found that beginning students could deliver a karate chop at about 20 feet per second with just just enough energy to break a one-inch board. But a Black Belt like McNair deliver the karate  chop at 46 feet per second. Hitting a piece of wood at that speed, a 11/2-pound hand can deliver up to 2,800 newtons (one newton is roughly equal to the force exerted by the weight of an apple). Splitting a typical concrete slab 11/2 inches thick actually takes less, about 1,900 newtons. Brute strength is not the only requirement for breaking concrete blocks, the strike must have maximum momentum and the follow through, just like in golf and tennis.

To understand how follow-through works, Jearl Walker, a former tae kwon do student who now teaches physics at Cleveland State University, set up a study much like Feld’s and McNair’s. He found that a well-thrown fist reaches its maximum velocity when the arm is about 80 percent extended. “That’s exactly what my tae kwon do master had taught me,” Walker says. “You focus your punch in your imagination so that it terminates inside your opponent’s body, rather than on the surface. To deliver the maximum power, you want to make contact before the slowdown begins.”

What happens after contact is also very important. All materials are at least slightly elastic: Whack them in the right spot and they will start to oscillate. “If you tweak a rubber band it goes up and down, and the same is true if you tweak a board or a brick with a much greater force,” Feld says. “When they reach their elastic limits, they start to yield. In other words, they break.”

Fortunately for us says Feld, bones can withstand 40 times as much force as concrete, and a cylinder of bone less than an inch in diameter and 21/3 inches long can withstand a force of 25,000 newtons. Because skin, muscles, ligaments, tendons, and cartilage absorb a great deal of impact, hands and feet can take even more abuse. A well-kicked foot can absorb about 2,000 times as much force as concrete before it breaks. Michael Feld has never fractured a finger in karate, even though he once broke eight one-inch-thick boards together at the same time.

If students want to break boards with a karate chop, they need to increase their speed and improve their aim, but they also need to toughen up their hands and feet by striking a makawari, (padded striking post wrapped in foam, canvas or rope). “In the beginning, your skin is so soft you may end up cutting it. And then comes the blood,” says tae kwon do grandmaster, Sihak Henry Cho. “This is not recommended.” Over time, the shuto, or knife-edge of the hand, develops a callus that acts exactly like a car bumper, absorbing and diffusing the force of a collision.

The important lesson for would-be breakers is that physics needs a little help from perseverance. “Tiger Woods didn’t just wake up one morning and start hitting a ball 320 yards, and we don’t just walk in and shatter a cinder block [with a karate chop],” Cho says. “Everybody has to work at it.”

*Ronald McNair was born in 1950 in South Carolina. He became an MIT-trained physicist who specialized in laser research before joining NASA in the late 1970s. In February 1984, he became just the second African American to reach space, serving as a mission specialist aboard the Space Shuttle Challenger. On January 28, 1986, he was one of the seven crew members killed when the Challenger shockingly exploded 73 seconds after liftoff. The accomplished physicist and astronaut was highly skilled in karate. He won the 1976 AAU Karate Gold Medal and five regional championships, eventually achieving the rank of fifth-degree Black Belt.

This original content of this article was written by Curtis Rist is from Discover Magazine and was published on August 5, 2008.

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