Have you ever wondered how the human hand can split a block of wood or concrete without damaging itself? Physicists Michael Feld, Ronald McNair, and Stephen Wilk, also wondered about the same thing. But they were not just physicists; they were also karate enthusiasts. Feld was a brown belt, while McNair was a fifth-degree black belt. In 1979, the trio published a paper about this curiosity, and the answer that they found was, well, found in physics.
As it turns out, there’s no trick—the perfect karate strike is nothing more than a precise application of Newton’s laws.
Feld, McNair, and Wilk placed wood and concrete in a hydraulic press to determine the amount of stress (force) needed to crack the underside of the objects. A wood plank can bend by about one centimeter before it breaks, which requires a force of 500 newtons. Concrete blocks only need to be deflected one millimeter before breaking, but since the material is less bendable than wood, that displacement requires 2,500 to 3,000 newtons. And because some energy is lost upon collision, the fist needs to exert even more force than that in order to actually break the blocks.
Thankfully, the human hand is capable of generating a very high degree of force in a very short period of time. The impact from a typical strike lasts only about five milliseconds. Through a combination of theory and experiment, the team discovered that within this brief flash of time, “the hand of the karateka, or practitioner of karate, can…exert a force of more than 3,000 newtons, a wallop of 675 pounds.” The team’s model indicates that the hand must reach a speed of 6.1 meters per second to break wood and 10.6 meters per second to break concrete. “Such speeds agree with our observation that beginners can break wood but not concrete,” they write. “A hand velocity of 6.1 meters per second is within range of the beginner, but a velocity of 10.6 meters per second calls for training and practice.”
Good to know!
(Image Credit: Pixabay)
But a lot of board breaking is psychological. The most common instinct is to bring your fist (hand, foot, elbow, whatever) right up to the board and then stop. You have to crash through the board and not care what happens.
Speed breaks, which is that the last quoted paragraph refers to, requires finely-tuned fast twitch muscles. They're a wonder to watch. My roundhouse kicks, on the other hand, are slow enough that I might as well be delivering them through ground mail.