eBooks - Martial Arts - Physics of Striking

eBooks - Martial Arts - Physics of Striking - Volume 1...

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Volume 1 JOURNAL OF HOW THINGS WORK Fall, 1999 © 1999 Jon Chananie 1 THE PHYSICS OF KARATE STRIKES JON CHANANIE University of Virginia, Charlottesville, VA 22903 1 Introduction In recent years, the ancient eastern art of Karate-Do (a Japanese word, literally translated as “the way of the empty hand”) has become popular in the western world. Karateka—practitioners of Karate—often break boards, cinderblocks, and other solid materials in order to demonstrate the strength that their training develops. Much can be said of the history and culture associated with the expansion of martial training, but this essay—it is, after all, a physics paper—will examine the collision mechanics of a hand strike to a solid target like a board. 2 Force, Momentum, and Deformation Energy That large objects moving at high speeds hit harder than smaller objects moving more slowly goes without saying. In attempting to break a board, a karateka seeks to hit the board as hard as possible. It therefore follows that the karateka should move his or her weapon (for the purpose of this paper, the hand) as quickly as possible in order to hit as hard as possible. But what makes for a “hard” strike? Two ways exist to answer this question, both equally accurate. The first looks at the collision in terms of force and momentum ; the second looks at the collision in terms of energy . Force (F) is acceleration (a) times mass ( m ): F = a. Momentum (p) is mass times velocity (v): p = v. Since acceleration measures change in velocity over time (t) (put another way, acceleration is the derivative of velocity with respect to time), force is the derivative of momentum with respect to time. Equivalently, force times time equals change in momentum, or impulse ( p): p=F· t. This is significant because momentum is a conserved quantity. It can be neither created nor destroyed, but is passed from one object (the hand) to another (the board). The reason for this conservation is Newton’s third law of motion, which states that if an object exerts a force on another object for a given time, the second object exerts a force equal in magnitude but opposite in direction (force being a vector quantity) on the first object for the same amount of time so the second object gains exactly the amount of momentum the first object loses. Momentum is thus transferred. With p a fixed quantity, F and t are necessarily inversely proportional. One can deliver a given amount of momentum by transferring a large force for a short time or by transferring small amounts of force continuously for a longer time.

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This note was uploaded on 06/21/2009 for the course COMP gc taught by Professor Ferry during the Spring '09 term at Abraham Baldwin Agricultural College.

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eBooks - Martial Arts - Physics of Striking - Volume 1...

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