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The Physics of Karate Strikes

Volume 1
JOURNAL OF HOW THINGS WORK
Fall, 1999
why martial artists seek to use as tiny a striking surface as possible in not only hand
techniques, but also kicks, elbows, and other strikes as well.
4 Point of Focus
Karate black belts often advise white belts before their first attempt at breaking
not to try to break the board, but to break the floor under the board. This is to ensure
that the hand does not decelerate prior to contact with the target, a mistake that
beginners, fearful of injury and therefore mentally hesitant, often make. High velocity
of the hand is critical to successful breaking, and data taken from high-speed movies
of karateka show that maximum hand velocity is achieved when the arm reaches
approximately 75% of extension. Intuitively, this makes sense. Since the hand cannot
move forward a distance greater than the length of the arm, it must have a velocity of
0 at full arm’s length extension. It follows that the hand must decelerate well before
the arm is fully extended. Advising beginners to attempt to hit an imaginary target
25% of their arms’ length on the far side of their targets would therefore be more
precise than the typical encouragement to aim for the floor, but the physical principle
is the same: maximum hand velocity is achieved when the point of focus of the strike
is well beyond the surface of the target.
5 Use of Body Mass
Note that mass is a co-efficient in the formulae for force, momentum, and energy
transfer alike: all three are directly proportional to mass. Since a human being’s mass
for the time it takes to deliver a strike is constant—a karateka with a body mass of 70
kilograms before a strike will have a body mass of 70 kilograms after the strike—
mass is often and erroneously dismissed as a constant in the equations for force,
momentum, and impulse. What matters is not the karateka’s body mass, but how
much of that mass is involved in the strike. A body mass of 70 kilograms is beyond
the karateka’s immediate control; how many of those 70 kilograms contribute to the
strike is very much within the karateka’s control. It is therefore crucial not to use the
arm alone to extend the weapon and hope for sufficient force and energy to break the
target. The entire body should be used by snapping the hips and pushing with the legs
in the direction of the target. This explains why boxers are seldom knocked
unconscious by jabs, where little more than the mass of the arm contributes to the
punch, but are frequently knocked out by hook punches where the entire mass of the
body is thrown behind the punch. The same principle of using the entire body mass to
deliver a blow applies in breaking techniques as well.
6 Specifics of Impact
Consider now the breaking process from the perspective of the target. When the
force of the strike is applied to the board or cinderblock, it accelerates in response to
that force. The key is that it does not accelerate uniformly—those areas where the
force is applied (the center of the target, if the strike is properly aimed) accelerate
much more than the outer regions of the target which are held in place by large
cinderblocks. This localized strain, the response to influence of stress imposed by the
strike, initiates the rupture. Strain is functionally the loss of height of the target that
occurs when the top surface is compressed and the bottom surface stretched. Because
© 1999 Jon Chananie
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