Jeet Kune Do. Teri Tom

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СКАЧАТЬ The object is not only to move your fist or foot at a high velocity, but to have that velocity maximally increase at the point of impact. This is known as Newton's Second Law of Motion which states that the acceleration produced by a net force on a body is directly proportional to the magnitude of the net force, is in the same direction as the net force, and is inversely proportional to the mass of the body.² In mathematical terms, it is expressed as:

      Force = mass x acceleration

      From the equation, we see that force production increases with acceleration. We also see that an increase in mass of the object that is accelerating also increases force. This is central to all the techniques that comprise JKD, which I affectionately call "the art of how to best throw your weight around." For every punch and every kick, you should be asking yourself how you can get as much of your body weight into the technique without compromising balance and mobility.

      In the following chapters, we'll explain what Bruce Lee believed to be the best ways to maximize acceleration and body weight into each technique. We'll explain how a 135-pound Bruce Lee could generate such incredible power. Bruce himself wrote in the Tao:

      "The principle is to preserve the maximum acceleration up to the last instant of contact. Regardless of distance, the final phase of a movement should be the fastest."³

      IMPULSE AND SNAPPINESS

      Related to the equation for Newton's Second Law is an equation that accounts for a change in velocity—and in the case of throwing punches, a change in direction. Momentum is defined as the following equation:

      Momentum = mass x velocity

      To change an object's momentum, the velocity must change as well. When you throw a punch, the punch does not continue forever in the same direction unless you're Mr. Fantastic. No, you have to retract your hand at some point. This requires a change in direction. For straight punches, this means throwing the punch straight out, hitting the target, and then bringing the hand back to the on-guard position. We've already established that force is a product of mass and acceleration. But there's another variable here—how long do we apply force to an object to maximize force production?

      Let's use the equation for Newton's Second Law and for acceleration, we'll use the average acceleration. This is the initial velocity (v_j) (which in the case of our punch is when the hand starts moving towards the target) minus the final velocity (v_f) (the velocity at impact) divided by the time duration of force application:

      Remember Newton's Second Law is expressed as:

      Force = mass x acceleration

      If we substitute average acceleration into the equation, we have:

      Multiply both sides of the equation by the time interval:

      Force x time interval = mass x (final velocity — initial velocity)

      This product of force and the time interval is what we call impulse and is expressed as:

      Impulse = Ft

      What does this have to do with JKD? In Bruce Lee's words, this is what we'd call the equation for "snappiness." You'll see quite a few references to snappiness in Lee's writings, as he was distinguishing between a forceful punch and a push. If your hand stays in contact with the target too long, it becomes a mere push.

      This is why: from the equation, you can see that force is inversely related to time. The more time spent during force application, the less force is required to cause a change in momentum. When you throw a punch, at some point, you are going to have to stop and retract the hand. When you stop your fist, this is a change in momentum. The time that it takes for you to change that momentum is inversely related to the amount of force required to make that change.

      A good example would be that of the shoulder roll versus running straight into a punch. To decrease the amount of force coming your way, you roll with a punch. You move in the same direction with the punch to increase the amount of time you're in contact with it. This lessens the impact. On the other hand, say you run into a punch. It takes a lot less time, and therefore, a lot more force, to stop you in your tracks. I wouldn't recommend trying this one out.

      Another example would be the difference between training gloves and fight gloves. Training gloves have a lot more padding. This lessens the impact of punches because it takes more time to for your fist to connect with its target. Fight gloves, have less padding and therefore require less time to change the momentum of your fist, and less impact time means a greater force is required to change that momentum.

      In general, Bruce favored a punching depth of about 2—4 inches past the target. This was just enough to penetrate the target without devolving into a push:

      "All punches should end with a snap several inches behind the target. Thus, you punch through the opponent yet end the punch with a snap." ⁴

      This goes for all punches and not just straight ones. Even with angular punches like hooks and uppercuts, you move straight through the target for only a few inches before leaving the target with a tearing motion as your hand continues to travel along its angular path.

      PROJECTILE MOTION

      In discussions of martial arts technique, you hear a lot about torque, force, mass, acceleration, and stability. But you never hear about projectile motion. Yet this is a concept central to most JKD techniques, and it has to do with footwork. A projectile is any object that has been thrown or dropped into the air and once in the air, the only force acting on it, barring significant air resistance, is gravity. A lot of the time in JKD, the projectile is you! Every time you push step or push off, you are momentarily—even if it's only for a millisecond—in the air. Your toes might still be barely touching the ground, but the majority of your body weight is airborne. Every time you throw a straight punch, and almost any time you throw any punch, your body itself becomes a projectile giving you more force production by allowing you to throw your body weight into it.

      So let's look a little closer at projectile motion. Once you've thrown an object—in this case, your body—into the air, that's it, you cannot change directions in midair. The only force acting on you at this point is gravity, which we know to have an acceleration of 9.81 m/s/s downward or —9.81 m/s/s.

      I won't bore you with the derivation of the equations for projectile motion, but there is an excellent explanation of it in McGinnis' Biomechanics of Sport and Exercise for those of you who are interested.⁵

      For our purposes, just knowing what the equation is for vertical velocity of a projectile should be enough:

      Where:

      Now, if you look at this equation carefully, it should look familiar. Remember from algebra class:

      y СКАЧАТЬ