Jeet Kune Do. Teri Tom

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      You'll see this idea of cheating inertia again and again. In the footwork chapter, we'll discuss certain moves that are aided by a slight shift in weight. For example, when reversing direction with a side step, if you lean slightly with the upper body in the reverse direction and then sidestep with both feet momentarily off the ground, you fall towards the desired direction (Figure 1.27). If you were moving in one direction, left for instance, remember this is a form of linear inertia. To move right, then, you've got to impose a force. You won't have to work so hard to impose that force if you let gravity do a lot of the work for you.

      The same principle applies to defensive moves like bobbing and weaving. When you weave, a slight lean at the waist sets the majority of your body weight into motion. Again, gravity will be pulling on you and helping you along. In the section on kicking, you'll see that a slight lean forward and the placement of your body weight in the front foot will help you generate the torque needed to get your leg up quickly to kick.

      The law of inertia applies to both linear and rotational motion. In both cases, mass is directly proportional to the degree of an object's inertia. The more massive an object, the more force is required to move it. Think of heavyweights versus featherweights and notice how much faster the smaller fighters are. It takes a lot more energy and force to move around 200 pounds as opposed to 126 pounds. This explains how smaller fighters are still able to generate tremendous power. They are capable of accelerating their body weight better than larger fighters. Remember that force increases with acceleration. For every fighter there is an optimal balance between weight, force, and speed. Greater weight, or mass, may mean more force, but at some point, too much will compromise speed, and consequently, force. Too little weight may mean not enough force behind punches or enough muscle to move body parts fast enough to accelerate adequately.

      In rotational, or angular motion, there is another factor besides mass that impacts inertia. With angular motion, an object rotates about an axis. In JKD, our angular punches include hooks and uppercuts. The most important thing to remember about rotary inertia is that the greater the distance between the rotating object and its axis of rotation, the greater the rotational inertia. This is also referred to as the radial distribution of mass14 (Figures 1.28—1.29).

      To illustrate this, think of a figure skater going into a spin. To increase his spin rate, he draws in his arms and legs, decreasing rotary inertia, or resistance. This increases angular velocity. To slow down and come out of the spin, he spreads his arms and legs out, increasing rotary inertia. His body mass is distributed farther away from the axis of rotation. There are notes in the Tao that address this principle in relation to other sports:

      "After momentum in a throwing or elliptical striking movement has been generated by a long radius and a long arc in the swingg, the speed may be increased without applying additional force by suddenly shortening the radius of the arc. This effect is seen in the 'pull-in' at the last of the arc in the hammer throw, in the backward thrust against the forward leg by the batter in baseball, and so on. Snapping a towel or a whip are common examples of the same 'shortened lever' principle."¹⁵

      The same rule applies to fighters throwing hook punches. The tighter your hook, the closer your arm is to your body, the faster you can turn into the punch. And since hooks are usually for close quarters work, you want to make yourself small anyway. Keeping your hooks tight makes you more elusive, faster, and more powerful. This is all related to the radial distribution of mass. And once again, developing your punches so as to minimize motion and maximize efficiency is a matter of refining proper technique.

      TORQUE

      Since we've just been discussing rotary inertia, now would be a good time to introduce the subject of torque. Torque is a force that is specifically rotational and results in a turning effect. Mathematically, it is represented by the equation:

      Torque = lever arm x force

      Just as the velocity of a rotating object is dependent on a distance variable, torque depends on the distance from the line of force to the axis of rotation. This distance is called the lever arm. It is also referred to as the moment arm or the perpendicular distance. A good example of this would be a door swinging on its hinges. If you exert a force on the door by pushing it on the side of the door close to the hinges—the axis of rotation—notice how hard you have to push to open the door. If, on the other hand, you push the door at the point furthest from the hinges—where door knobs and door handles are always placed—notice how relatively easy it is to open the door. In accordance with our equation, you can produce the same amount of torque with a large force and a small perpendicular distance, or a small amount of force and a greater perpendicular distance.

      The angular force, or torque, produced by a hook punch is very similar. In any sport, torque, particularly hip rotation, is an elemental component of technique. Think of the golf swing, the tennis forehand, baseball's tabletop swing, and the football pass. All involve some kind of hip rotation, which is initiated by force produced by the body.

      In our discussion of rotary inertia, recall that there is a trade-off between mass, velocity, and force production. Similarly, with torque there is a trade-off between the lever arm, force, and angular force. When throwing a hook, then, the tighter the hook, the more force must be applied to generate a certain amount of torque. When throwing a loose hook, where the arm is more extended, you can potentially throw a punch with just as much angular force and less effort. For strategic purposes, though, even though it requires a tremendous amount of energy, it is usually more advantageous to throw tight hooks for reasons we've already outlined—speed, explosiveness, and evasion. However, there will be times, when a loose hook, with its increased lever arm, increased torque, and whip-like action, will be an effective choice.¹⁶ Torque is not just important to the angular punches like hooks and uppercuts. Hip rotation is crucial to all punches—including straight ones—and all kicks, as we'll see in upcoming chapters. The increased perpendicular distance in kicking is one of the reasons why kicks can generate so much more power than punches. The distance from your hips and the axis of rotation to your foot is much greater than the distance between the axis and your hand. However, most people move their upper body limbs much faster than their lower limbs because the legs carry so much more mass. It's the old mass-versus-speed balancing act.

      In addition to the torque generated by hip rotation, there is also a kind of torque that is very important to kicking. It is generated by what we call a force couple, which consists of two forces acting in opposite directions. To illustrate a force couple, think of a book lying flat on a tabletop. If you were to push the book to the right at the lower left corner and simultaneously push it to the left at the top right corner, the book would spin counterclockwise. The two forces are moving in opposite directions and are noncolinear. (If they were colinear, you would be pushing at both lower corners in opposite directions and the book wouldn't move).

      What do force couples have to do with kicking? Well, to get your front leg off the ground quickly you'll actually need to generate a bit of torque. We'll get into more detail in the kicking chapter, but what you're essentially doing is pushing slightly upward with the back foot and pulling by first digging into the ground with the front foot. At the same time, you are redirecting your center of gravity from a forward more upright position to your back leg and downward. You are doing the same thing the book does on the tabletop. Your limbs are rotating about your center of gravity as you shift from having the weight in the front foot to placing it in the back. The push-pull action helps you generate the torque that makes it possible to get your leg up into kicking position (Figures 1.30—1.34)

      BALANCE AND STABILITY

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