Jeet Kune Do. Teri Tom

property of stability. As we mentioned earlier, this is the degree of resistance required to disturb one's balance. There are three variables that affect stability: the height of the object's center of gravity, the size of the base of support, and the object's weight. And, yes, there is an equation to represent this:

      Toppling force x moment arm of toppling force = object's weight x moment arm of object

      Technically, it's not the width of the base of support that determines stability. More accurately, it's the horizontal distance between the line of gravity and the edge of the base of support in the direction of the toppling force that determines stability.²¹ This is the moment arm of the object. The "toppling force" is the force required to unbalance the object. The moment arm of the toppling force is dependent on the object's center of gravity. The higher the object's center of gravity, the longer the moment arm is for toppling the object. Remember, a longer moment arm requires less force to produce torque. Therefore, the lower the object's center of gravity, the shorter the moment arm of the toppling force, and a greater force is required to unbalance the object.

      This is a pretty technical explanation of stability. What's important to know is that lowering your center of gravity and widening the base of support usually result in greater stability.

      So we've already established that widening the base of support, or in our case, widening the JKD stance, increases stability. But remember Bruce's quote about "movement without loss of balance." Movement. In JKD, your base of support is constantly shifting. As we'll see in the next chapter, the JKD stance is designed to strike a balance between stability and mobility. The wider your stance the more stable and less mobile you'll be. A narrower stance means less stability but more mobility. There is always a trade-off between the two.

      In certain situations, you'll want to lower your center of gravity and widen your base of support. In close quarters, for example, when you duck, you widen your stance and lower yourself to the ground. Not only does this get you out of harm's way, but you've also hunkered down so that follow up blows are less likely to knock you over. Similarly, if a grappler shoots in to throw you, you'll go into a wrestler's crouch. Again, this widens your stance and lowers your center of gravity. In this position it will take a lot more force to topple you.

      Widening the base of support, though, is not just for defensive maneuvers. As we'll see in the next section, whenever you apply a force, an equal and opposite force comes back at you. The more stable your stance, the less likely you'll be thrown off balance by the opposing reaction force.²² Examples of widening the base of support to apply force can be found in almost all sports. When a pitcher throws a ball, he takes that giant step after the wind up just before releasing the ball. If he didn't, he wouldn't be able to apply as much force to the ball without being thrown off balance. Try hitting a heavy bag while standing on one foot. That's a pretty narrow base of support. You're likely to be knocked off balance. Now stand in a regular stance and hit the bag. You've widened the base of support and can absorb the opposing force of the bag while maintaining stability.

      NEWTON'S THIRD LAW OF MOTION: MINIMIZING WEAR AND TEAR

      In our analysis of stability, we've just introduced the concept of action and reaction. This is more formally known as Newton's Third Law of Motion, which states:

      "To every action there is always opposed an equal reaction: or the mutual actions of two bodies upon each other are always equal and directed to contrary parts."²³

      Going back to our heavy bag example, remember how a stable stance keeps us balanced after we've hit it. When we apply force to the bag, it sends an equal and opposite force right back at us. If our stance lacks stability, we'll be knocked off balance. Think of the recoil of a rifle. The force the rifle exerts on the bullet causes an equal force that acts on the rifle, causing it to kick. The rifle, however, does not move with the same acceleration over the same distance as the bullet, though, because it is so much more massive than the bullet.

      In our heavy bag example, then, notice the jarring effect you feel when you hit it. That's the same amount of force that you applied to the bag coming at you. In JKD, we're often in the business of hitting things. Over days, weeks, years, decades, that's a lot of hitting! And if you're connecting, that's a lot of force coming back at you—every time you make contact with a target.

      Because of all that pounding, you'll want to construct a stance and develop techniques that help you absorb that reactionary force with the least wear and tear on your body. If you plan to stay in the martial arts for any real length of time, it's in your best interest to adopt practices that minimize the physical stress you'll incur.

      This leads us right into the next chapter, where balance, stability, mobility, force production, potential energy, and martial arts longevity all begin—the JKD stance.