with shot-putters that the resultant velocity increases—and, thus, acceleration and force increase—when the shot is released at an angle of less than 45°.6 To maximize horizontal velocity and minimize "hang time," then, the same goes for you when you "release" yourself as a projectile from the ground. This has to do with air resistance. The higher you launch yourself, the more you are actually held up, to some degree, by air resistance. This is exactly what we don't want. What we're aiming for is to cover as much ground as possible with the least amount of time in the air. You'll have to experiment with this on your own to find what is most efficient for you.
WORK AND POWER
From our discussion of impulse, you know that the impulse is the product of force and time. We can also measure another variable—the distance an object moves when a force is applied to it. The product of this distance and the force is known as work and the equation is:
Work = force x distance
Whenever you step and slide and cover a certain amount of ground, work is being done. Whenever you throw a kick, your muscles contract, producing forces that pull on your tendons and bones. This causes your leg to move through space. Your leg covers a specific distance. That's work.
Crucial to the biomechanics of the martial arts is the rate at which you can do work. We call this power. In layman's terms, we often interchange this with the word force and we will do so in this book, but in "biomechanicspeak," they are two very different things. The equation for power is as follows:
Power, as you can imagine, is very important to us. It doesn't really help us to throw a kick if it's so slow it never reaches the target. Just think of Bruce Lee and how fast he moved his limbs through space. He was power personified.
KINETIC ENERGY
In the realm of biomechanics, energy is defined as the capacity to do work. As you may recall from high school physics, mechanical energy comes in two forms: 1) kinetic energy, which is energy of motion and 2) potential energy, which is energy due to position.
When an object moves, its motion gives it an ability to do work. The movement gives it kinetic energy. If you hit a heavy bag, your moving fist has kinetic energy, the ability to displace the bag. Kinetic energy is determined by an object's mass and velocity. The mathematical equation is:
Kinetic energy = ½ (mass x velocity2)
This equation makes measuring kinetic energy much easier than measuring force, as we often know the mass and velocity of objects. Measuring acceleration is not always so easy.7
In the case of hitting the heavy bag, the equation makes sense. The faster you hit the bag, the more capacity you have for moving it. And the more body weight—or mass—you put behind your punch, the more you'll displace it.
POTENTIAL ENERGY: THE ENERGY OF GOOD FORM
In scientific terms, potential energy is often defined as energy of position. How appropriate! Throughout Bruce Lee's writings, you'll see reference after reference to good form, alignment, position. In the following chapters, we'll be spending a lot of time describing the proper stance, and the mechanics of each technique. Some of this is strategic, of course, but the underlying principle is that we are trying to create the most potential energy without sacrificing efficiency or safety (i.e. mobility, stability, etc.)
There are two types of potential energy that we'll be discussing repeatedly. The first is gravitational potential energy. Elevating objects against gravity requires work. So once an object is elevated, it has additional potential energy. Just as we explained in the case of projectile motion, we want to use gravity to our advantage as often as we can. The equation for this is:
Gravitational potential energy = mass x gravitational acceleration x height
So, in our example of the straight lead and projectile motion, when you push off, you temporarily elevate yourself above the ground. In that airborne position, you have more gravitational potential energy to direct in your punch. This is essentially what Jack Dempsey described as the "falling step." By "falling," you allow gravity to take you downward and into the punch. In his book, Championship Fighting, Dempsey uses the analogy of a sled to explain gravitational potential energy:
"In a sense, the boy and his sled are falling objects. But the slope of the hill prevents them from falling straight down. Their fall is deflected to the angle of the hill. The direction of their weight-in-motion is on a slant. And when they reach the level plain at the bottom of the hill, they will continue to slide for a while. However, the direction of their slide on the plain—the direction of their weight-in-motion—will be straight out, at a right angle to the straight-down pull of gravity."8
We'll come back to this idea of gravitational potential energy again and again in our chapter on footwork. By positioning your upper body in a certain way, you create more gravitational potential energy for yourself with accompanying footwork. In many cases you offset your weight just enough to help you move in a particular direction with more speed and less effort, all compliments of earth's gravitational pull.
Bruce Lee spent a lot of time in developing the JKD stance. Of course, he was incorporating strategic factors (e.g. narrow and closed stance, stability, etc.), but the stance also was designed to maximize potential energy, specifically for throwing the straight lead. As we'll soon discuss, the correct on-guard position, is positioning of your body to throw the most effective punch—for example, hip position for uncoiling of the body during rotation, foot position to maximize the push off and, thus, gravitational potential energy, and a slight lean forward to cheat inertia. All are examples of maximizing potential energy.
STRAIN ENERGY: THE SLINGSHOT EFFECT
The other type of potential energy important to understanding JKD is called strain energy. This is potential energy generated by the deformation of an object. Think of a slingshot or a rubber band. The further you stretch you it, the further you deform it, and the more capacity it has to do work. In discussions of the martial arts, we sometimes interchange the term strain energy with leverage. Strain energy is dependent on the degree of the object's deformation and the stiffness of the object, which may also be referred to as the spring constant of the material.
Strain energy is mathematically represented by the equation:
Where:
From the equation, we see that the greater the deformation, the greater the potential energy. In the Tao, Bruce Lee describes strain energy as it relates to throwing a ball:
"The arm is kept sofar behind that the chest musclespulling against it are tensed and stretched. The final wrist snap is postponed until the last instant before release or in striking, before contact. In football, the punter puts the last snap into his knee and foot as, or a shade after, he makes contact with the ball."10
In JKD, for example, strain energy is especially important for throwing hook punches. As we'll discuss in a future chapter, you never want to let your arm overtake your hip as you rotate into the punch. In a later chapter, we'll be referring to a "catch" you should feel on your shoulder as your hip momentarily rotates away from your arm. This creates tension, or strain, on the tendons of your chest and shoulder. You are stretching, or deforming, those tendons, so you can store more potential energy for the punch. The same goes for hook kicks. Your knee should never move ahead of your hip. Keeping the knee a hair behind the front hip increases the strain on the tendons of your leg at the hip. By keeping