The Science Behind It

In looking through many of the essays on our website, the reader will notice a considerable reliance upon facts, formulas, and expressions from physics. We expect our students to have some capacity to understand the science behind the techniques and concepts, and provide an introduction to the physical sciences are part of the learning process. Students expecting calculus-level discussions of ballistics or advanced optical calculations will be directed elsewhere, but we do expect some familiarity with the real explanations of their training.

The value of this is two-fold. First, understanding the science of any technique allows the student to apply its fundamental principles. In effect, if the student needs to vary his or her application to meet the circumstances, this understanding allows for considerable alteration of the basic technique yet have some confidence it will still work. For example, a student may find himself being pinned to the ground using a breath-taking new hold. A certain escape technique uses a specific type of leverage against the opponent; while it may not be possible to escape this new hold with the standard technique, he knows that applying the right type of leverage will work. He can instantly modify the technique to slip out of the pin.

The other benefit is to understand that many claims found in some styles of hand-to-hand combat may not be true. The techniques may work, but often the explanations for how do not make sense. A stylist believing his technique works due to a mystical energy force will always be at the mercy of someone who knows how the techniques actually works. Removing the magic from our techniques makes them easier to understand and significantly reduces learning time.

Many fighters understand this, but not all use their terms correctly. What follows is a list of basic concepts in physics that pertain to hand-to-hand combat. Some of them may be new to the reader, while some may better explain a misunderstood concept. Feel free to apply these definitions or examples to your own training.

Force. Force is any change in combination of mass and speed. The word change is very important in defining any force. Specifically, a change in speed results or requires a force. A change in mass also results or requires a force. For example, if an opponent runs toward you, and you wish to change his speed, you need to apply a force to him. Many arts, such as Judo, refuse to change the opponent’s speed but redirect it. It is a true statement then that these arts attempt to minimize the use of force. It requires a force for you to move your own mass from a standstill. It requires you to apply a force to get your body mass to stop moving once it starts. Incidentally, a bullet’s force comes from a change of mass: as the gunpowder ignites in the cartridge, the bullet’s total mass changes downward, resulting in an increase in force.
Momentum. A person’s momentum is his mass combined with his speed. Note that no change is required here. Momentum is purely mass times speed. But momentum is a conserved property, meaning that when two objects collide, the momentum stays the same. So if a 300-pound person drives his body into a 150-pound person at 5 miles per hour, the 150-pound person will bounce in the same direction at 10 miles an hour. Similarly, a 150-pound person at 5 miles an hour will be able to knock a 300-pound person back at only 2.5 miles per hour. This is why the common claim that a smaller person can do as much damage or more damage than a larger person is not correct. The larger person will generally always produce more momentum than a smaller person at the same speed. Linebackers are bigger than receivers for a very good reason: they’re harder to move. A smaller linebacker is at a serious disadvantage.
Kinetic Energy. Moving objects have energy. The simplest example is a fist or foot travelling through the air. Each has energy that can be measured: take half the mass of the object and multiply it by the square of the speed. Many martial artists use the word energy in many contexts, but this is the term they generally mean. However, even those with a background in simple physics make a mistake when trying to estimate how much energy their strike produces. For example, they use the mass of their hand, and figure their fist to move at huge rate of speed. In reality, you need a portion of the mass of the arm that travels with it...which means the fist doesn’t move as fast as people think it does! The most important item to take away from this is that it is usually kinetic energy that does the real damage in a fight.
Potential Energy. Objects that are not moving also have a type of energy: potential energy. The potential energy of an object is simply its mass multiplied by the effects of gravity multiplied by its height. Your opponent has potential energy as he stands: you see it best when he is thrown down to the floor. Potential energy is able to trade places back and forth with kinetic energy: a roller coaster gains potential energy when it runs up a slope, and gains kinetic energy (but loses potential energy) when it races downward. As it races up the next slope, it slows down...indicating that kinetic energy is being lost. When an opponent races toward you, and you take him down to the ground hard, he is injured. His racing toward you is kinetic energy; your takedown converts his kinetic energy to potential energy; his fall converts this back to kinetic energy which causes the injury. The expression “the bigger they come, the harder they fall” is mathematically proveable.
Work. In physics, work is a defined concept. It is simply force multiplied by distance. For example, a push or shove is a force extended over a couple of feet. Work can also be a change in kinetic energy: when a fist strikes a target, work is performed by the deformation of the target’s skin tissue, bone, and muscle. There are many other definitions of work in physics, but remember this: it takes force to change mass, speed, or momentum. It takes work to change kinetic or potential energy. In our example above, of taking an opponent to the ground, the force is supplied by gravity; the distance is the height of his fall.
Power. One of the more misused terms in fighting circles is power. Power is a specific concept in physics with many definitions but one meaning: power is work divided by time. Hitting someone hard is about energy, not necessarily power. Pushing someone backward is work, not necessarily power. Tackling someone is about momentum, not necessarily power. Instead, power is about doing it quickly. Two people can throw opponents using the same technique; however, whoever does it faster is actually more powerful. Do not confuse power and energy. Power is about getting work done quickly.
Impact. Power is work divided by time; impact is force divided by time. This rarely explored concept is the basis for snapping-style techniques, like jabs and snap kicks. The less you stay in contact with with the object being struck, the more impact there is. For example, if you push your fist slowly into a person’s jaw, you’ll probably push his head slightly as you do work. But if you do the same motion very, very fast (such as a jab), you’ll notice his head jerk backward violently. This is impact, and has an obvious advantage.
Impulse. Even less familiar as a concept is impulse, or force multiplied by time. An even easier way to define it is a change in momentum. Chongdo follows the principles of ûm-yang: to counter a hard powerful punch, our students will use a soft, tapping block to redirect the strike away (rather than try to stop force with force). This works through impulse: by applying a little force on the X-axis of the strike, we change the momentum of the strike on the Y-axis, away from us. The slower the strike, the more impulse we need to provide, so that we might use a hard, pushing block against a softer-style grab or choke. The hard-soft, soft-hard strategy we use is the result of impulse.
Angular Momentum. In physics, the term angular is usually synonymous with rotating. A person charging at you has momentum. A person spinning into you has angular momentum. The concepts are similar: if you wish to change a person’s angular momentum, you need to use force, and so on. And angular momentum also likes to be conserved: if you wish to stop a spinning kick, you could either to push against it backward (which requires a lot of force), or you can simply check it with a linear block (which requires less force). A familiar example is riding a bicycle: conservation of angular momentum is what keeps you moving upright despite gravity’s attempt to tip you over. It isn’t balance that keeps you up: otherwise, you could stop the bike and stay upright as long as you wanted. Instead, it’s your angular momentum: gravity simply doesn’t supply enough force to counter your angular momentum. This is why a motorcyclist moving at high speeds can take turns with the bike leaning almost completely over without spilling: his angular momentum is so high. Further, angular momentum can be conserved in a roll to prevent you from being injured when flipped or thrown.
Torque. Torque is an angular (or rotating) force. It’s measured most simply as force multipled by the radius of the turn. For example, when a straight-charging opponent comes at you, you rotate him over a hip or around your pivot point to throw him to the ground. Torque is what you used: he supplied the force, but you provided the radius. It’s essential in most of the throwing arts. However, one common usage by striking arts is mostly incorrect: many Japanese and Korean systems teach their students to rotate their fist 90°-180° during delivery in order to “increase the torque.” It does require torque to turn your wrist, but this torque is not sufficiently transferred to the opponent. The reason for the twisting of the wrist is more biomechanical (it can be more comfortable and therefore more efficient to extend the muscles when striking off the hip), and with certain strikes will increase impact by about 5%. Overall, many of the best punchers do not turn the wrist, because turning the wrist does not increase force...and therefore, has no benefit to torque, power, kinetic energy, or impulse. Again, it can be more comfortable to do turn the wrist, so should not be discarded if you already do this: however, it is important to understand that it has nothing to do with torque because it has nothing to do with force.