You may have the erroneous idea that force is not necessary in judo, especially when you see a sixty-year-old instructor throwing many young- and strong men seemingly without effort. Dynamics, how­ever, denies this illusion. A body begins to move only when an external force works on it, as will be explained later. A human body is a phys­ical entity. Therefore, if you want to break your opponent's posture and make him fall down or hold him down on the mat;, you must apply the proper force to him.

In the preceding chapter we studied how to make use of the force of the waist and abdominal region to produce the largest possible force and speed. We must now study how to apply force effectively. It is properly applied force that enables you to gain victory over your opponent.

Newton's "three laws of motion" can be considered the foundation of modern dynamics. As a human body is a physical solid, its motion must be studied under these laws. Therefore a complete understanding of these law.-; may- well be the first step toward learning- the dynamics of judo. In this chapter let us create a conception of force from these laws of motion.

First law of motion

What is force? A conception of it can be derived from the first law of motion. Newton states that a body at rest remains eternally at rest, and a body in motion remains eternally in uniform motion unless acted upon by an external force. When we push a book that rests upon a desk or stop a ball in motion with our hands, we receive resistance from these objects. This resistance is called inertia. Therefore, to give motion to a body at rest or to stop a body in motion, we must overcome the inertia of that body. We may define force as an action to overcome the inertia of a body.

According to the first law of motion, a rolling ball would continue eternally in motion after it had once been put in motion. Therefore, if we were to throw a ball at the moon, the ball would continue its flight until it reached the moon. In reality the ball eventually falls to earth. The reason for this is that the attraction of the earth's gravity acts on all objects in the same manner. How can we apply this law to judo? We know that if an opponent at rest makes a motion it is the result of an external force—that is, of something already indepen­dent and separated from his will—even if originally produced by his own will. Therefore you can exploit the force of your opponent if you have a good understanding of the nature of force.

For instance, assume that your opponent moves to his left with the object of making you lean to your right front corner. Suppose that the force with which he moves is equal to five units and that you can throw him in the same direction by adding a force of five units to his five. Then the total force of ten units will throw him easily, even if he tries to resist. To keep from being thrown, he must exert additional force against the force of ten units that is being placed on his body. We know that there is a force that can be exploited whenever the opponent makes a motion.

Second law of motion

Newton states in the second law of motion that when a force acts on a mass, the mass acquires a certain acceleration proportional to, and in the direction of, the force acting on it and that the accelera­tion is inverse to the magnitude of the mass. The law can be easily understood by means of the following experiment.

Suppose that there are two balls placed on the floor. One is made of iron and the other of wood. Now let us roll the balls simultaneously by applying two forces equal in magnitude and direction. The wooden ball goes farther than the iron ball because its velocity is greater. Try the experiment again, this time exerting a larger force than before. The wooden ball will roll proportionately farther and faster.

Now let us see how this law works in judo. You know that if you exert a force on your opponent and that if his body weight is less than yours, you will be able to make him move or fall with compar­ative ease. On the other hand, if he is heavier than you, you will have more difficulty in moving him and breaking his posture. If you throw him with a larger force, he will fall faster to the mat. If he strikes his body against yours to push you down backward, the larger his body, the more difficult it becomes for you to stop his body's movement before he applies his throw.

These phenomena are all due to the second law of motion. It is this law that shows us why a large man has an advantage over a small man in competition.

Third law of motion

The third law of motion is called the law of reaction. The law states that to every motion there is a reaction. We find numerous examples of this in our daily experience. If a man in a rowboat pushes another rowboat, the pushed boat moves in the direction of the force applied, while the pushing boat simultaneously makes a corresponding motion in the reverse direction. Again, if a man in a boat pushes the shore with a pole, his boat gets clear of the shore. (See Figure 32.) If a person standing in a natural posture before a pillar pushes it with his hand, he will tend to fall backward. This is due to the force of reaction that the pillar exerts against his force. (See Figure 33.)

Let us consider walking. Look at Figure 34. When a man strikes the ground backward with his lower extremity, there is produced a force of reaction, F, inverse in direction and equal in magnitude to the force K, with which he kicks. The force F is produced because the force K can kick the ground. If there is no ground to step on, there is no force F, either. It would be like trying to walk on the sea.

32. Law of reaction: force P produces reaction Q.

The reason that the man who pushes the pillar cannot exert a force greater than 0.5 kilograms without falling backward is that the pillar has strength enough to exert a reaction sufficient to overpower him. He can lift 75 kilograms from the ground because of the same fact; there is the ground that his lower extremities can press against with the force of 75 kilograms. Therefore we find that to pull or push an object horizontally there should be a base strong enough to exert the required reaction.

Here, too, let us see what the third law of motion teaches us. Look again at Figure 33. The stability of your body can be considered as 0.5 kilograms when you stand in natural posture. Therefore, if you push the pillar with your hand, you must fall backward if your push­ing force is larger than 0.5 kilograms. Nevertheless, you know that you can exert a large force horizontally on your opponent in judo practice. And the horizontal force is extremely effective in making your opponent fall.

How can you apply such a large force horizontally in spite of the third law of motion? The answer is one of the technical secrets of judo. It is that when you make your body strike against his, the force exerted on him is not only that of your hand but also that of the momentum produced in your body when it is set in motion. We shall study this momentum in Chapter 5.

The third law of motion—the law of reaction—proves that a dy­namically produced force is more important than the force of the muscles in breaking the posture of the opponent and making him fall

33. Law of reaction: force P produces reaction Q.

34. Kicking force K produces reaction force F. G: gravity

In grappling, why is it difficult for your opponent to get up when he is turned over on his back? It is because he has nothing to push against effectively with either of his feet. Consequently, he can use only the force of his arms and shoulders, but this is not enough to make all parts of his body cooperate.

Look at kesa-gatame (side collar hold) in Figure 105. A is holding B down on his back under his arm. Since A, as you can see, rests his upper body on B's chest, the pressure is considered to be the weight of A's upper body. Thus, however heavy he may be—even if he weighs 90 or 100 kilograms—B will not feel any pain because only half of A's weight is pressing him down.

When an instructor holds you down, you will feel that his body is unbelievably heavy. There are reasons for this phenomenon. One of them is that he is trying to exert the heaviest pressure possible on you. Another is the action of the third law of motion.

Look again at Figure 105. Picture yourself as A. After taking the position shown in a, press your chest against your opponent before you take the position shown in b. Then take the position shown in b and raise your upper body, pressing his body against your chest. To take the position shown in b, you raise your upper body with your chest pressed against his, at the same time continuing to hold the pressure on him. The reaction to the force with which you raise your body is produced to press heavily against his chest.

In kesa-gatame (side collar hold) the force that holds your opponent down results from the reaction described above, as well as from the weight of your upper body. How to apply this reaction dexterously upon your opponent is the key point of kesa-gatame.

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