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.
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. 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.
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 as the result of a fast and powerful movement of your body.
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.
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