In judo it is important to throw your opponent by making use of his loss of balance, the law at work here is the law of gravity. We know that Sir Isaac Newton discovered the law of gravitation by seeing an apple fall from a tree. All bodies in the universe attract one another. For instance, the earth attracts the bodies near and around it. In turn they also pull the earth.

Since the power of attraction, according to Newton, is proportional to the mass of the body that attracts, the larger the mass of the body, the larger its attraction. All bodies near the earth fall to the earth because it has an immense mass. An airplane or a bird starts to fall to the ground as soon as its power of flight is exhausted. The force of attraction between the earth and a two-pound weight is twice that between the earth and a one-pound weight.

The attraction between the earth and a body is called gravity. When gravity is represented by weight units, it is called weight. Now let us consider the application of the law of gravity to judo. The heavier the opponent, the more difficult it is for you to move him horizontally. It is even more difficult for you to move him vertically. On the other hand, a larger gravity acts on him to make him fall.

In judo gravity may be represented as a force pulling the opponent downward. If you want to make him fall, you make him lose his balance;that is, you cause his center of gravity to go outside the base. Then the gravity that acts on him works for you to make him lean or fall. Let us study the action of the law of gravity by illustrations.

Lets assume that you and your opponent are standing face to face. When your opponent advances towards you to take hold of you by the left lapel. At the same time you withdraw as much as your oponent advances. If your oponent is mentally or physically unable to let his advanced foot advance again, he will lean forward, lose his balance, and fall.

Also, it is obvious that the same thing will happen when the stability of the legs supporting the trunk is taken away. When your opponent takes a larger step forward than usual, you merely sweep his advanced foot away in the direction of his advance,(de-ashi-harai, or advanced foot sweep). By doing this, you will drop him with the gravity acting on him directly.

Finally let us consider a case in which the opponent is downed by the nullification of his resistance to gravity. It may be difficult for you, because of the weight advantage of your opponent, to lift him with your arms. But it is easy to support him at the center of gravity with your loins as he leans forward. Look at the seesaw in the above figure.

A long board is supported at the center of gravity, so that the gravity on one side is equal to that on the other. Thus a slight force can rotate the board around the fulcrum. After making your opponent lean forward, support him at the center of gravity with your loins. No matter how much weight he may have, a slight pull can rotate him around your loins. To support your opponent completely at the center of gravity with your loins is the key point of such hip throws as o-goshi, o-tsuri-goshi, ko-tsuri-goshi, hane-goshi, etc.