Moment of Force

A moment of force occurs when a force is applied anywhere except at the center of mass. It is what makes a fulcrum work. The product of the acting force and the distance between the axis and the line of action of the acting force is called the moment of force. For example, to lift a rock with a board, you slide one end of the board under the rock, place a small log under the board, and push down on the other end of the board. The closer you place the log to the rock, the greater the moment of force, and the easier it is to life the rock. To lift a standing opponent, pull the opponent's upper body forward with one arm, lean in and grab the opponent around the waist, and continue to pull the opponent over onto your shoulders so you may lift the opponent off the ground. The lower you grab the opponent below the waist, the greater the moment of force and the easier it will be to lift the opponent.

Moment of a Couple

A couple is two forces equal in magnitude and opposite in direction acting on a body. It is similar to equal forces pushing in opposite directions on the ends of the two blades of a propeller. The moment of a couple is the product of one of the forces and the distance between lines of action of the two forces. When you punch, if you pull one arm backward as you punch forward with the other arm, the body rotates along its vertical axis and the moment of the couple adds to the power of the punching arm.

Surface Area

The striking pressure of a technique is inversely related to the surface area of the striking object (pressure = force/area). If two objects strike a body with equal force, the object with the smaller striking surface area will strike with the greater pressure and thus be the more effective blow. In other words, the smaller the striking area of a blow, the greater the damage to the opponent. A strike with the outer edge of the foot is more effective than a strike with the sole of the foot.

Dissipation of Force

The person on the receiving end of a strike may reduce the effect of the blow by absorbing the force of the blow over a distance. For example, if you try to catch a falling, heavy object with rigid, outstretched arms, you may injure yourself or drop the object. However, if you bend your arms and let the arms slowly stop the object over a distance, you may avoid injury and successfully catch the object.

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