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Force is a push or pull that causes motion of a body or object. More specificially, a force changes the motion of the body it is acting on. See the Newton's Laws of Motion page for the direct relationship between force and motion (acceleration). Force is a vector quantity, having both magnitude and direction.

In a golf swing, multiple forces are normally acting on golfer's body simultaneously: muscle forces, gravity, friction, ground reaction forces, air resistance, etc. Muscle forces are generated by the skeletal muscles through both the passive (stretching) and active (contraction) mechanisms. Muscle forces are an essential part of all human body motions. Gravity (weight) is caused by earth's gravitational pull. Weight is always acting vertically downward (toward the center of the earth) throught the center of mass of the body. Air resistance is due to the friction between the body and the air during a fast motion of the body and club. Ground reaction force is the reactions to the forces the golfer exerts to the ground. The horizontal GRFs are equal to the frictions between the feet and the ground.

A force has three important properties: magnitude, direction, and point of action. These properties determine the outcome of the force exertion. Figure 1A shows three forces acting on the same point of action in the same direction. These forces yield different outcomes (distances in particular) because their magnitudes are different. In Figure 1B, three forces of same magnitude are acting on the same point of action but the the shot outcomes (spins in particular) are different because their directions are different: horizontal vs. down blow vs. up blow. In Figure 1C, three force of same magnitude and direction are acting at three different points of action. Again, as a result, the shot outcomes become quite different. First two properties are essentially the properties of a vector quantity. The third property (point of action) is related to the moment of force (or torque) generated by the force about ball's COM. See the Moment of Force page for details.

Force 01

Figure 1. Properties of the Force: Magnitude, Direction, and Point of Action