to the air through which the ball is moving when there is no wind. Another is the velocity of the wind relative to the ground. A third velocity is that of the air relative to the moving ball when there is a wind blowing. Two of these velocities determine the third as shown in Fig. 8.2. In this figure the line OP represents the velocity of the ball relative to the ground. The radius of the circle represents the speed of the wind. If the ball is hit directly into the wind, the velocity of the air past the ball will be greater than if there were no wind. The velocity of the air past the ball will then be represented by the line AO. This velocity is the sum of PO and AP. If the ball is hit with the wind, the velocity of the air past the ball will then be the difference of PO and BP which is BO. If the wind is directly across the intended path of the ball and from the left, the relative velocity of the air past the ball will be represented by the line CO. The line DO represents the relative velocity of the air past the ball when the wind is quartered toward the ball from the left. The aerodynamic forces on the ball, the lift and the drag, are determined by the velocity of the air past the ball and not directly by the velocity of the ball or the velocity of the wind relative to the ground. If we remember how lift and drag depend on velocity of the ball through the air, we realize at once that a ball hit against the wind will have greater lift and more drag and one hit with the wind will have a smaller lift and less drag. The experienced golfer will keep his drives against the wind low and will hit them with less spin. He will hit his drives with a following wind somewhat higher than otherwise and will impart more spin to provide the necessary lift when the ball is moving more slowly relative to the air. For a 30 mile an hour wind, the drag, estimated on reasonable assumptions, may differ by a factor of 5 or more for a ball hit against and wit...
