sense that the bottom of the ball appears to be moving away from the golfer while the top of the ball appears to be coming toward the golfer. Sometimes, when a ball is hit above center, it spins the other way. Such a ball is said to have been "topped." The flow of air past a spinning dimpled ball should be something like that shown in Fig. 8.1(d). The air is assumed to flow from left to right if we think of the ball being at rest. The ball is moving from right to left if we think of it moving through still air. The turbulent boundary layer is now moving with the surface of the ball as it spins. This means that the air over the top of the ball is moving more rapidly relative to the ball than at the bottom of the ball. According to Bernouilli's Principle the pressure above the ball is less than that directly below the ball. There should thus be a force, called lift, perpendicular to the direction of the ball's motion. The existence of this pressure difference has been shown to have more than a theoretical basis. Professor J.J. Thomson, the discoverer of the electron, was interested in golf and reported on an experiment, performed at a public lecture, in which he demonstrated a pressure difference in the direction to account for the Bernouilli lift on a spinning ball in flight. The turbulent boundary layer as it is pulled along over the top of the ball stalls farther down on the backside of the ball while the boundary layer on the underside of the ball is prevented from remaining next to the ball and stalls even before it reaches the lowest point of the ball. The wake behind the ball thus starts down lower than the wake behind a non-spinning ball. The flow pattern takes on a downward component. The air thus receives some downward momentum and the ball recoils in the upward direction. This is another way of looking at the origin of the Bernouilli lift on the ball. Measurements have been made on the aerodynamic lift and drag on spinning golf...
