The Wind Tunnel Results (Page 6)

When a ball rotates, the air that is in contact with the balls surface rotates with the ball. The air a little farther from the ball gets drawn along too but not as fsat. Farther away from the ball the rotation does not affect the surrounding air. The layer of air around the ball where the fluid is drawn along with the spinning surface is called the "boundary layer".

Let's look at topspin - the offense stroke in tennis. If the ball wasn't rotating as it flew through the air then both the top and bottom sides of the ball would meet the air rushing over it at the same speed.

But the ball is spinning. In topspin the top of the ball spins into the oncoming air and the front of the ball is moving downwards. Because the boundary layer draws the air down along with the front of the ball, more air gets pulled under the ball than goes above it. Now, since more air has to pass under the ball it gets squashed together and has to move faster. This means there needs to be a higher velocity on the lower side of the ball, and subsequently a lower velocity on the top of the ball.

On the top side of the ball this lower velocity creates a higher pressure and at the bottom the higher velocity creates a lower pressure. (This is an application of Bernoulli's Law.) With high pressure on top and low pressure on the bottom there is an imbalance in the forces on the ball which curves it downward from its straight line path.

This effect is called the Magnus effect. It affects all sports balls (the hook and slice in golf, the infamous curveball in baseball) and ballistics.

To view the wind tunnel tapes follow this link.