s with a consequent lowering of the air pressure in this region. The air pressure on the other side of the card is greater and there is thus a resultant force from the high pressure region holding the card firmly against the end of the spool. There are many other illustrations of Bernouilli's Principle. A light ball such as a ping-pong ball may be supported on a small jet of air. Larger rubber balls may be supported on an air jet from a vacuum cleaner used in its blowing mode. In each case the flow of air over the top of the ball produces a low pressure region. The low pressure resulting from the increase in speed of the air over the top side of the wing of an airplane in conjunction with the high pressure below the wing produces the lift on the wing. An aspirator, whether one to apply medicine to the nose and throat or one in a spraying outfit, uses the low pressure, produced in a constricted tube where air is speeded up, to lift the fluid being sprayed from the supply vessel to the air stream. As we shall see, the lift on a golf ball is another example of the Bernouilli effect. UNDERSTANDING FORCES ON A SPINNING BALLWe shall develop the understanding of the aerodynamics of a golf ball by easy stages. It is a complicated subject and as far as I know is understood only in a qualitative manner. In other words, one does not sit at a desk and, with the use of a computer, calculate the optimum design of the dimples on a golf ball. Consider first a ball, similar to a golf ball, except that it has a smooth surface. Let it be at rest and let it have air moving past it with a small velocity. This is equivalent to having the ball move with the same small velocity through air at rest. However we shall consider in our discussions that the ball remains at rest and the air moves past the ball because it appears that this relative motion is easier for most of us to visualize. A possible flow pattern of the air past a ball is shown in Fig. 8.1(a). The...
