Golf balls

In the early days of the sport, golf balls were made with a smooth surface. It was soon realized, however, that when the surface became worn the ball travelled farther when driven, and subsequently golf balls were manufactured with a dimpled surface to simulate the worn surface. The reason for the increase in driven distance with the rough surface is as follows.

The diameter of a golf ball is about 42 mm, which gives a critical velocity in air, for a smooth ball, of just over 135ms-1 (corresponding to Re = 3.85 x 105). This is much higher than the average flight speed of a driven ball. In practice, the critical speed would be somewhat lower than this owing to imperfections in manufacture, but it would still be higher than the usual flight speed. With a rough surface, promoting early transition, the critical Reynolds number may be as low as 105, giving a critical speed for a golf ball of about 35 ms-1, which is well below the flight speed. Thus, with the roughened surface, the ball travels at above the critical drag speed during its flight and so experiences a smaller decelerating force throughout, with consequent increase in range.