Science Puzzle
Why Do Golf Balls Have Dimples?
Hit a golf ball with a perfectly smooth surface and it flies only about half as far as an ordinary dimpled one struck the same way. A modern ball has somewhere between 300 and 500 shallow dents in its surface.
You'd expect a rough surface to make the air drag harder, not less. So why do dimples make a golf ball fly further?
The Answer
Dimples shrink the pocket of swirling, low-pressure air behind the ball, and that pocket is what causes most of the drag.
As a ball flies, air flows round it in a thin layer that clings to the surface. On a smooth ball that layer is calm and smooth, and it peels away early, not long after passing the widest point. Behind the ball it leaves a wide, swirling wake of low-pressure air. Higher pressure in front and low pressure behind add up to a strong backwards push.
The dimples stir the clinging layer into tiny swirls. A turbulent layer like that carries more energy, so it doesn't let go as easily and hugs the curve of the ball for longer before breaking away. The wake behind the ball is much narrower, and the drag is roughly halved. Dimples help in a second way too. A golf ball leaves the club spinning backwards, and the dimples help the spin drag air round with it, which gives the ball lift and keeps it up for longer.
Golfers didn't work this out from theory; they found it by accident. Balls in the mid-1800s were made of smooth gutta-percha, a hard tree sap, and players noticed that old, scuffed balls flew further than new ones. Soon they were roughening new balls with hammers on purpose. In the early 1900s an Englishman, William Taylor, patented a ball covered in regular dimples; golf balls have had them ever since.
The principle: Drag and the wake. Most of the drag on a fast ball comes from the low-pressure wake behind it, and a thin turbulent layer of air clings to the surface for longer, which shrinks the wake.