Why Does A Golf Ball Have Dimples?

So why does a dimpled golf ball travel further than a smooth golf ball? The answer has everything to do with drag, even though that may seem a bit counterintuitive at first. As air moves past a sphere, two types of drag develop. The first is drag due to friction, or the drag of the air hitting the front of the ball. The second and more prominent type of drag, however, comes from the separation of the airflow behind the golf ball (see top of figure). As the air loses its ability to stick close to the ball (known as the boundary layer), it separates from the ball and creates lots of little eddies that basically try to pull the ball backwards. Thus, to make a ball fly further, the separation of the boundary layer must be delayed as long as possible. To do that, there are basically two options: increase the speed of the ball through the air (which isn't really possible since a player can only hit the ball so hard) or find a way to force the air to stay in the boundary layer longer. And that, Mr. Hobson discovered, could be done by roughing up the ball.

Why does a rough ball help delay airflow separation? The rough parts (or dimples) create very small amounts of turbulence near the ball's surface that actually add energy to the air in the boundary layer. More energy means more sticking power which means the airflow separation occurs later. The later the separation, the fewer eddies behind the ball thus decreasing the force trying to pull the ball backwards and allowing the ball to fly further. (See bottom of figure).

Obviously, the physics involved are more complex than this basic explanation, but hopefully you get the idea. Golf ball manufacturers are constantly doing more tests to figure out the ideal number of dimples, but it's generally accepted that between 300 and 450 dimples is ideal. The USGA also stipulates that the dimples must be symmetrically placed around the golf ball because asymmetrically placed dimples help to keep a ball spinning upright and thus nearly eliminate the chance for hooks and slices.