# Dimples on a Golf Ball

Q:
Why does a golf ball have dimples on its surface?
- jack
RGHS, Bastian, VA 24314
A:

Very good question, and one that has kept golf experts and people who make golf balls busy for many years.

When a golf ball is flying through the air, it has three forces on it. Two of them are gravity and drag from the air. In addition, if the golf ball is spinning, there will be a Magnus force on it (which may point up or down or either side, depending on how it is spinning -- the Magnus force cannot point along the flight path of the ball however -- see our for more information).

The main goal of making a golf ball go farther however is to reduce the force of drag as it flies throught the air. In general, turbulence increases drag, because the energy needed to stir the air up and make it swirl around is energy that the ball has lost. One might think that a rough ball will induce more turbulence in the air than a smooth one, but it depends on how fast the ball is going. For balls going slowly through a viscous fluid, then the fluid just moves a bit to the side as the ball passes, and then it returns more or less to where it was. If the fluid motion is smooth, we call the motion "laminar", otherwise it is "turbulent." A ball moving quickly through a fluid like the air will have air flowing in a laminar fashion in some places and in a turbulent fashion in others. Directly behind the ball there will be a turbulent "wake", and surrounding that will be smoothly flowing air. The whole idea behind reducing the drag is to make the turbulent wake small.

The air that slides past the ball very close to it is called the "boundary layer". At the place where the turbulent wake starts is called "separation of the boundary layer" where the smoothly flowing air departs from the ball and does not close up behind the ball nicely but rather swirls around in small vortices. If the boundary layer can be encouraged to stick to the ball a little longer, then the turbulent part of the wake can be reduced. It turns out that adding a little extra turbulence in the boundary layer itself all over the ball allows the main smoothly-flowing air currents to stay closer to the ball and delays the separation of the boundary layer. Some nice pictures of balls in wind tunnel showing this effect can be found .

There is also an increase in the Magnus force, giving the ball some lift when it is spinning in the correct direction. This force helps keep the ball in the air longer, allowing it to travel farther.

People have thought of putting dimples on everything from swimsuits to cars to airplanes. You only get an advantage from these dimples if the boundary layer can be made to stick longer to the object. Some cars just have vertical flat ends to them where the trunk comes down and there is no way to reduce the turbulent wake of these no matter how dimpled the paint is. And the boundary layer stays with airplane wings except maybe a bit at the ends (some gain can be made by putting small rods out on the tips or on the trailing edges of the wings).

Tom

(published on 10/22/2007)

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