# Why Do Golf Balls Have Dimples?

Topics: Aerodynamics, Fluid dynamics, Reynolds number Pages: 3 (851 words) Published: November 24, 2011
Why do golf balls have dimples? - Oct 2011

Historical Background
Before golf balls started being produced with dimples, golf players originally played with smooth balls. However, eventually players started noticing that the older balls that had more dents and scratch marks on them would fly further. Golfers were drawn towards this advantage, and so more golf balls were intentionally grooved by manufacturing companies. More research went into shaping balls to fly further and more efficiently up until today where modern day golf balls have dimples on them to have an average of 330 dimples [1]. One would think that the extra scratch marks would give golfer’s a disadvantage during their gameplay, but it was discovered that this is not always the case. So if dimpled golf balls do really give the edge to make them travel further distances what is the physics behind this phenomenon?

So how does having dimples on the golf ball reduce drag?

Firstly we have to look at the aerodynamics around golf balls, since the only force that can slow down a golf ball while in mid-air is the air flowing around the surface of it. We call these resistive forces drag forces, and there are two kinds which inevitably slow down spherical golf balls; pressure drag and skin friction drag. Skin friction drag is the shear forces on the sphere from the air molecules. Pressure drag is a resistive force caused by turbulent airflow behind the sphere in the wake area (explained later), which is only experienced once golf ball reaches a certain speed as to create turbulence in the system. When a ball moves slowly it experiences laminar airflow and no pressure drag since the pressure distribution is symmetric – the pressure in front and behind the ball is the same. As the ball speeds up the ball eventually experiences turbulent airflow which breaks the symmetry of pressure where the front of the speeding ball experiences higher pressure than behind, creating a drag force on the sphere. We can...

References: [1] Number of dimples on a golf ball, http://www.golfblogger.com/index.php/golf/comments/purpose_and_number_of_golf_ball_dimples/
[2] Separation of boundary layers, http://www.see.ed.ac.uk/~johnc/teaching/fluidmechanics4/2003-04/fluids14/separation.html
[3] Fluid Dynamics, http://electron9.phys.utk.edu/phys136d/modules/m1/bernoulli.htm
[4] Aerospaceweb.org – Ask Us Golf Ball Dimples and Drag http://www.aerospaceweb.org/question/aerodynamics/q0215.shtml
[5] Why are golf balls dimpled? – http://math.ucr.edu/home/baez/physics/General/golf.html
[6] Classic Moth Boat Association (Figure 1) – http://www.mothboat.com/Building/waxon.html
[7] Louis A. Bloomfield, How Things Work: the physics of everyday life, Wiley. 4th Ed., 2010.