QUESTION #423

# Why does a golf ball slice or draw? What is the difference in the flight of a golf ball hit with back-spin and one hit with top-spin?

No less than four principles are used to explain the movement of objects such as golf balls as they travel through the air. Since air is considered a fluid, then fluid dynamics, or the characteristics of moving fluids or objects moving through them are described using a Reynold's number. This relationship is part of the research done by British physicist Osborn Reynolds for which it is appropriately named, and is a function of the viscosity of the air, the speed of the air and the size and shape of the object moving through the air.

A characteristic of air as an object passes through it is such that it is best described by using a layers model. These layers, particularly the boundary layer around the object itself is a critical determining factor as to how the object will behave through the fluid. Since the boundary layer adjacent to the ball is most subject to friction from the surface of the ball, the smoothness of the surface obviously plays a part. A rougher surface causes air to 'grip' the ball for a longer period of time before passing, creating turbulence and a thickened boundary layer. A smoother surface will allow the air to flow easier over the ball creating what is called laminar flow. Unfortunately, laminar flow, while initially having less drag, is also prone to separation, which produces an increased drag. By inducing turbulence in the boundary layer through the use of dimples in a golf ball, or seams on a baseball, greater layer adhesion is realized, and surprisingly enough, a decrease in overall drag as compared with that said, now on to the focus of your question: What causes a golf ball or any projectile for that matter, gravity excepted, to deviate from its initial trajectory? That was basically the same question that a German engineer, G. Magnus, was asking himself when studying cannon ballistics. He noted that a cannon with a barrel bent to the left actually made the cannon ball curve to the right. Further research revealed to him that the barrel bent to the left imparted a clockwise spin on the ball. This discovery led to an entire field of study explaining the behavior of rotating objects as they travel through the air.

'Golf ball with backspin [rotating CW] with air stream going from left to right. Note that the air stream is deflected downward with a downward force. The reaction force on the ball is upward. This gives the longer hang time and hence distance carried.' - from Lift and Air Resistance by Tom Steiger, Department of Physics, University of Washington.

Simply put, this is what his research found: A rotating ball traveling through the air will create relatively low pressure, explained in Bernoulli's principle, on the side of the ball rotating the same direction as the air stream (faster air speed). High pressure will occur on the side of the ball rotating against the flow of air (slower air speed). Higher pressure (side rotating against the air stream), will induce premature laminar separation around the ball. Lower pressure air (side rotating with the air stream), has better adherence to the ball and deflects the air stream toward the area vacated by the high pressure separation, creating a 'wake.'

It is very much like turning the tiller on a boat to deflect the wake on the boat and alter its course. Turn the tiller to deflect water to the left boat causes the rear of the boat to move right. Deflect the water to the right, and rear of the boat turns left.

Since the golf terms of slice and draw are particular to the left or right-handedness of the golfer, we will simply use left or right to describe the deflection of the ball. If an imperfect hit on the golf ball causes the ball to spin clockwise, the ball will deflect air left causing the ball to curve to the right. If the hit imparts a counterclockwise spin, the ball will deflect air to the right causing a curve to the left. Top-spin, deflects air upward forcing the ball downward; while backspin will cause the ball to rise above its normal gravity determined parabolic arc.

The amount the ball will deviate from its initial trajectory is a function of the density of the air, the velocity of the ball, and the rpm of the spin on the ball.

The description of these principles is aptly named the Magnus Effect.

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Answered by: Stephen Portz, Technology Teacher, Space Coast Middle School, FL