Cricket and the physics behind swing bowling

Ball hitting cricket stumps
Hit the stumps The more scientists understand ball behaviour, the more developed the game becomes. (Courtesy: iStock/simonkr)

Cricket and golf have little in common. Sure, there’s a tradition of wearing knitted jumpers while playing both sports, but from the point-scoring system and the number of players, to the size of the ball and the playing area, they are very different games. There is one feature the two share though – both cricket and golf involve hitting a ball that has a textured surface. This apparently minor detail allows golfers and cricketers to exploit the principles of aerodynamics to help them win.

In golf, the ball is manufactured to have a uniform covering of hundreds of dimples. These create pockets of turbulence, which make the air flow pass closer to the ball’s surface than if it were smooth. The effect reduces the low-pressure zone behind the ball, thereby lowering the drag and allowing the ball to travel further.

Another benefit of this dimple design is that it amplifies the “Magnus effect”, a phenomenon that occurs when a ball spins as it travels through the air. Named in honour of the 19th-century German physicist Heinrich Gustav Magnus, it is a result of the pressure difference across a spinning surface, between the side where the ball’s motion opposes the air flow, and the side where it is in the same direction.

This pressure difference causes an overall force across the ball in the direction of low pressure. In the case of a golfer creating backspin – where the “top” of the ball rotates towards the golfer – the net force is upwards so the ball travels further than it would if it wasn’t spinning.

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The physics of a cricket ball is even more interesting. It’s manufactured to be smooth and glossy, with a raised stitched seam encircling it. The onus is on the cricketers themselves to alter the texture of the leather surface (provided they do so within the laws of the game). It’s a responsibility with interesting consequences, and a history of scandals.

In cricket, there are many styles of bowling, but they all fall into two broad categories – fast and spin. Spin bowling is a slower delivery but by rotating the ball rapidly, the bowler can get the ball to bounce at unusual angles, making it difficult for the person batting to predict its incoming pathway. In contrast, fast bowlers try to fire the ball as quickly as possible at the batter to force a mistake.

But within fast bowling there’s another discipline called swing bowling, where the aim is to make the ball deviate from a linear trajectory. The idea is that this will confuse the batter and not give them enough time to adjust their shot, making a wicket (meaning the batter is out) more likely. A fast bowler can achieve this delivery from a shiny new ball by angling the raised seam away from the intended direction of travel.

In scientific terms, swing is a net force acting sideways on the ball, resulting from a pressure difference across it. When a ball is bowled, a thin layer of air – the boundary layer – surrounds part of the ball. This detaches from the surface at two locations, known as separation points, “behind” the ball relative to the direction of its motion.

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A turbulent boundary layer detaches from the ball later than a laminar one (where the air flow is smooth), and that later separation point leads to a lower pressure on that side. By having both laminar and turbulent boundary layers on opposite sides of the ball, the separation points become asymmetric, resulting in a pressure gradient across the ball.

The bowler will angle the seam away from the direction of delivery, which will disturb the air flow over one side of the ball

So how do you create both types of boundary layer on the same ball, especially when that ball is a smooth new cricket ball? It is here that the prominent seam of the ball comes into play. The bowler will angle this seam away from the direction of the delivery, which will disturb the air flow over one side of the ball. The boundary layer on the other side stays laminar and thus you have your asymmetry and your swing; in this case, in the direction of the seam.

A brand new, beautifully firm and glossy cricket ball doesn’t retain its shine for long however. Being hit and bounced all over the pitch for potentially hundreds of deliveries creates cracks, wrinkles and general scruffiness. While it may seem that angling the seam on a uniformly scruffy ball should serve the same purpose as it does for a smooth new ball, this is not the case. As the ball ages, the seam too will undergo wear and tear, and become less prominent. Essentially, it will be less effective at tripping the air flow to make one side even more turbulent.

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In contrast, having half the ball smoother than the other means that the bowler doesn’t need to create laminar and turbulent boundary layers themselves – instead these will form as per the surface that they flow over. Players therefore try to maintain the ball’s physical asymmetry, which requires the bowling side to keep one half of the ball as smooth as possible. This is usually done by polishing the ball on their clothes, creating the distinctive red streaks on a cricketer’s white kits, or smoothing it with their sweat before bowling it.

As scientists develop the technology to quantify every variable of a ball’s trajectory, athletes and their coaching teams are understanding more about these aerodynamic phenomena, and how to manipulate them. The game is therefore constantly developing, with boundaries being pushed further and further in the pursuit of trophies.

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