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Polish the Ball or Create a Weight Imbalance?

Apart from being former international cricketers, Shane Warne, Ashish Nehra, Michael Holding, Madan Lal, Sarfraz Nawaz, Imran Khan, and Snehal Pradhan have at least one thing in common: they have all expressed, at some point, that reverse swing is achieved when one side of the cricket ball becomes heavier than the other. That view is totally incorrect, of course. It has no basis in science. But, it has persisted, perhaps because it was largely the accepted explanation in the early days of reverse swing and it came from some of its foremost exponents.

Since we have all been enlisted in the fight against Covid-19 recently, we have had to make changes in the way we behave. Shaking hands, for example, something we did habitually, is now strongly discouraged and we have had to come up with other ways of greeting each other.  In cricket, the use of bodily fluids to aid in the care of the cricket ball has recently come under scrutiny as well. This is something bowlers did without really thinking about it. With the Coronavirus now rampant, however, it is an activity that is under revision. Sweat and/or saliva are usually liberally applied to shine one side of the ball in order to aid movement through the air. But with the use of saliva now frowned upon, alternative methods are being investigated.

One suggestion that stoked its fair share of interest -- especially since it was tendered by none other than Shane Warne -- was to make one side of the ball heavier than the other. Warne raised the issue while speaking on Sky Sports’ Cricket Podcast: “Why can’t the ball be weighted on one side so it always swings? It would be like a taped tennis ball or like with the lawn bowls.”

The view that the ball swings towards its heavier side has been discredited by science. But like other debunked myths – the belief that the world is flat, to state one common example -- it still has currency in some quarters despite evidence to the contrary.

For the record, a half-taped tennis ball swings very effectively, but NOT due to an imbalance in weight. It swings for the same reason the ball that is rough on one side and shiny on the other swings: the difference in surface conditions, as discussed below.

Another erroneous idea is that sweat or saliva applied to the shiny side of the ball makes that side heavier due to the liquid absorption, and this creates a bias that prompts the ball to deviate towards the heavier, shiny side.

For one thing, applying sweat or saliva to one side of the ball does not really create much of a weight imbalance. But, more importantly, the aerodynamics of the ball is NOT affected by creating an imbalance in the weight. The swing properties of a ball are totally determined by the condition of the ball’s surface which affects the flow very close to the surface (boundary layer). This boundary layer can have two states: laminar and turbulent. The laminar boundary layer separates from the ball surface near the ball apex, whereas the more energetic turbulent boundary layer stays attached to the surface beyond the apex. This asymmetry in boundary layer separation locations leads to asymmetric pressure distribution, and hence side force, which makes the ball swing. For a ball released with an angled seam, the boundary layer on one side is “tripped” by the seam into a turbulent state whereas the other side separates in a laminar state. For a ball with a contrast in surface roughness, the boundary layer is turbulent on the rough side but remains laminar on the smooth side.

Swing has been a troublesome subject in cricket and many are the myths and misconceptions surrounding it. Take the belief, for example, that the ball’s tendency to swing is dependent on the weather; that the ball is more likely to swing in damp conditions under cloud-laden skies. Scientists have done their best to smother this theory. Moreover, one of the finest swing bowlers in modern times, Jimmy Anderson was quoted as saying: “There have been days when the sun is out and the ball swings and some days when it is freezing cold at the end of April and it swings. I couldn’t tell you when it is going to swing, NO!” (ECB Video: Wings to Fly, Part XII: Making a Difference, 2015). And yet the belief lives on, even in the minds of highly respected experts.

They ought to know better. Since there have long been studies that have dispelled these misapprehensions of what makes a cricket ball swing then there really is no reason pundits should still be spouting them. What is the cricket fan to believe, who may have read somewhere that the weather has no effect on the ball deviating through the air, only to hear a highly-respected commentator remark, matter-of-factly, that given the overhead conditions there should be some swing about? How will they know not to take seriously some leading expert suggesting that making one side of the ball heavier will not only aid swing but will also eliminate the need to use sweat and/or saliva to shine the ball.