Physics behind swing


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Physics behind swing

As a former cricket fanatic (I lost interest when it seemed there was more politics in cricket than the game itself), when I was required to write a short project on a physics effect of my choice in my undergraduate degree, I decided to research the physics behind the swing of a cricket ball. This was back when Wasim Akram, Waqar Younis and Shoaib Akhtar used to dominate the Pakistan fast bowling scene with Azhar Mehmood and Abdul Razzaq being the first change medium pacers. It was an exhilarating watch. Wasim, Waqar and Shoaib would terrorise the opening batsmen with their pace and some superb swinging deliveries and then Razzaq and Mehmood would pick up some early wickets as the batsmen started to relax and hit out.

It was a superb display of the exploitation of some basic physics phenomenon by highly skilled sportsmen. Given the recent string of cricketing events, I thought it would be a good time to resurrect that research project and explain some of physics behind swing bowling.

When given the new ball at the start of an innings, the fast bowlers generally use the raised seam of the ball to create swing. The ball is held with the seam at an angle of around 20 degrees to the direction in which the ball is headed. This creates the following effect as the ball traverses the air; the side with the seam represents a rough surface that causes the air flowing over it to become turbulent, whereas the air flows smoothly over the other side of the ball with the smooth surface. Turbulent air is at a lower pressure than smooth flowing air, causing a pressure difference on the ball, which creates a side force resulting in the ball moving sideways.

So an in-swinger to a right-handed batsman is bowled by holding the ball with the seam pointing slightly to the right causing the ball to move inwards and towards the batsman. An out-swinger to a right-handed batsman is bowled with the seam pointing to the left causing the trajectory of the ball to move outwards before coming back sharply to the batsman. In short, for conventional swing, the ball swings in the same direction as that in which the seam is pointing.

Waqar Younis was famously known to cover the ball in his run-up so as not to give away the direction of the swing!

As the innings progresses, the ball goes through wear and tear and every effort is made to polish the ball on one side so it is as smooth as possible while letting the other side deteriorate. This has the same effect as above, causing turbulent air to flow on the rough side and smooth air to flow on the polished side resulting in a sideways drift. For this type of swing, the seam is aligned in the vertical position with the rough side of the ball on one side and the polished side on the other. The direction in which the ball swings is largely determined by the bowling speed.

Another type of swing, particularly attributed to Pakistani bowlers, is reverse swing. This generally tends to occur later in the game when the ball is older and has somewhat deteriorated. This effect is usually only significant at very high speeds, however, it can also occur at lower speeds if the ball is considerably roughened. Of course, this type of swing has also garnered significant media attention as numerous players have been accused of deliberately scuffing the ball with foreign objects. The rougher the surface of the ball becomes, the lower the speeds you need to bowl to achieve reverse swing. At high enough bowling speeds and as the ball becomes rougher, it swings in the direction opposite to the alignment of the seam.

Indeed, Waqar and Wasim were some of the finest exploiters of reverse swing.

Wasim Akram’s dismissal of Lamb and Lewis in two successive balls in the World Cup final of 1992 was a magnificent display of reverse swing bowling and considered a turning point in the match. One delivery swung in and then straightened to hit the stumps, the other produced a more pronounced swing in the same direction, both virtually unplayable!

All swing bowling techniques essentially take advantage of the asymmetric way in which air flows over different sides of the ball whilst making use of the orientation of the seam and the differences in surface texture.

The skill of bowlers to exploit these basic physics effects makes for an exciting display of cricket. So, next time you’re ooh-ing and aah-ing at those in-swinging yorkers, remember, it’s all physics!

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Dr Sarah Alam Malik is a postdoctoral associate in Experimental Particle Physics. Her research interests can be found at http://www.sarahalammalik.com

The views expressed by this blogger and in the following reader comments do not necessarily reflect the views and policies of the Dawn Media Group.

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