Opinion | Swing now, sweet cricket ball, for India
A cricket ball’s seam churns the air, causing a degree of turbulence around it. Swing bowling is a matter of controlling and directing that turbulence
Spent a depressing hour or so watching India bat—or try to bat—against England in the recent Lords cricket Test. If you follow cricket, you will remember that the first day of the Test was rained out and there were rain interruptions on the second day, too.
What that meant was one of those cricket clichés—that conditions were perfect for swing bowling. England has several bowlers adept at that particular skill, and they got to work.
The result was a shipwreck they called the Indian first innings: all of 107 runs. Next day, England piled up a big score in reply. The day after that, the English bowlers got to work again, and the result was a shipwreck they called the Indian second innings: 130 runs this time. Victory to England by plenty.
Despite that beginning, this is not a cricketing lament, I promise you. After all, this is a column about mathematics and science, didn’t you know?
So as I watched the shipwrecks, I thought I should try to learn something about the science of swing bowling. I mean, there was a time when I dreamed of being a fast bowler; but in those distant days, I never did figure out the secret of swinging the ball. Who knows, if I can do it now I may yet bowl for India, swing for India…
Back in the real world. What we’re talking about here is the way fast bowlers are able to get the cricket ball to move in the air, to deviate from the path it starts out on. A ball can also deviate from its path after bouncing—think spin bowling—but here I’ll focus on movement in the air. How does that happen?
First, let’s remember that balls deviate in the air in other sports, too. Football and tennis, for two examples. When Rafael Nadal hits a ball, it’s always with some serious topspin. The result is that the ball’s path curves, dropping towards the ground faster than you’d expect from gravity alone. Why?
Imagine the ball as it shoots through the air off Nadal’s racquet, topspinning as it flies, and think of how the air “splits” to go past, above and below the ball. Because the top surface of the spinning ball is moving against the direction of the onrushing air, it slows down that portion of the air; conversely, the bottom of the ball speeds up the air on that side. (Try this thought experiment that might help: suppose you put the ball into a shallow basin of sand, and then rotate it clockwise. What happens to the sand above and below the ball? What if the sand is moving right to left across the basin as you turn the ball?)
Bernoulli’s principle, that we all learn about in high-school physics, tells us there is a link between air speed and pressure: the faster the speed, the lower the pressure. Thus the speed differential between the top and bottom of the ball causes a pressure differential. There’s higher pressure above the ball, lower below. That produces exactly what you would expect, a downward force on the ball. Voilà: it curves downward as it flies. More generally, a spinning ball will tend to curve in the direction of its spin. This is why some of Nadal’s shots curl left or right, or why footballs will sometimes swerve out of reach of hapless goalies and slam home for a spectacular goal.
The way that spin on the ball produces a force in a particular direction is called the Magnus effect. It explains the way all kinds of balls move in the air in all kinds of sports.
But not the way the cricket ball swung at Lords, bamboozling so many Indian batsmen.
Remember that England’s fast bowlers were not spinning the ball, or at least not in the way Nadal does while playing tennis. So there’s no question of the Magnus effect. But the ball did swing one way or another as it travelled; so James Anderson, Stuart Broad and their pals were somehow producing a force on it in one direction or another. How, if not by spinning it?
The vital factor here is the seam that holds the two halves together, the reason a cricket ball is not a smooth perfect sphere. As the ball travels, the raised stitches of the seam agitate the air that flows around it. This is not a particularly unfamiliar notion. There’s a reason the prow of a ship is smooth and streamlined—that allows the ship to cut cleanly through water. If the prow had all manner of protuberances on it, the ship would not move quite as smoothly, because the protuberances would churn up the water. In the same way, the ball’s seam churns the air, causing a degree of turbulence around the ball.
Essentially, swing bowling is a matter of controlling and directing that turbulence. You do that by changing the orientation of the seam as you bowl. You won’t get the ball to deviate in the air if the seam is upright and aimed straight down the pitch. You want some asymmetry in the way air flows around the ball, and you get that by having the seam point in one or the other direction. This is why the seam is vital. (It would be next to impossible to get a smooth, seamless ball to swing.)
So now let’s say you deliver the ball with its seam pointing slightly to your left. As you watch it hurtling toward the batsman, think of the air splitting to go around the ball. The air on the right side of the ball (as you look at it from behind) encounters no part of the seam, only the smooth surface of the ball. Its flow around the ball stays largely smooth, and this is called “laminar flow”. The air that flows to the left, though, immediately hits the seam and becomes turbulent.
There’s a crucial difference between these two kinds of flow in this situation: turbulent flow tends to stick to the surface of the ball just a little longer than laminar flow does. Why this is so is a tale for another time, and will anyway need more fluid mechanics than I ever managed to grasp. For now, consider: If it wasn’t so, the air from both sides would leave the ball at the same time, at the back of the ball, heading straight at you the bowler. But instead, the laminar air leaves the ball a little earlier, thus off to your right. The turbulent air hugs the ball longer, thus also leaving it off to your right.
In effect, what you have is the ball leaving a “wake” of air that streams to the right and behind, like the plume of a rocket. Apply Newton’s third law—every action has an equal and opposite reaction—and you know that the wake produces a force on the ball that’s directed ahead and to the left. Just enough of a force, if conditions are right and the bowler positions the seam properly, for the ball to swing to the left.
That is, you have bowled an outswinger (to a right-handed batsman). Maybe he tried to drive it, missed the line and has edged a catch to your first-slip fielder. Congratulations!
We can similarly break down inswing or reverse swing, though reverse in particular involves some further intricacies, which is why it is a skill found much more rarely in cricket.
Besides, other factors also help swing. The wear and tear on the ball, for one, especially if it’s more on one side than the other, which is why bowlers and fielders shine the ball on one side. The temperature and humidity, for another. The way you deliver the ball, for a third: the angle of your wrist, the speed of the ball, any spin you can give the ball.
But while all those things play a role, swinging a cricket ball is, when stripped to its seams, a matter of exploiting certain properties of the way air flows past the ball. So is getting a tennis ball or a football to curve as it flies.
Now I don’t mean to make all this sound easy. It’s not. But since I’ve managed to get a sense of the theory of how these balls move in the air, I’ve been wondering. Returning to tennis after a long break, I’ve promised myself I’ll learn to hit with topspin. So maybe I can also learn swing bowling and offer my services to the Indian team in England. Bowl for India, swing for India…
Once a computer scientist, Dilip D’Souza now lives in Mumbai and writes for his dinners. His Twitter handle is @DeathEndsFun
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