Hello, and welcome to the Aftermatter for the first instalment of a series on the physics of sport in the lead up to the Olympic Games, this week talking about Swing Bowling in cricket. Any of you who have watched cricket will have seen this effect; fast bowlers bowling balls that seem to swerve in mid-air. But why does this happen? And why do they constantly rub the ball against their trousers? And what about the mysterious ‘reverse’ swing?

Ali’s bowling there was an example of conventional inswing, that is, swinging towards the right-handed batsman. But how exactly did Ali make this delivery swing?

First let’s look at a new cricket ball. It is spherical, with a leather outer layer, and has a “seam” forming a circumference around it, which sticks out a bit.

A new cricket ball. Note the prominent seam

The air hits the smooth face of the ball first, and a laminar boundary layer is formed in both directions. However, as we see at the top of the diagram, the seam ‘trips’ the air particles of the laminar layer, and a turbulent layer is formed. Now, turbulent layers cling to the surface of objects much more than laminar layers, which leave the object relatively early. This means that there is more air resistance near the top of the ball in the diagram above, and the bottom of the ball will travel faster. Since one side of the ball is travelling faster than another, the balls direction will curve towards the side travelling slower. This starts to happen at about 30 mph, and reaches a peak at about 70 mph, depending on the ball. After 70 mph, this effect is gradually reduced, until it comes to a complete standstill at about 90-95 mph. This is because, as the ball is bowled faster, the air becomes turbulent immediately upon impact of the ball, and thus the air is turbulent on both sides, and there is no force pushing the ball in either direction. But what happens if you bowl even faster than this?

If you did begin to bowl at 100 mph or more, then you may begin to notice reverse swing with even a new ball. The air becomes turbulent on impact with the ball, but when the air is tripped by the seam this time, the layer becomes even more turbulent! Very turbulent boundary layers actually break away from the object earlier than less turbulent ones, meaning that exactly the opposite of what happens in conventional swing happens! The ball swings the opposite direction, with no change of action on the part of the bowler. This can be very difficult to face for a batsman; top batsman can often predict by the seam direction which way the ball will swing, but with reverse swing it does exactly the opposite of what they would expect.

In practice, no bowler has ever bowled faster than 100 mph, so Reverse Swing with a new ball would be almost impossible to achieve. However, as the ball becomes older and rougher, the rough surface causes the air to become turbulent at lower speeds, so with an old ball, reverse swing can be achieved at speeds of about 80 mph, which most international fast bowlers can reach. It’s for this reason that the rough side faces the batsman in reverse swinging deliveries rather than the shiny side. The devastating effect of reverse swing can be seen in the video below, skip to 3:10 for the best bit.

There is one more type of swing which isn’t mentioned as often, as it is used less in top level cricket, but is perfect for amateur players. In many ways, it is the most simple type of spin. In order to bowl it, one needs to have one side smooth, and the other rough.

It uses the same principles of laminar and turbulent layers as conventional and reverse swing, but rather than use the seam to trip up the boundary layers, it uses the rough surface of one side of the ball to create turbulence

At less than 70 mph, the boundary layer over the smooth side will be laminar, and over the rough side it will be turbulent, meaning the ball will swing towards the rough side as with conventional swing. You can even try this out at home quite easily: get a tennis ball, and cover one half with duct tape, and throw it straight with the duct tape on the left, and the uncovered half on the right, and you should find that it swerves right.

However, bowling at higher speeds, more than 70 mph, like in reverse swing, the boundary layer will be turbulent on both sides of the ball. The rough side makes the air even more turbulent, meaning it detaches from the ball sooner, and the ball moves in the opposite direction to how it would at lower speeds. This happens for exactly the same reason that conventional swing switches to reverse swing at a certain critical speed.

In conclusion, we have seen that physics can explain the seemingly mysterious arts used by bowlers to try and deceive batsmen, and that “Reverse” swing is just a natural continuation of conventional swing. It is so powerful because the bowler changes nothing, it is just the physics that changes, and this creates some of the greatest contests in international cricket.

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