Explained: What makes a cricket ball swing and how cheats tamper it to gain an edge

Australia's Josh Hazlewood celebrates after dismissing England's Jonny Bairstow during the first day of the fifth Ashes cricket test match. (REUTERS)

In light of the recent ball-tampering controversy involving players from the Australian cricket team, particularly skipper Steve Smith and vice-captain David Warner, many cricket fans would be wondering why was the incident made such a big deal.

Common questions like “Why does a ball swing in a particular way?” or “How does reverse swing work and how is it different?” and “How does ball tampering change or improve swing?” may seem like stupid questions to ask. But even those who ardently follow the game and are up to speed with its nuances find it quite difficult to understand how swing really works.

The science of swing

For starters, tampering with the state of a cricket ball is primarily done only on one side of the ball to extract more swing or drift from it.

Usually, when one side of the ball is continuously polished by the fielding side, it remains smooth and shiny. When the ball is delivered, it starts swinging in the direction opposite to that of the shiny side, which is how conventional swing works. This occurs when the ball is in motion and the air reacts differently to the rough and smooth sides.

When a bowler releases the ball from his hand, with its seam pointing in the direction he intends to swing it, the seam splits the air into two. The air that passes over the smooth side of the ball creates some pressure on that side, while the air that passes over the rough side does the same, but to a smaller extent.

The general perception would be that the air passing over the rough surface would exert more pressure because of more friction. However, when the ball is in the air, the rough side traps pockets of air on the surface, the dents and scratches. So when the ball cuts through the air, the portion of air acting on the rough side passes over it relatively easily when compared to the other side as it meets the trapped air on the surface and not the surface itself.

As a result, the pressure on the smooth side is greater than that on the rough side, and the ball moves from a region of high pressure to a region of low pressure, causing it to swing in the direction opposite to that of the shiny side.

How does reverse swing work?

Reverse swing generally occurs with a relatively older ball that has probably been used for 40 overs or more. Because the ball is old, both its sides are rough, so much so that the air it cuts through passes over both sides in a similar fashion.

However, the fielding side continues to polish or shine only one side of the ball, which remains in a relatively better condition. As a result, when the ball is travelling through the air, the seam separates the air into two, but earlier on the rougher side than on the smoother side. In the absence of a hard shiny side, like in the case of conventional swing, the higher friction on the rougher side is enough to overpower the pressure being exerted on the other side. The ball is then sucked from a region of high pressure to one of low pressure, thereby making it swing in the direction of the smoother side.

Also, because the ball is older and the seam is relatively battered as compared to a new ball, it takes longer to separate the air it travels through. This is why reverse swing typically occurs later in the air than conventional swing, making it more difficult to deal with as a batsman.

Ball-tampering, and how it can change a match

The illegal act usually involves scratching or roughing up a ball intentionally so that a rough side is created sooner. It can involve tampering the ball using objects like one’s chewing gum, or Vaseline to the shiny side. But the bottom line is the same - the condition of the ball is artificially altered to make it swing beyond a certain extent, or sooner than the batsman would expect it to.

If the history of ball tampering incidents is carefully examined, one would notice that teams or players usually resort to it when conventional tactics and methods are not fetching them wickets. If caught, it amounts to a serious offence because it is considered against the spirit of the game and is viewed as cheating by many.

When a player tampers with the ball, it potentially alters the course of the match. When the recent Australian controversy came to light, many started speculating that although only a few players were named in the incident, the rest of the team may have known about it. This is because it would be virtually impossible for a bowler to not notice and recognise the condition of a ball when he has it in his hands.

Until now, Australian bowlers were not known to be great proponents of reverse swing. So when they win a match with it and incidents like this come around the same time, one can’t help but wonder if it was because of tampering that the ball was swinging against South Africa, or England.