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[ruchir]

http://www.timesonline.co.uk/tol/sport/cricket/article6740828.ece


August 6, 2009
Nasa proves why cricket balls really swing

Mark Henderson, The Times Science Editor, with the assistance of Nasa, offers a study in contrast to the established belief of swinging certainties



When clouds roll in over Headingley, swing bowlers smile in expectation. The venue for the fourth npower Test has long been known for extravagant movement in the air and James Anderson, Graham Onions and Ben Hilfenhaus will be hoping for overcast conditions.

The notion that the ball swings when skies are grey is part of cricket folklore and players and commentators have invoked the weather to explain why the ball moved ferociously on the second and fourth mornings at Edgbaston, but not at other times.

Science, however, has a different view of swing. Its causes have been well established by experiments that explode the myth that cloud cover and humidity have an effect.

Rabindra Mehta, a Nasa scientist and schoolfriend of Imran Khan, the former Pakistan all-rounder, has studied the aerodynamics of cricket balls for three decades and has advised Troy Cooley, the Australia bowling coach. And he has been watching the Ashes on television from his California home in exasperation.

“What the commentators, cricketers I much admire, have been saying about swing is plain wrong,” he told The Times yesterday. “They’ve been talking about the clouds, how the new ball won’t swing until the lacquer has come off, and it’s just rubbish.”

Contrary to common belief, there are three types of swing bowling, not two, he said. Seam position and bowling speed are critical to achieving all of them, but overcast weather conditions are not.

As the ball moves through the air, a thin “boundary layer” of air hugs its surface before breaking away. If the boundary layer breaks later on one side of the ball than the other, pressure will be reduced, causing the ball to swing that way.

For conventional swing, the ball is gripped with the seam angled towards slip for an outswinger and fine leg for an inswinger. Part of the shiny side must face the batsman with the bowlers’ fingers resting next to the seam. As air catches the raised seam, it creates turbulence on that side only. The boundary layer breaks later on the other side of the seam, causing swing in that direction.

Reverse-swing works, as the name suggests, in reverse. The seam is pointed away from the direction in which the bowler wants swing and part of the rough side of the ball faces the batsman.

The roughness means the boundary layer starts off turbulent on both sides. The seam, however, weakens this turbulence on its side, causing the boundary layer to break earlier and sending the ball swinging in the other direction.

The key to normal and reverse-swing is to bowl the ball with backspin so that it revolves along the seam. A bowler such as Anderson, with a high action and strong wrist, will achieve this more reliably than one with a low arm such as Mitchell Johnson.

The condition of only one side of the ball is important: the shiny side for conventional and the rough side for reverse. “The new ball is perfectly capable of swinging this way: that the other side is shiny doesn’t matter,” Dr Mehta said.

Also critical is speed. For conventional swing, the optimum pace is about 70mph and balls of above 80mph will not move at all; very fast bowlers must rely on reverse-swing. Dr Mehta thinks the radar used in the present series is exaggerating the pace of all bowlers.

Reverse-swing works with the new ball only at high pace, of 90mph or more, but as one side roughens, the speed at which it will reverse drops.

The third type of movement, often confused with reverse-swing, is contrast swing. This is achieved with a vertical seam and works when one side of the ball is rough and one smooth, and speed of delivery determines which way the ball moves.

Slower balls move towards the rough side, because of greater turbulence on that side. Faster balls, however, will move towards the shiny side: as with reverse-swing, there is initially turbulence on both sides, but this weakens over the rough surface.

“Commentators often say it’s reversing when it’s actually contrast swing,” Dr Mehta said. “True reverse-swing occurs only when the ball moves in the opposite direction to the seam.”

Pitch conditions can help: a grassier wicket protects shine for longer, assisting conventional swing, while an abrasive one will cut it up, promoting reverse-swing. A prevailing wind can alter turbulence around the ball, which may explain why some grounds and ends are better for swing.

Dr Mehta says the reason why it is believed weather conditions matter is psychological. The idea that the ball swings under cloud is so ingrained that we remember occasions that confirm this impression and forget those that do not, and overcast conditions may also affect bowlers’ confidence.

“My latest theory is that when bowlers show up and the conditions look ideal, maybe they subconsciously orient their wrists better and bowl a bit fuller,” he said. “Perhaps they just bowl better.”

[kban1]

Dr. Mehta should look into the change in atmospheric conditions - humidity, wind speed, moisture, wind direction and how that affects airflow to understand how weather can be incorporated into his analysis rather than stating that

a) his analysis and the impact of weather are mutually exclusive events.
b) only his analysis is correct and the weather is a non factor

[RicePlateReddy]

Same fellow who made a noise about contrast swing as opposed to reverse swing a few years ago.

Harsha Bhogle defended him here -
http://www.indianexpress.com/news/why-mehta-matters/1466/

What I love in this article:

"Kapil Dev: What is all this contrast swing..I know swing bowling, not rocket science. I used to play cricket, not mathematics. These are new-fangled terms (not an expression Kapil Dev would use, hence I must advise some caution with the quote) for things we have been doing since way back.

[kban1]

Not sure why harsha is taking to task the cricketers who say that they do not know about contrast swing --or any other swing when talked about in terms of core physics. It is almost natural that barring the handful of cricketers who possess / possessed an advanced scientific degree (kumble, Cheeka, Prasanna), these cricketers would not know the intricacies of the science behind the art they practice.

Just as it is not right to deride mehta, neither is it right to scoff at the cricketers.

If tomorrow, a physicist (or some new fangled biomechanist --with or without the aid of new age Power Point presentations) tried to explain to Sehwag the physics behind a bat swing, or the balls impact with the bat, or the principle of the lever, it is but natural that Sehwag would say "I dont understand all this --I know how to bat, not the physics behind it".

Unless Bhogle also subscribes to the theory of making cricketers better human beings by imparting to them a higher education in physics, I fail to see what is wrong in whatever kapil and the other cricketers have said.

And as far as Mehta is concerned, I am surprised that he talks about turbulence, etc etc but fails to take into account the fact that swing (which is made possible through the ball's path being affected by air viscosity) would also be affected by cloudy conditions precisely because such conditions change the air viscosity, thereby making less or more swing possible.

[broadbat]

Science, however, has a different view of swing. Its causes have been well established by experiments that explode the myth that cloud cover and humidity have an effect. 

Really tough to believe this one. Perhaps Mehta has not found out why this happens?
Though rest of the article does make sense.

[WicketView]

I find it rather hard to believe that the weather conditions do not play a role, and this entire business is psychological.

Incidentally, I think it is worth pointing out that the headline of the article is rather misleading. NASA and an individual at NASA are hardly the same thing. Typically, if a well-known organization makes a statement, it is on the basis of work/cross-verification by a number of experts. Hence, one believes to a larger extent statements issued by well-known organizations. Most of what I could see in this and other articles, seems to suggest that this is the individual view of (probably born of a personal interest) this one man. Not that this makes his views wrong, but they headline is misleading for sure.

[dhruvdeepak]

Dr. Mehta should look into the change in atmospheric conditions - humidity, wind speed, moisture, wind direction and how that affects airflow to understand how weather can be incorporated into his analysis rather than stating that

a) his analysis and the impact of weather are mutually exclusive events.
b) only his analysis is correct and the weather is a non factor

yep agree