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December 2002

Feature

 


Will it bounce?

G Venkatesh

If you wish to see beyond the obvious and enjoy a game of cricket not just for the runs scored or wickets taken or catches spilled, but for something more also, read onů

How the ball behaves in the air is largely determined by the relative roughness of both halves. The wind is also factored in. But how it behaves after hitting the turf depends, if not entirely, at least to a considerable extent on the state of the wicket. Else, the Perth pitch would not see a lot of bouncers being bowled and the Trent Bridge track will not enable swing bowlers to deviate the cherry appreciably in the air, while not bouncing the ball often, above the height of the bails at the batsman's end.

It is intriguing at first, but elementary analysis will sort things out. And if you have been avoiding the pitch report so lovingly given by Tony Greig and Micheal Holding often, before the start of cricket matches, you will realise what you had been missing all along!

Impact dynamics

In a solid particles are densely packed together, in a liquid they are freer to move.Let us start from first principles - the atomic level. In a solid, the atoms are packed together tightly in a crystal structure. The energy levels of these atoms are lower as compared to liquids and gases. They would not be able to vibrate freely and are held closely together by the inter-atomic forces that are more pronounced owing to the proximities involved. (see figure to the right).
When there is a blow on the surface of such a solid, the force is immediately resisted by the inertia of the atoms. The first atom receiving the impact, would not be able to vibrate much as its movement will at once be impeded by the others in the vicinity, which will repel it back. This cumulative repulsion manifests itself as the force of resistance exerted by the solid on the impactor. If the impactor is an object, this force will tend to push it away.

Vapour content of the air above the pitch goes up as the Sun goes up overheadNow consider a case when the solid is not very hard. Some water has seeped into the spaces between the closely-packed atoms. Now, when there is an impact, even though the atoms do not vibrate freely, the energy is transferred to the water molecules. Water is a liquid, and the molecules in it are freer to vibrate as compared to the solid. Hence, the impact moves from atom to atom, via the water medium, till it dies down. In other words, the energy which is supplied to the solid surface by the impactor is absorbed or cushioned. This is lost or 'spent' energy, which manifests itself as the movement of the molecules/atoms in the solid/water and finally is dissipated as heat energy. (See figure above)

Contrast this with the earlier case, when the energy which was transmitted was not absorbed but given back to the impactor, owing to the resistance offered by the interfering atoms. However, it should be noted that no collision is perfectly elastic. Some energy is lost nevertheless in the first case also, and this manifests itself as a little heat.

In both cases, it is this loss of energy that results in the ball losing velocity on the rebound. That is how one may define the coefficient of restitution. That is why the coefficients of restitution of different pairs of impacting surfaces are different.

Applying the basics

Ball bounce is affected by moisture content of pitchExtend that to the pitch and the ball bouncing on it. Consider a track which has just received rains or a track in a city where the ground water level is not very deep. To add to this, if there is a fall in pressure in the atmosphere, by capillary action, the ground water may seep towards the surface. With the passage of time, the amount of water present to absorb impacts received by the track from the ball, will go up. The ball will start bouncing lesser and lesser.

When the sun comes out in the afternoon, the moisture will evaporate from the surface of the track. This will add to the water vapour content of the air over the pitch. The addition to the content of water vapour in the air, will increase the partial pressure of the same and thereby the air pressure. Hence, as the match progresses, fast bowlers will be able to get more swing, if they manage the roughness/shine factor well.

England

On English tracks, the moisture content is higher as the Westerlies bring rains practically throughout the year. The air remains saturated with water vapour and hence exerts more pressure on the cherry. Hence, when Greig tells you from the commentary box at Lords, that the ball would start swinging after the 15th over, he simply means that by that time, the moisture below the soil would have evaporated. However, the bounce component would be nullified as long as the track is wet and all the moisture has not evaporated. Once all the moisture goes off, bowlers would also be able to get a slightly greater bounce. This however, will also depend on how hot or cold the weather is. In England, which is in the north temperate zone, it seldom gets hotter than 25°C. Hence, the pitches there will predominantly assist swing bowling.

The Tropical cricket venues

Now take the case of the West Indian wickets. The Carribean islands lie between the Tropic of Cancer and the Equator and hence have a hot, wet climate. The ground water level is high enough, due to the heavy rains they receive, but the temperatures too are very high - 30°C to 40°C. Hence, in the mornings, when it is relatively cooler, both bounce and swing would be a wee bit less. Hence, the bowlers would have to rely on sheer pace to dominate. However, as the day progresses and it gets warmer, the moisture seeps out and evaporates quickly, adding to the water vapour content of the air. This is when one can expect the cherry to swing a bit. At 3.00 pm - the hottest time of the day - the pitch dries out totally and the water vapour by that time, dissipates itself over a wider area. Hence, the effect of the extra pressure is diminished a bit, while the ball starts bouncing more. The same can be extended to wickets in the Indian sub-continent - Colombo, Islamabad, Karachi, Mumbai, Chennai et al.

Australia

Down Under is a dry place. Perth for one, lies on the western fringe of a large sandy desert. These dry tracks are hard and owing to the reasons mentioned earlier, if you give the cherry a nice thud, it responds by rebounding well and bounces high. As the day progresses, and the temperature rises, the hard tracks start getting drier and impacts leave behind cracks on the surface. This is when people like Warne and McGill make merry! Pitches at Melbourne and Sydney also are dry and hard, though these two cities on the South-eastern coast are a bit cooler as compared to Perth, and receive more rains.

Green-tops

Sometimes, you get a greentop to play on. Christchurch and Wellington are good examples. There is grass on the pitch. Presence of vegetation is a sign of presence of moisture below the soil (a sure-shot divining tool!). When there is grass, the bounce would be less, and one can assume that the ball would start swinging after a few overs.

Use of rollers

Many of you may have noticed that the skipper of the team which is about to commence its innings, asks for a roller to be rolled over the track a few minutes before his opening pair would walk in to bat.

There are two rollers - a light one and a heavy one. When a heavy one is used, some of the water that may be there below the surface of the wicket, is squeezed out in the process, like taking water out of a sponge. This water evaporates immediately if the weather is warm and hence by the time the batsmen walk out, the wicket is neither completely dry nor very wet. Hence, the batsmen will find the ball coming on to the bat. The ball will neither keep very low nor will it bounce too high.

On the other hand, if a pitch is dry and hard, a heavy roller will do damage by introducing cracks on the surface (and widening the ones which may already be there.) (Remember that substances which are not elastic enough and which are not able to absorb impacts, fail by breaking or cracking.) These cracks may widen as play progresses and turn out to be bonuses for spinners who may exploit them. Hence, a light roller is generally preferred to gently even out any irregularities that may have developed.

So, the next time you switch on your TV set, and see someone squatting and explaining how the pitch would behave during the course of the day, and also trying to reason on whether the decision of the captain winning the toss was the right one or not, please lend your ears!


More Information: 
G.  Venkatesh has an MSc in mechanical engineering and works in the publishing field as editor and writer.

 

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