|
|
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 on…
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
 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.
 Now 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
 Extend 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.
|
