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The batsman's defence
Brick and mortar of his innings
The physics of a great batting stroke
G Venkatesh
Ideally, any coach starts with the forward / backward defence stroke. But of
late, the Australians have resorted to a novel idea. The player is first taught
the uninhibited swing of the bat. The idea is that anyone who gets accustomed to
swinging the bat freely will find it easier to go a few rungs down - for the
forward / backward defence strokes. This is by no means an undermining of the
importance of the defensive strokes. In truth, they form the very brick and
mortar of a batsman's innings. A Gary Kirsten in the top order of the South
African batting line-up is very much the hardened cement which lends solidity to
the innings. If Kallis and Klusener can display fireworks with gay abandon, it
is thanks in part to Kirsten's dour stints at the crease.
How to play the forward and the backward defensive strokes is common knowledge
to anyone who has been initiated into the game. But, why does a coach tells you
to play these in a particular way?
Defensive strokes
Refer to the diagram which illustrates the bat-ball contact when you play a
defensive stroke, and as you read on, keep an eye on the diagram, to help you
understand better…just as you have to keep an eye on the ball all the time to
play your strokes well!
Let us reduce the analysis of the defensive stroke to a collision between two
bodies, where one is supposed to resist or impede the forward motion of the
other. The resisting body is quite like a wall, which sends a ball thrown
against it, ricocheting back. The difference however is that the wall is fixed
at both the top and the bottom and cannot move in the direction of impact, while
the bat is free to swing back towards the batsman. The challenge that the
batsman faces here is to present as rigid a bat as possible while defending the
ball, with the grip firm and controlling.
When the ball is moving fast, recoil is inevitable, but then, a good defensive
stroke minimizes the recoil of the bat, as much as possible.
In the figure, the ball strikes the bat and imparts a force equivalent to
Fball-bat on it. The faster the ball, the greater is this force. Now, this force
makes an angle 'B' with the horizontal and can be resolved into its cosine
component along the horizontal and sine component along the vertical. The bat
resists the ball with a force equal to Fbat-ball , which is directed
perpendicular to its surface. The bat makes an angle of say 'A' with the
vertical. Resolve this force into its cosine and sine components.
From the figure, one can arrive at the following:
Fbat-ball * cos A resists Fball-bat
* cos B
Fbat-ball * sin A resists Fbat-ball
* sin B
Deductions galore
1. Assume a case when the bat is held loosely and does not resist the
momentum of the ball. According to the law of conservation of linear momentum,
the bat would have recoiled and the ball would have moved on towards the stumps.
The velocity of the ball would have reduced after the impact, but this loss in
momentum of the ball, would have resulted in a gain in momentum of the bat in
the same direction of the ball. Well, this defeats the whole purpose of a
defensive stroke!
2. Now consider a case when the batsman stretches well forward to a spin
bowler and encounters the ball just as it is taking off after pitching. At this
instant, the value of angle 'B' is high and so is the velocity of the ball. On
impact, the force exerted in the horizontal direction is going to be lower as
this would be directly proportional to the cosine of angle B. The vertical
component, therefore is going to be higher. Now to effectively resist and bring
the ball to rest on the track, the countering force Fbat-ball should have a
larger vertical component. This means that the value 'sine of A' will be
greater. In other words, the inclination of the bat will have to be more with
the vertical, when you are moving well ahead to defend a ball close to where it
pitches. Just the right amount of force will arrest the momentum of the ball and
bring it to rest soon.
However, practically, the force of resistance instead of exactly countering
the forward momentum of the ball and making it drop down vertically under the
force of gravity, imparts some extra 'positive' energy to it. This results in
the defensive prod becoming a 'push'.
This is alright as long as the ball is moving along the ground after the
defensive prod. In other words, excess of momentum in the upward direction, will
make the ball pop up and close-in fielders are ever waiting to gobble up those
half-chances! Hence, greater the angle 'A', higher will be its sine value and
the batsman can rest assured that the ball is not going to be spooned up in the
air.
3. Consider a case when the ball is being encountered on the back foot, when it
has ascended a bit higher on its upward path after pitching. At this instant,
the angle 'B' is substantially smaller as compared to Case 2. A smaller value of
'B' would mean that the horizontal component of the force exerted by the ball
will be greater. To counter this, angle 'A' will have to be smaller. The smaller
it is, the more effectively can the backward defence stroke be played. Ideally,
this stroke is played with a perfectly vertical bat. A perfectly vertical bat
would mean that 'A' equals zero and hence all the resistance offered is utilized
to counter the horizontal force of the ball. However, if the ball is still
climbing up towards the highest point of its flight, it has a significant
vertical component on account of its velocity (though the angle 'B' is small).
Here, the bat has to be inclined a bit. This would detract from the horizontal
resistance a bit, but then, the batsman rests assured that the ball does not
gain height after leaving the bat.
In all these cases, if exactly countered, the ball drops down under the force
of gravity, beginning from zero velocity at the top. However, this is rarely the
case. Hence, one would find the ball tracing a parabolic descent and landing
close to the bat in front of it.
When you defend, the bat was either vertical or inclined with the handle
leading. Extend the very same principles to an inclined bat with the handle
trailing at the point of impact. What you get is a lofted stroke.
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