There are two types of variations in seam bowling - leg cutter and off cutter. A lug cutter is bowled by placing the middle finger on the seam with the index finger about two centimetres away.
The ball cuts into the right hander here. While bowling an off cutter, the entire process is reversed. In this case, the ball veers away from the right-hander. Decks play a pivotal role in generating seam movement. The bowlers are able to produce more movement on a hard strip that is studded with cracks. Cracks give rise to deviation as the varied surface constitutes movement. On such tracks, the ball changes its path after it hits the crack.
Green-tops also enhance this effect, owing to the extra grass cover. For a ball to deviate off the pitch, the seam position should be straight. If the ball lands on the pitch with a straight seam, there is a good chance it will change its trajectory. The condition of the pitch matters a lot for the bowler to get the ball seaming.
The pitch with cracks or grass covering will aid the seam bowlers. One of the interesting parts of seam bowling is that even bowlers don't always know in which direction the ball will change its trajectory, unlike swing in which a bowler deliberately bowls an inswinger or outswinger. Know more about the difference between a Duke ball, a Kookaburra ball, and an SG ball. These are external factors but there are certain points that are within the control of the fielding side.
Players are allowed to apply saliva and sweat to the ball with a view to keeping it shiny on one side. The other side of the ball is left alone to allow it to rough up naturally.
The players are not, however, allowed to alter the condition of the ball by any artificial means. Swing bowling is a difficult art to master on a consistent basis. At times, the condition of the ball will combine with overcast skies to provide a perfect situation for most bowlers to obtain some swing. However, the best swing bowlers can use this art, even when those conditions are not in their favour. When a ball swings prodigiously it is very difficult for the batsman to defend his or her wicket.
When a fielding side shines one side of the ball while leaving the other to deteriorate naturally, they are applying a rule of physics which can help that ball to swing. Any form of swing bowling is dependent on that air flow. As the bowler delivers the ball, a thin layer of air forms around it. This is known as the boundary layer but it cannot form to cover the entire surface of the ball and it will separate at certain points. The point of separation will determine the pressure on the ball.
Crucially, swing will be generated if the pressure is different on either side of the ball. This is why the fielding side treats the two individual hemispheres of the ball differently. With conventional swing bowling, the bowler is either looking for inswing or outswing.
Inswing bowling sees the ball swing in to the batsman while outswing leads to it moving away. The seam is important here because it acts as a rudder, guiding the ball in the direction that the bowler wants it to swing. If the seam is angled to the slips on the point of delivery, the bowler is looking for outswing, away from the batsman. In contrast, with the seam angled into the batsman, the bowler is aiming for inswing which would take the ball into the stumps.
There is a second factor to consider and this is reverse swing. This is a relatively new phenomenon which is believed to have been discovered in the early s.
There are also speed ranges in which flow may be turbulent or smooth. These speeds are given by Reynolds number. A medium pacer uses this middle critical region of speed, where the flow may be smooth or turbulent, to swing the ball. The rough side of the ball has turbulent flow whereas the smooth side of the ball has smooth flow in this critical range of speed. Hence, air on the rough side fails to catch up with the air on the smooth side creating a low pressure region on the rough side.
This tends to deviate the ball on the rough side in normal swing. Reverse swing can only occur when both sides of the ball has turbulent flow. This is only possible at high speeds. Pakistani bowlers did research to achieve these high speeds only and nothing else.
Wasim Akram even advised Irfan Pathan to achieve high speeds back as always after the T20 world cup. Bernoulli principle comes into effect at these high speeds and it makes air speed low on the rough side and fast on the smooth side. Faster speed on the smooth side leads to air being vanished more quickly on the smooth side creating a low pressure region as explained by Bernoulli principle. Hence, the reverse swing. Bernoulli's Principle states that for an ideal fluid low speed air is a good approximation , with no work being performed on the fluid, an increase in velocity occurs simultaneously with decrease in pressure or a change in the fluid's gravitational potential energy.
But how does it help a pace bowler playing cricket? It helps pace bowlers who know the art of swing bowling. A ball bowled thrown down the length of a cricket pitch at high speed "swings", i. To understand this, let us look at the cricket ball :.
The newball is smooth on all sides to begin with, but after a few overs are bowled, the surface becomes rough, thanks to the beating it receives from the batsman and the bowler who slams the ball down on the rough earth.
The swing bowler takes advantage of this condition of the ball by applying saliva and rubbing only one side vigorously on a piece of cloth usually his trouser. What results is a ball with a surface that is smooth on one side of the seam and rough on the other.
When such a ball is bowled at a high velocity with the seam vertical and in the same plane as the moving ball, it moves right if the smoother surface is on the right and to the left if the smoother surface is on the left, as seen by the bowler.
The reason is as follows.
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