Swing bowling

Swing bowling is a technique used for bowling in the sport of cricket. Practitioners are known as swing bowlers.

Physics of swing bowling

The essence of swing bowling is to get the cricket ball to deviate sideways as it moves through the air towards the batsman. In order to do this, the bowler makes use of three factors:

Related Topics:
Cricket ball - Batsman

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• The raised seam of the cricket ball.
• Asymmetry in the ball caused by uneven wear of its surface.
• The speed of the delivery

The asymmetry of the ball is encouraged by the constant polishing of one side of the ball by members of the fielding team, while allowing the opposite side to deteriorate through wear and tear. Over time, this produces a marked difference in the aerodynamic properties of the two sides.

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At speeds around 80 mph, the airflow around the ball is in transition between smooth, or laminar flow, and turbulent flow. At speeds of 90 mph and above, all the flow is turbulent. A medium-pace bowler, working at 75 to 80 mph, takes advantage of this. In this critical region, the raised seam and other minor imperfections in the ball's surface can induce turbulence while air flowing over other parts of the ball remains laminar. Turbulent air separates from the surface of the ball later than laminar flow air, so that the separation point moves to the back of the ball on the turbulent side. On the laminar flow side it remains towards the front. The result is a net force in the direction of the turbulent side.

Related Topics:
Laminar flow - Turbulent flow

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Thus by keeping the seam and roughness to one side, the bowler induces the ball to swing in that direction. Skilled bowlers can even make a ball swing one way, and then 'break' the other way upon bouncing, with an off cutter or leg cutter hand action.

Related Topics:
Off cutter - Leg cutter

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Note that swing of a cricket ball is not caused by the Magnus effect, which gives rise to a force perpendicular to the axis of rotation (in this case up or down). The swing of a cricket ball is parallel to the axis of its rotation.

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