Swing of a Cricket ball

Its time to write about something which I love the most - Cricket! When we [members of CEG cricket team] were sitting at the sidelines while our batsmen were doing the job against IIT Madras 'B' Team, one of my team mates told me that my bowling is quite effective as I deliver the ball with an upright seam which eventually aids swing. But how does it swing???

The Physics

The fundamental elements involved in the physics of swing of a cricket ball are turbulent and laminar flow of air around the ball. The swing caused is NOT BECAUSE OF MAGNUS EFFECT as the movement is along the axis of rotation of the ball and not perpendicular to it.

Inswing and Outswing

For conventional swing, the ball will move towards the rougher side. When the seam is held such that it points towards the slip and the ball is held with the rougher side on the left side, the flow of air around the ball will be turbulent on the left side and laminar on the right side. This happens when the ball is bowled at about 120 kmph or less. As the pressure on the turbulent side is lesser, in accordance with Bernoulli’s principle, the ball swings towards the left(outswinger). Inswinger occurs in a similar way when the rougher side is on the right.

Reverse swing

When the ball becomes old and if the bowler is able to generate pace higher that 120 kmph, the swing characteristics differ. The ball begins to swing in the direction of the polished side and hence reverse swing. Incase of reverse swing, the magnitude of the ball’s movement is stronger than conventional swing. It also occurs late after the ball is delivered. At higher speeds of the ball, the flow is turbulent on both sides, but the separation of airflow from the ball occurs earlier on the rougher side. So the smoother side will have constant turbulent flow and hence have a low pressure. Thus the ball moves towards the shiny side. The raised seam aids separation of airflow(also for conventional swing).

Well, as we all know Zaheer and Ishant are capable of getting reverse swing consistently(remember Ishant’s 8 over spell against Ponting?). Hope India wins the series against New Zealand