How does a Cricket Ball Swing?

Indian fast bowler Mohammad Shami has appealed to the International Cricket Council (ICC) to reconsider the ban on using saliva to shine cricket balls. The ban was introduced as a Covid-19 precautionary measure.

What is Swing?

• Swing refers to the lateral movement of a cricket ball in the air before pitching.
• It occurs due to an air pressure difference on either side of the ball.

How Does a Cricket Ball Swing?

• When a bowler releases the ball, a thin air layer (boundary layer) forms on its surface.
• This boundary layer separates from the surface at different points on either side of the ball.
• The location of this separation determines the air pressure on each side.

Role of Seam in Swing

• Bowlers tilt the seam in one direction to disturb airflow.
• The raised seam side creates turbulent airflow, which sticks to the ball longer and moves faster.
• The smooth side has laminar flow, meaning the air travels slower.
• According to Bernoulli’s principle, faster airflow on one side reduces pressure, causing the ball to swing in that direction.
• If the seam is perfectly straight, the ball won’t swing as both sides have equal airflow.

Why is Saliva Important for Swing?

• Cricketers traditionally use saliva to shine one side of the ball.

Benefits of saliva

• Smoothens one side, increasing swing contrast.
• Adds slight weight to the shiny side, aiding reverse swing.
• Sugary saliva (from mints or candy) is heavier and more effective.

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