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Physicist brothers demystify spin-bowling secrets

Clever mathematical calculations from two Aussie brothers reveal how slow bowlers use small variations in ball rotation to cause problems for cricket batsmen.

Ian and Garry Robinson – honorary fellows at Victoria University and the University of New South Wales respectively – have used mathematics to demystify the secrets of spin-bowling.

Spin bowlers in cricket are masters at making the ball loop slowly through the air to confuse batsmen.

Legends of the game know the magic combinations of top-spin, side-spin and off-spin necessary to fool the opposition, but shrewd calculations by the academic duo will share this knowledge with the punters.

In their latest work, published in the journal Physica Scripta, the brothers highlighted the significance of small variations in different spin types.

To calculate the various flight paths, they considered a number of forces acting on the ball – including gravity, drag force and a Magnus force.

Top-spin causes the ball to dip in flight, side-spin causes the ball to move side-ways through the air and, perhaps most importantly an off-spin can cause the ball to drift across the pitch towards the end of the delivery, drawing the batsman into a more vulnerable position.

When their numerical analysis confirmed some of the more well-known details of the game, the duo took to examining spin-bowling at a subtler level.

“We found that if the total spin is kept constant and a small amount of top-spin is added to the ball at the expense of some off-spin, the length at which the ball pitches can be reduced by as much as 25 cm,” Ian Robinson said.

“It is an amount that batsmen can ignore at their peril – despite little change being observed in the side-ways drift. A small amount of side-spin introduced to a top-spin delivery does not alter the point of pitching significantly, but can produce 10 cm or more of side-ways drift.”

The physicists hope their analysis will give newcomers to spin-bowling a helping hand in mastering the variety of deliveries necessary to keep batsmen guessing.

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