Spin bowling: Physics' knowledge can teach the tale of turn

Batsmen facing up to spin bowlers have to factor in the turn, the flight and the guile of the bowler in question, but new research indicates that they may well have to take physics into account, with wind direction playing a key role in where the ball pitches, how it dips and how it holds up.

A pair of brothers from Australia have been exploring the physics behind the spin of a cricket ball. While physicists are much more accustomed to measuring the spin of electrons, protons and neutrons, Garry and Ian Robinson, Honorary Visiting Fellows at the University of New South Wales and the University of Melbourne respectively, have presented equations that govern the trajectory of a spinning ball as it moves through the air in the presence of a wind.

Their paper will be published in Physica Scripta - a journal brought out by IOP Publishing on behalf of the Royal Swedish Academy of Sciences for the Science Academies and the Physical Societies of the Nordic Countries.

According to the research, the presence of a crosswind from either side of the cricket pitch can cause the spinning ball to either slightly "hold up" or "dip", depending on which direction the wind comes from and which way the ball is spinning. This, therefore, changes the point at which the ball pitches on the wicket.

"Our results show that the effects on a spinning ball are not purely due to the wind holding the ball up, since a reversal of wind direction can cause the ball to dip instead," said Garry Robinson. "These trajectory changes are due to the combination of the wind and the spin of the ball.

"The effects of spin in the presence of a crosswind, and how to fully exploit it, may or may not be completely appreciated by spin bowlers. Either way, we have provided a mathematical model for the situation, although the model of course awaits detailed comparison with observations."

Physica Scripta said the researchers show that when a 14 kph crosswind interacts with the spinning ball, the point at which it hits the ground can change by around 14 cm, which they believe may be enough to deceive a batsman.

The equations take into account the speed of the ball, gravity, the drag force caused by air resistance, and the Magnus or "lift" force, while at the same time incorporating the important effect of wind.

The Magnus force is a commonly observed effect, particularly in ball sports, when the spin of a ball causes it to curve away from its set path. This is observed in football when players purposely put spin on the ball to make it bend around a defensive wall.

The researchers also show that a spinning cricket ball tends to "drift" in the latter stages of its flight as it descends, moving further to the off-side for an offspin delivery and moving further towards the leg-side for a legspin delivery, effects which are well-known and regularly utilised by spin-bowlers.

With the Ashes set to start in less than a week, the spinners in the England and Australia teams will hope the unpredictable British weather brings plenty of wind in the series.