Unravelling the secrets of swing in cricket with the physics of friction

Cricket matches under day-night conditions are associated with shifts in humidity and moisture. However, most cricket teams’ captains have not displayed awareness that choosing to bowl first in a day-night game – because dew is expected later – carries specific risks.

Fans of the sport are aware of the ‘dew factor’: water vapour condenses on the ground in the evening, creating a slippery surface. As a result, spinners have a harder time getting the ball to grip and fast bowlers have more trouble producing swing and seam. Fielding on a slippery ground is also obviously harder.

As a result of these changes in ground conditions, batters appear to have an advantage under dew, as they face less swing, less spin, and less lateral movement of the ball. (Sometimes, fast bowlers release the ball at a certain angle into its flight path. As a result, air flow is turbulent on one side of the ball and streamlined on the other. This causes a sudden pressure difference that causes the ball to deviate from its path in a motion called its swing.) Batters also expect the ball to skid off the bat under dew, and expect opportunities to maintain a higher run rate with less effort.

For these reasons, most captains winning the toss in day-night matches prefer to bowl first and bat second.

Friction isn’t so straightforward

A closer analysis of the physics of friction suggests that the belief that dew always increases slipperiness is scientifically flawed. Friction is reduced only when the water film in between is thick enough to reduce the amount of physical contact between two surfaces. When the thickness is below a certain threshold, it increases the overall friction because the water molecules interact more strongly with the two surfaces due to adhesive forces.

A recent study led by Liang Peng of the University of Amsterdam found that the coefficient of friction doubled when humidity was increased by 20% and decreased only thereafter. The scientists attributed this to hydrogen bonds that formed as a result of electrostatic forces.

Thus, in moist weather, the coefficient of friction increases, advantaging the bowler. This may have been why India’s batters lost three early wickets in the match against Australia in Chennai.

Bowling speed also has some impact on the friction. The work of German physicist Richard Stribeck on friction has shown that for a given layer of lubricant – such as a film of water on a bat or a ball – friction increases when the speed of interaction between two surfaces is higher than a threshold value. So in wet conditions, fast bowlers can use this feature to force the ball to grip more by launching it at a higher speed.

Effects of weather conditions

Cricketers have also displayed the belief that the dew content negatively influences swing. In specific weather conditions, there is one optimum bowling speed, one optimum seam angle, and one desirable spin rate. If the delivery speed is less than the optimal value, the spin will need to be increased to generate a certain amount of swing.

The ball’s trajectory through the air also creates an asymmetric flow field around its surface, which produces the so-called Magnus force. The strength of the force increases when the temperature is lower and there is more moisture in the air. That is, changes in air density have a strong influence on the swing.

For example, if the temperature drops from 25º C to 15° C, the air density will increase by 4%, and the ball’s deviation due to swing can increase by an inch. The effect is minor but the outcomes can be significant.

Here’s another relatively less-known fact: when the air temperature drops, sunlight causes less turbulence in the air above the pitch, giving bowlers more control. The success of Indian bowlers Mohammed Shami and Jasprit Bumrah in the ongoing ICC Men’s Cricket World Cup may well be due to this effect.

A misunderstanding of the impact of dew can leave batters overconfident, and get out caught when trying to hit what they believe to be ‘easy’ balls for boundaries. Instead, their chances can improve if they maintain a particular level of moisture content on their gloves and soles, while avoiding six-hitting.

Ins and outs of the DLS method

Cricketers playing a game in wet weather also need to contend with the peculiarities of the Duckworth-Lewis-Stern (DLS) method. The prospect of rain forces captains to prefer risk-free play that preserves wickets. This is because the DLS method works with the ratio of runs scored to resources used, and the resources are the number of overs and wickets available.

According to the DLS method, when setting a target, Team A’s score per unit resource is multiplied by Team B’s resources. The ‘worth’ of a ball and wicket in percentage terms are derived from data in a sliding four-year window.

For example, if Team A batting first scores 150 runs in 25 overs and loses four wickets, it will have used 50% of its resources according to the DLS method. If the team had known that it would bat only only for 25 overs, its members may have tried to score more even at the expense of the remaining wickets. In this setting, the DLS method assumes that Team B has more resources, and sets it a higher target to compensate for the denial of resources to Team A.

If rain shortens the game such that Team B has 25 overs left plus 10 wickets in hand, the method estimates Team B to have 66.5% of its resources remaining. So it is expected to score 16.5% more runs than Team A did in 25 overs. Thus we have a target of 175 in 25 overs for a draw and 176 runs for a win.

Key drawbacks of the DLS method are that it can’t factor in the quantitative values of each team and that it favours teams that maintain a low run-rate and save wickets in hand. In addition, the method also neglects the fact that, when after rain, Team B will have to play with a very damp pitch, which will influence its run-making abilities even while advantaging Team A, which can reap more gains if it knows how to use friction to achieve its goals.

For these reasons, our cricketers must be made fully aware of the intricacies of playing with dew and moisture, and ensure future wins.

The author holds a doctorate in electrical engineering from the University of Cambridge, U.K. His areas of expertise are microsystems, sensors, and antennae.

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