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

Cricket matches are associated with changes in humidity and moisture during day and night conditions. However, captains of most cricket teams have not shown awareness that choosing to bowl first in a day-night game – as dew is expected to arrive later – carries specific risks.

Sports fans are aware of the ‘dew factor’: water vapor condenses on the ground in the evening, creating a slippery surface. As a result, spinners have difficulty getting a grip on the ball and fast bowlers have more trouble generating swing and seam. Fielding on slippery ground is also obviously difficult.

As a result of these changes in ground conditions, batsmen 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 to its flight path. As a result, the air flow on one side of the ball is turbulent and on the other side is streamlined. This causes a sudden pressure difference that causes the ball to deviate. Its path in motion is called its swing.) Batsmen also expect the ball to slip off the bat under dew, and expect opportunities to maintain a high run rate with less effort.

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

Friction is not so simple

A thorough analysis of the physics of friction shows that the notion that dew always increases slippage is scientifically flawed. Friction is reduced only when the film of water in between is thick enough to reduce the amount of physical contact between the two surfaces. When the thickness is below a certain limit, it increases Aggregate friction because water molecules interact more strongly with two surfaces due to adhesive forces.

A recent study Led by Liang Peng of the University of Amsterdam, they found that when humidity increased by 20% the coefficient of friction doubled and then decreased only then. Scientists attributed this to hydrogen bonds formed as a result of electrostatic forces.

Thus, in humid weather, the coefficient of friction increases, giving the bowler an advantage. Perhaps this is the reason why India’s batsmen lost three wickets early in the match against Australia in Chennai.

The speed of the bowling also has some effect on friction. The work of German physicist Richard Striebek on friction showed that for a given layer of lubricant – such as a film of water on a bat or ball – friction increases when the speed of interaction between the two surfaces exceeds a threshold value. it occurs. So in wet conditions, fast bowlers can use this feature to get more grip by launching the ball at higher speed.

Impact of weather conditions

Cricketers have also demonstrated the belief that the amount of dew has a negative impact on swing. In specific weather conditions, there is an optimum bowling speed, an optimum seam angle and a desirable spin rate. If the delivery speed is less than the optimum 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 low and there is more moisture in the air. That is, changes in air density have a profound effect on the swing.

For example, if the temperature drops from 25º C to 15º C, the air density will increase by 4%, and the deflection of the ball due to swing may increase by up to an inch. The effect is minor but the consequences can be significant.

Here’s another relatively lesser known fact: when the air temperature drops, sunlight creates 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 be due to this effect.

Misunderstanding the effect of dew can make batsmen overconfident, and lead to being caught out attempting to hit a boundary on what they consider to be an ‘easy’ ball. Instead, their chances may improve if they maintain a particular level of moisture content on their gloves and soles while avoiding hitting sixes.

Ins and outs of DLS methodology

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

According to the DLS method, when setting goals, Team A’s score per unit resource is multiplied by Team B’s resources. The ‘value’ of a ball and wicket in percentage terms is derived from data over a four-year sliding 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 as per the DLS method. If the team had known that it would bat only for 25 overs, its members would have tried to score more even at the cost of the remaining wickets. In this setting, the DLS method assumes that Team B has more resources, and sets it a higher goal to compensate for depriving Team A of resources.

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

The main drawbacks of the DLS method are that it cannot take into account the quantitative values ​​of each team and it favors teams that maintain a low run-rate and save wickets in hand. Furthermore, the method also ignores the fact that, after rain, Team B will have to play with a very damp pitch, which will affect its run-scoring abilities while still giving an advantage to Team A, who will gain more. Can if he knows how to use friction to achieve his goals.

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

The author holds a Doctorate degree in Electrical Engineering from the University of Cambridge, UK. His areas of expertise are microsystems, sensors and antennas.