Cricket: The Physics of How the Ball Is Bowled
Cricket: The Physics of How the Ball Is Bowled
As with most confrontations that can sometimes take on epic stature, the tools of the conflict between a cricket bowler and the batsman are very simple. The batsman strides into the crease, prepared to defend a three-posted wooden wicket with a flat-sided willow bat, equipped with modest protective equipment and his reflexes. The opposing bowler has a single weapon, a hard-wound, leather-skinned ball, bound with a single raised seam, that is delivered by the bowler with a variety of spins and speeds.
The manner in which a cricket ball may be delivered is well defined by the rules of the game. The bowler is not permitted to flex the elbow of the arm used to deliver the ball, in the manner of a baseball pitcher. The arm of the bowler must generally be straight, with the elbow extended. At the conclusion of the delivery, as the bowler approaches the limit of the crease, the bowler will often incorporate a dynamic leap to bring greater force to the ball as it is released from his hand. The ball is not required to be bounced in front of the batsman, but is done so invariably to assist in the deception as to the ball's movement as it is thrown toward the wicket.
The pitch on which the bowler delivers the ball is 66 ft long (19 m); the ball must be delivered within an 8 ft (2.6 m) wide crease. There are two general methods for the delivery of a cricket ball, and two corresponding types of bowlers, representing terms as well as strategies: fast bowling and spin bowling. The type of bowling to be used will depend on both the nature of the opponent and the conditions of the pitch.
A fast bowler is one who delivers the ball at a high rate of speed; a typical international caliber fast bowler will reach speeds of between 85 mph and 90 mph (140 km/hour and 150 km/hour). While the ball is delivered to achieve a measure of spin and resulting break when it bounces off the surface of the pitch in front of the batsman, the velocity of the ball is its primary feature.
By contrast, a spin bowler uses a variety of techniques to induce the ball to spin sideways after contact with the pitch, to fool the batsman as to the ball's trajectory. A typical spin bowler delivers the ball at speeds ranging from 45 mph to 60 mph (70 km/hour to 100 km/hour).
By rule, an innings of a cricket match will commence with the use of a new, unmarked cricket ball. A new ball, and its tendency to bounce harder and faster from the surface of the pitch, makes it a desirable ball to be thrown by the fast bowlers. As the match goes on and the ball is marked by the effects of play, as well as the permitted scuffing of its surface by the bowlers, a ball is created that is preferred by the spin bowlers.
There are distinct physical principles underlying each of the fast bowling and the spin bowling techniques. Fast bowling is further subdivided into two categories: the seam bowling method and the swing bowling method. Seam bowling involves a delivery where the seam of the ball is vertical to the ground, and the rotation imparted to the ball on delivery is horizontal. When this ball strikes the surface of the pitch, it tends to move unpredictably and therefore is difficult for the batsman to hit.
Swing bowling is a delivery intended to make the ball move in flight, through a combination of three different physical factors. These factors include:
- the speed of the ball at delivery
- the imperfections of the surface of the ball induced by the rubbing of the surface by the bowler, the application of sweat or saliva by the bowler, and the effects of prior play
- the use of the seam
As the ball leaves the bowler's hand, the surface of the ball is exposed to two different types of airflow; the laminar flow is that of the air moving on the smooth, polished portion of the ball surface, and the turbulent flow is that directed to the roughened side of the ball. The combination of these effects is a net increase of forces directed to the turbulent side of the ball, which causes it to move in the air, or "break," in that direction. There are further variants of swing bowling effect achieved through the imparting of different spins on delivery, which will result in the ball breaking in one direction in the air, and moving in the opposite direction after contact with the pitch.
Spin bowling relies more on the technique of the bowler on delivery than it does on the velocity of the ball. There are a number of different mechanisms for the imparting of spin by the bowler; each involves the application of the same physical principles. Using either the action of the wrist or the fingers being drawn along the surface of the ball, the ball is delivered with a sideways spin. This spin is intended to induce "drift," the expression describing the deviation of the ball from a straight trajectory. Spin bowling allows for the generation of the "Magnus effect," whereby the spin of the ball creates different velocities on each side of its surface, causing the path of the ball to be deflected. The Magnus effect, coupled with the different imperfections on the pitch when the ball strikes it, can create a very unpredictable ball for the batsman to contend with as it approaches. The rougher the ball surface, the better the ball may adhere at the point of impact, causing an even greater potential for erratic movement on the bounce.