The ball changes its direction during flight in Swing Bowling. This trajectory changing delivery is a beautiful sight. In this part, we uncover the forces causing the ball to move in a curved path.
Man has always been fascinated by things that are unconventional and out of the ordinary. Even a simple phenomenon common in nature but unknown to man becomes a mystery and amazes him. When it comes to a sport like cricket, one of the most watchable things is to see the cricket ball swing and fox the batsman. The change in trajectory of the ball makes lives for the batsmen difficult but is surely enjoyed by everyone else. And it is then that a bowler becomes the magician and the spectators feel amazed.
But did we all really know that swinging the ball was a phenomenon of science, one that can be easily taught and explained? You might say that if it were that simple then most of us could bowl in-swings like the great Wasim Akram and reverse swing the ball like Waqar did. Well, you might not be able to bowl like they did but knowing the Physics around it would certainly help you with your abilities. Of course, Wasim and Waqar practiced the art to perfection but chances are that even they don’t understand the science behind the art of swing bowling. In fact, it is said that the proponent of reverse swing bowling, Imran Khan discovered it accidentally.
Science Behind Swing
Swing bowler throws the ball with smooth side towards the batsman and seam at an angle. There is laminar flow of air on the smooth side of the ball. On the rough side, air flow encounters the seam of the ball and this starts turbulence which continues far back. Laminar flow separates with the ball earlier while turbulent flow continues with ball surface, separating much later. The layers of air moving on the smooth side flow much closer to the solid surface of the ball. The air layers on the rough side move much farther from the solid surface (due to a layer of turbulence in between). So the velocity of air on the smooth side is lower than the velocity of air on the rough side. This creates a pressure difference on both side, with higher pressure on the smoother side. This results in net force towards the rough side, moving the ball sideways (see the image below).
Other Factors Affecting the Swing
The raised seam of the cricket ball: The seam of the cricket ball provides irregularities to the surface of the ball. If the ball is pitched on the seam then these irregularities will cause the ball to move in a certain direction. The direction in which the ball moves depends on how the bowler holds the ball in his hand before throwing. The seam needs to be held upright to achieve swing.
The wear and tear on the ball: The irregularities on the surface of the ball change its aerodynamics. A cricket ball will swing best when it is new, i.e. when it has lesser irregularities. The smoother surface of the ball should face the batsman. When the ball gets old, it allows reverse swing. The smoother surface of the ball should then face away from the batsman.
The polishing liquid used on the ball: After some overs on the field when the ball gets rugged, the fielding team tries to keep the surface of the bowl new by applying their saliva and by rubbing the ball with their clothes since a smooth surface will help the bowl swing.
The speed of the delivery: The maximum swing effect is achieved when the bowler bowls it around 70 to 90 mph. So, one has to be a medium pacer or a fast bowler to make the ball swing.
The bowler’s action: The bowler’s action affects the release of the ball from the bowler’s hand. Example, a side on action will assist an out-swinger.