Swing Bowling

 HOW DOES A BALL SWING IN AIR??

In a cricket-frenzy country like India, cricket lives in almost every street of the country. Every fast bowler wishes to master swing bowling, and would love to see his delivery baffling the batsman. Ever wondered how swing bowling actually occurs? How is it that the ball just swirls around in the air as if it is magically controlled by the bowler? What are the techniques followed?

As usual, SCIENCE has an answer! Science has its way of lurking around everywhere, be it sports or in the walks of everyday life. Now let us try and understand swing bowling.

For understanding this phenomenon, we need to understand a few basic concepts in aerodynamics. Although the word “AERODYNAMICS” sounds real hi-fi, it just refers to understanding of air motion or Fluid motion in general.

So imagine a regular cricket match and to make things interesting we have a match of India against Australia with Brett lee bowling to Master-Blaster Sachin Tendulkar. Usually, before bowling a usual observation is that the bowler keeps “shining/smoothening” one side of the ball. He lubricates it with saliva, and rubs it against a towel or his thighs. Now why is that done? Just to scare the batsman? Just to keep the ball clean? NO. This is one of the factors that govern SWING BOWLING.

The fielding side keeps polishing one side of the ball alone and leaves the other side to roughen from wear and tear. This produces a marked difference in the “aerodynamic properties” of each side. Or in simpler words, they respond differently to air currents incident on them.

While delivering the ball, the bowler keeps the shiny side in one particular side (say for example, facing his left) and the rough side in the other direction (facing his right side). When the ball is released in this manner, we get an IN-SWINGER to right-handers. And if the bowler had released the ball in the opposite way, an OUT-SWINGER would be the result.

INSWINGER

Now what is the underlying principle? What makes the ball execute such curved motion?

Suppose the ball is held in the INSWINGER position and is released. It is cutting through the surrounding atmosphere. We simulate this situation by taking the ball to be stationary and the wind to hit the ball. As soon as air hits the ball, it parts into two separate flows. One flow moves over smoothly on the shiny side. It remains stuck to the shiny side for a while and then parts at some point (after travelling certain distance). The flow on this side is smooth and ordered – called LAMINAR flow. Now the other layer has to go over the ROUGH side.

The rough side has miniature ‘ups’ and ‘downs’ which can be imagined like HILLS AND VALLEYS. As the air flow goes over them, it is trapped by these valleys and remains stuck to them for a longer time. So it does not part easily from the rough side, but moves over it for a longer distance. Moreover, this flow is not smooth but more disordered – called TURBULENT flow.

This kind of difference in flows creates different SPEEDS of air flow on both sides. The turbulent layer flows faster. Hence, air is moving fast on rough side while it is moving slowly on the shiny side.

The air moving slowly has higher pressure. Hence, the shiny side has more pressure on it than the rough side. So we get a net force towards the rough side. This force is the factor that causes the SWING of the ball.

So you must note that the atmosphere plays a very important role in this case. If the ball was bowled in vacuum, it would make absolutely NO DIFFERENCE, and the ball would go straight.

For swing to be observed high bowling speed is required and that is why only fast bowlers can swing easily.

 Now then, try to think about this one: How is REVERSE SWING possible??

 - article by Sameer