The Science Of Swing: How Do Bowlers Swing the Ball In Mid-Air?

Glenn McGrath, Wasim Akram, Dennis Lillee, Dale Steyn and Irfan Pathan. While you probably know all of these names, what do all of these famous cricketers have in common?

Yes, they’re all great bowlers, but the thing that connects all of them is swing bowling.

They are able to change the direction of the ball in mid-air, without the ball ever touching the ground. In the cricketing world, this is what we call “swinging” the ball.

Cricket Ball

Credits:Brian A Jackson/Shutterstock

How Does a Ball Swing?

“Swinging” a ball depends on these factors:

  • The raised seam of the cricket ball
  • The wear and tear on the ball
  • The speed of the delivery
  • The bowler’s action

When the ball moves through the air, a thin layer of air is formed around the surface of the ball that is called the “boundary layer”. This layer does not remain attached around the ball throughout the journey of the ball in the air and becomes separated from the ball at a certain point. The point at which this layer is separated determines how the ball will move as it goes further through the air.

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Normal Swing (Out-Swinger)

The boundary layer has two states: laminar and turbulent.

In the laminar layer, the air flow is smooth and regular, whereas in the turbulent layer, there are rapid fluctuations in velocity and pressure. When the ball is released at a speed between 50km/h and 112 km/h (approximate values), the laminar layer along the bottom of the ball separates at the top of the ball. However, the boundary layer at the top is sent into a turbulent state due to the seam of the ball; therefore, the separation of this layer is delayed.

This results in a pressure difference between the top of the ball (where the pressure is low) and the bottom (where pressure is high); hence, a side force makes the ball swing in the direction in which the seam is pointing (upwards).

569px-Magnus_effect_svgA similar situation can explain the case of reverse swing.

You have almost certainly seen players rubbing the ball on one side. This helps to achieve reverse swing, as the surface of the ball becomes rough at one side and smooth at the other.

Here, the turbulent boundary layer is on both sides of the ball, which causes the effect of the seam to be reversed. It now pushes the turbulent air away from the ball and causes the boundary layer to separate sooner. This makes the pressure on that side higher, forcing the ball to swing towards the batsman. To achieve reverse swing with a new ball that is smooth on both sides, experiments show that the bowler has to hurl it at 130-145 km/h.

Watch this fascinating video to learn more about how to bowl an in-swinger by properly gripping the ball:

References:

  1. Wikipedia
  2. MOTION – Sprott’s Gateway
  3. How to Reverse Swing a Cricket Ball – Wiki How
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