• Question: why does the ball swing in cricket

    Asked by ixabdulxi to Andrew, Ben, Beth, Heather, Louisa on 16 Jun 2010 in Categories: .
    • Photo: Andrew McKinley

      Andrew McKinley answered on 16 Jun 2010:

      The ball spins while it is in mid air, which means that the air flows differently over one side of the ball (the side that spins in the direction of travel) to the other side (that spins in the opposite direction). This means that the air on one side has a different force on the ball than the other side, giving an unbalanced force, making the ball to move sideways through the air.
      All balls do this – footballs, golf balls, baseballs; but it might be better left to our physicist in the room to go into more detail about the exact physics of it!

    • Photo: Beth Dyson

      Beth Dyson answered on 16 Jun 2010:

      As a bit of a cricket fan, I should know this! The ball swings based on asymmetry. If you throw a ball straight onto flat surface, it will bounce straight up. If you throw a ball and set it spinning, it will bounce in a different direction.

      Spin bowlers use this to make the ball swing lots. They also use the seam on the cricket ball to help, and try to make the ball asymmetrical by constantly polishing one side, which is why bowlers always have red stains on their pants. By making one side smoother than the other, they can try to affect how the ball moves through the air and how it bounces.

      Bowling takes an awful lot of practice though, and getting the ball to swing is one of the hardest things to do!

    • Photo: Ben Still

      Ben Still answered on 16 Jun 2010:

      By Swing do you mean curve or curl? If so then this is to do with the pattern of air stirred up by the ball as it moves toward the batter.

      The air is disturbed so much that we say that it is turbulent. Turbulent air patterns tend to move faster than nice smooth gliding air; this is the reason that golf balls have dimples and are not smooth, without the dimples they would not go anywhere near as far!

      This turbulence is manipulated by the bowler. By using spin (the seam on the ball creates turbulence) and/or roughing or smoothing the ball on one side the bowler can change the turbulence patterns.

      This is important because if air can travel faster across the ball on on side rather than the other then you have a change in pressure. There is negative pressure on (force toward) the side where the air is travelling fastest and so the ball is pulled in this direction, this makes it curve.

      An example of negative pressure like this can be seen when a door that is just slightly open slams shut when a gust of wind blows through the door. The small gap between the door and frame squeezes the air and causes it to travel faster through this gap. This produces negative pressure and cause the door to shut, even the air is blowing in the direction to open the door.

      A good example at the moment is the new World Cup Ball because the pattern of turbulence is dictated by the seams of the balls used, the imperfections from a perfect sphere. The footballs used in the world cup have a new pattern and this means that top class strikers have to get used to this new ball to ‘understand’ how to manipulate the turbulence pattern they produce.

    • Photo: Louisa Chard

      Louisa Chard answered on 16 Jun 2010:

      I always ask my husband this question when we go to watch the cricket! It is to do with the ball having a rough and a smooth side (you will see cricketers who want to bowl a swing ball trying to smooth the ball on one side before they bowl it). As the ball flies through the air, basically the rough side creates more friction than the smooth side, so one side flies a tiny bit slower than the other, causing it to curve or swing.

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