Swinging the ball and other such manoeuvres!

Yesterday, I heard a nice lecture by Dr David James of Sheffield Hallam University on Sports Engineering in general, and on the engineering aspects of cricket, in particular. Here is a summary based on my notes (and, my understanding, of course!)

Dr. James began his lecture with an introduction to sports Engineering and how the idea to use the scientific and engineering ideas to understand sports related mechanics is not really novel — apparently, Isaac Newton talked about the irregular flight of tennis balls (though, a cursory google search to locate the article tells me that it is Lord Rayleigh who wrote a piece titled thus — see here for example). He went on to mention some other recent pieces of engineering work that had been carried out at his university and elsewhere — helmut and bike design specialised for individual athletes (as in UK cycle racing team which, I understand is winning almost all the competitions across the globe, thanks to such design philosophy). However, sometimes the sports traditions are at variance with the engineering goals; for example, he feels that the tennis racket design has probably damaged the character of the game a bit. Also, sometimes, the sports engineer and the sports regulation authorities might not see eye-to-eye on issues; for example, a piece of research related to determining the 3D position of a football by triangulation using six receivers placed at different locations with the transmitter being at the centre of the football was discarded when the relevant authorities did not show enough interest in pursuing it.

After the preliminaries, Dr. James discussed the engineering aspects of cricket: here, he spent most of his time in describing the dynamics of ball delivery, the flight of the ball, the bounce of the ball off the pitch and the measures that ground staff can take to make the pitches neither too batsmen-friendly not too bowler-friendly. He ended his presentation with a short discussion on bat technology. The entire talk lasted for about one-and-a-half hours (though, I did not realise that at that time).

The first interesting result is the recording and analysis of how bowlers bowl the ball in a live match; using a high speed camera that records 600 frames per second located perpendicular to the pitch and focussed on one end, Dr. James recorded a huge number of ball bounces off the pitch and measured the impact speeds. Schematically, the curve of the frequencies of bowling at various impact speeds looked something like this:

cric-ball-bowling-dataThe bowlers thus use three ranges of speeds: 45-60 mph for spin, 60 to 70 mph for swing, and above 70 till up to 95 or so for fast. In his experience, nobody bowled at more than 100 mph.

After mentioning Rabindra B Mehta (of NASA) as the authority on ball aerodynamics (many papers of whose you can get via a google search), Dr James went on to discuss the swing and reverse swing in cricket balls; he also showed a couple of videos of windtunnel experiments to support his conclusions. As I understood, the usual swing (apparently, it is called the magnus effect in sports engineering in general) is due to the differences in the surface finish — for low velocities, on the smooth side, the ball has a laminar flow with a boundary layer that separates from the ball surface relatively early, while, on the seam side, the boundary layer is thicker, hugs the ball surface much longer, and the flow pattern is turbulent — resulting in a wake behind the ball which is at an angle — resulting in swing. By the way, this need for turbulence so that the wake behind the ball is minimal (and hence the drag) is the reason why gold balls have dimples, football is made up of patches, tennis balls have felt on top of them and so on. On the other hand, for balls which are bowled at much higher velocities, even on the smoother side, the flow patterns are turbulent, and now, soemtimes, the wake could be such that it is at an angle which produces the reverse swing effect. Dr. james also described that in simulations they have observed an effect similar to this: if a football is kicked with very less of spin — say, one rotation for the full length of its flight — since in games other than cricket, it is the spin that gives rise to the magnus effect — the ball might traverse a zigzag path in air. I understand, in volleyball too, some players can produce a reverse magnus effect while serving.

The section on the ball bounce and the pitch properties of the talk concentrated on answering the question, namely, that whether the ground staff, with a set of given number of experiments, determine the quality of the pitch and modify it according to their requirements. Apparently, the answer is yes; it involves a prescription in terms of the rolling regimes for the pitch (which bascially determines how hard the pitch is going to be — affecting how the ball bounces off its surface) and the water management in terms of keeping just the required amount of moisture (which affects the coefficient of friction between the ball and the pitch). The idea here is to consider the ball and the pitch as a system of springs and dashpots; determine the constants for these springs and dashpots by dropping the balls on rigis surfaces and by dropping hammers on the pitch; from these values, using an empirical model (which is obtained by fitting curves to actual experiments of bouncing balls off pitches), we can determine the coefficients of restitution and and coefficients of friction. During these discussions, Dr James mentioned about using clay tubes that are nearly a few feet deep but small in diameter to repair pitches — basically, by using these clay tubes as some sort of nails. It was cool!

Finally, he told why he does not think the bats will improve tremendously in the years to come — because of the restrictions — namely, that the blade should be of wood. The only thing that can be improved is by making the handles stiffer (and also energy absorbant — so that no vibrations are felt by the player — or, make it so stiff that the heavy vibrations are of such small amplitude that the player does not feel them either); however, the limitation here is the fact that the handle is to be connected to a blade which has very different stiffness properties.

After a couple of questions (and a discussion on how it is easy to tune pitches in places like Australia where you can put lots of clay and allow the sun to dry it to get the required pitch properties, which, can not be done in England given the weather conditions), we adjourned for refreshments.

PS: Dr. James has promised to send me some material that is in public domain — like his PhD thesis on pitches for example, which I will host in some page and leave the links here.


2 Responses to “Swinging the ball and other such manoeuvres!”

  1. raj Says:

    Guru, that was fascinating. Thanks for sharing.

    I wonder what’s the physics in the movement of the ball used in table-tennis. It’s quite smooth, has a regular outer surface and can be made to turn quite a bit off the table. Good players can impart a top-spin.

    • Guru Says:

      Dear Raj,

      The question of table tennis balls being smooth did pop up. In other games, we want the ball to travel longer distances; so, the drag on the ball should be less. That can be achieved by reducing the wake behind the ball; the reduction of the wake demands that the ball surface is rough so that the boundary layer surrounding the ball is turbulent and hugs the ball quite some time before separating. On the other hand, in TT, we want the game to be played on a table; making the ball surface rougher will make it fly larger distances. So, they are made smooth. Of course, then, TT players have to use speed and spin of the ball to manipulate swing — which, as you note, players do manage!

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