Swing Bowling Physics and other factors

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someblokecalleddave

someblokecalleddave

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We started to have this conversation off the back of discussing the swing that is obtainable with some of the Wrist Spin Variations bowled with an upright seam - Flippers etc.

Doctoran Said.................

Let's just explain this magnus effect terminology. A golf ball has crevices on it so that it can go further in the air. Smooth surfaces drag (or experience difficulty moving through the air). Hence on a cricket ball it is harder to go through the air on a smooth side compared to a rough side. The ball swings or seams towards what is rough. The roughest thing on a ball is generally not the side of a ball -- BUT THE SEAM (on a brand new ball)! The more pronounced it is the more effect you will see. To maximise the swing you want it as smooth as possible so that the force is directed onto the direction of the ball.

Drift happens the same way. With large rotations, a nice seam, and a ball that isn't too roughed up... the ball will move in the direction of the seam.

Air is heavier than people think. "One cubic metre of air at ground level weighs 1210 gm. A cricket ball weighs 160 gm. A room full of air weighs more than most cricket players." (University of NSW). Imagine the difference that humidity, and even differing air pressures and hence air densities could make. Wind is essentially caused by high pressure wind particles moving towards low pressure areas. If you're in a stiff gale you are in an area in between two largely different air pressure zones (could be hundreds or thousands of kilometres apart). High pressure means lots of air which means accentuation of the effects of swing and spin.

Therefore wind at the WACA can make the ball swing and drift more. Rough surfaces in India can make the ball drift less. And big seams do make the ball drift and swing more.

P.S. don't get me started on reverse swing

I then said in response -

That I'd just looked at a bunch of weather charts and the potential for his theory regarding atmospheric pressure affecting the outcome of the way that the ball moves through the air. My initial searches were inconclusive, so I've now gone back to some basic questions..............

So going back to the cricket ball scenario here we get the ball swinging when there's low pressure - anticyclonic weather, when the cloud cover is low and the atmosphere muggy. So that fits with every other situation. Needless to say then we struggle in Australia because the weather during your summer would be dry and sunny, high pressure and little moisture.

Does that then mean that as Brisbane is in a sub tropical area, at some times in the year you'd expect moisture laden starts to the day that would then quickly burn off and be replaced by blazing hot sunshine? At this time of year is that the case- does the day start muggy/humid and potentially offer swing conditions?

So lets get Doctoran started on swing and reverse swing and anyone else that wants to join in.
 
Lover of physics :D
Noone knows why a cricket ball swings more in humid conditions, theres alot of theories but you can't really explain a good swinging day... you just turn up and you can feel it and you just know its right.

A cricket ball swinging and drifting is different from a physics point of view.
Drift is probably easier to explain. Drift happens due to the magnus effect with a view towards newtons 2nd law of motion (simplified f=ma) and circular motion. Background info on circular motion is that an object will travel on a circular path when its acceleration acts perpendicular to the direction of its velocity eg it doesnt effect the speed but it will alter the direction (velocity= speed and direction). When a ball spins in air it will create a boundary layer of air around it, and this boundary layer induces a greater circular motion of the air around it creating a pressure difference between the top of the ball and the bottom of the ball. Hard to explain with out diagrams but basically if a ball overspins it will have higher pressure above then below due to the boundary layer. The most important point of the magnus effect is the pressure causes a force on the ball, the force acts perpendicular to the line of motion, so if a balls being bowled towards the batsmen on a straight line it will "drift" as there will be a force (so an acceleration) acting perpendicular to the ball.

Swing works again on the basis of boundary layers around a ball but this time aerodynamics of a ball comes into and a "rudder" effect of the seam. The important thing to remember is air is a fluid like water, the same outcomes will happen just there more visible in water because it happens slower.
I'm going to explain this for conventional inswing and reverse outswing (because i can use the same set up). For an inswinger we use the shiny side on the outside and rough side on the inside with the seam pointing towards leg slip. Now we all know its because the shiny side moves faster through the air then the rough side. The physics says its because as the ball moves through the air boundary layers build up, the boundary layers leave the shiny side faster due to the seam forcing it wider and it moving easier over the ball, on the rough side however, there is a much more scrambled boundary level due to the seam pushing it wildly into turbulents and gripping to the rough side causing the boundary level to stay on the ball for longer, making it travel slower than the shiny side... Ball swings.
Reverse outswing we reckon is done my holding the ball in the exact same way, "shiny side" out side, "rough side" inside, seam to fine leg,,, what happens this time is both balls on a microlevel have been destroyed just the shiny side has water pockets in it (making it look shinier) whilst the rough side has air pockets. We think it works on the basis air travels faster over air then air over water. On the WET shiny side the air if forced away by the rudder/seam but the boundary layer is even slower moving and becomes drastically more turbulent do to moisture and more is dragged round because it goes out of the seam and in and held... on the air filled rough side the air travels over it the same as on the shiny side above but it initially comes through scrambled of the rudder but its forced of faster so more air is pushed off.... this causes the rough side to travel faster ,,, therefore swings out

I know i explained that badly but the gaps i left i hope arent too hard to fill and if you do need anything clarifying just ask...
But as you can see thats why its so hard to explain why humidity effects it, it doesnt always fit, i just like the higher vapour % in air gives the potential for greater pressure differences, so greater resultant effects eg more swing, but by that merit it shouldn't reverse swing in very humid conditions because both sides get damp.
 
I think the line of questioning was going along trying to identify whether it was possible to identify whether there was any bearing on the ball by the air pressure. We've gone over spinning balls in relation to drift on the spinning thread to a satisfying degree and we're now looking at factors that may affect swing. I think it's generally agreed that overcast conditions which usually coincide with low pressure weather systems affect whether the ball swings?

Our line of thought was if you've got a weather pattern over a cricket ground of 1000 milibars which is at the upper end of a low pressure system would there be a noticeable difference to a day when the pressure was much lower e.g. 980 milibars. We all recognise that if the weather is fine e.g. 1028 milibars then swing doesn't happen? Is it known or suspected that the lack of swing is down to the lack of moisture in the air or the increased air pressure?
 
someblokecalleddave said:
Our line of thought was if you've got a weather pattern over a cricket ground of 1000 milibars which is at the upper end of a low pressure system would there be a noticeable difference to a day when the pressure was much lower e.g. 980 milibars. We all recognise that if the weather is fine e.g. 1028 milibars then swing doesn't happen? Is it known or suspected that the lack of swing is down to the lack of moisture in the air or the increased air pressure?
Click to expand...

See thats what i finished with, Physics says that a ball should swing always if the ball is in the right condition and is bowled properly. But we must presume a test team can bowl properly and get the ball in good condition.

Theres only really one formula i can think of and thats the equation for drag force:
Drag Force = 0.5 * density of fluid travelling through * velocity of object moving relative to fluid SQUARED * drag coefficiant for that specific shape * cross sectional area of object travelling thru fluid in

Now simple math manipulation: compare the ball moving thru "clean air" and "Damp air"... the drag coefficiants a constant, areas a constant, balls moving at same velocity and 0.5 is obv a constant....
SO Drag Force is proportional to the density of the fluid its travelling through eg F proportional to Densitiy

So if density of clean air is less then density of moist air, then the drag force is less in clean air then in moist air,,, and going of my comments above about boundary layers and drag, more drag more swing due to greater pressure differences...

I wnt lie to you i've learned about the stuff above so i can sort of talk about it even at a low level, but im nt a geography expert/ geophys/ meterologist so i don't really know about pressure systems but doesnt when a high pressure system moves in it gets a bit warm and sticky? If in a high pressure system i think we can presume the same effect as above where theres a more compact thicker air level so greater density (density = mass/vol if more air in smaller volume greater density) so drag increases from same proportion Drag<>density.... higher pressure more swing.
Thats the best iv gt.
 
A very interesting thread... I love the way Phil thinks :).

I am afraid my physics is limited to post grad cosmology and particle physics but I shall throw my tuppence in too... if that's OK.

Firstly, I only know of one published report regarding experiments with cricket ball swing (Barton, N. G. Proc. R. Soc. A379, 109−131 (1982)).

However an accurate correlation between swing, seam angle, spin rate and flow speed was ascertained in projection tests carried out by the Department of Aeronautics at Imperial College, London University, comprising spinning cricket balls projected into wind tunnels. These tests appear to conclude that for a good swing, the ball needs to be released between 15 and 30ms-1. The seam should be inclined at about 20 degrees, spinning steadily along the seam at about 11 revs s-1. These experiments did not find any increase in swing due to damp or humid conditions neither did they find any swelling in the seam during such conditions.

Although many of us believe we see a difference, there is no evidence that humidity has an affect at all. Of course, humid air is less dense than dry air, so you would expect less swing, however, the difference is minimal. In fact, there is no measureable aerodynamic difference in the ball from moisture. Having said this, I believe ground moisture can load a ball and possibly create contrast swing but it is a little haphazard. It will certainly affect the swing one way or another but not the way you would imagine.

I should add that I can get much better swing from side on bowlers ;).

We have all seen this video but it is always a good revisit...

http://www.youtube.com/watch?v=W4tGaoSz14g
 
Never saw them videos before, quite good arent they as a series :O a young andrew strauss there.

Just a thought Liz, at club level more people swing it from the hand rather then late swing and more of us bowl at 70mph then test cricketers so where bowling at the ideal pace to swing it, do you think thats why we start swinging it earlier from the hand or do you think its more how release it? My thought was maybe because our actions are less "linear" and our wrist position's aren't as good as county/test players we force it out at a weird angle, then newtons 1st law says "an object will move at a uniform state of motion unless an external force is applied" so by our poorer actions and set up do we cause the pressure differences (so a resultant force_ to be greater earlier causing much earlier swing?
The only thing i've noticed is these big swinging "dobbers" rip there fingers down the back of the seam alot and its like u wud for an incutter, how they get there wickets that big outswingers folloed by an injagger next ball.
Just something i've always wanted to know really.
 
i still think that swing bowling will occur regardless if you present the ball in the correct manner. there is no scientific evidence for weather playing a part, although it likely has an effect. the way i think of it is this (this is more philosophical than scientific, but i think simple logic is a lot more realistic than the voodoo nonsense you hear most retired swing bowlers come out with!)...

if youre a spinner, and you spin the ball hard and land it on the seam, then it will turn. full stop. there is no scenario where there is zero friction between the ball and the pitch, so even though the turn may only be slight on an unideal pitch, it will be present none-the-less (you can still generate turn on a damp grassy wicket, just to prove the theory). on some pitches though you might get massive assistance, and then the ball might turn massively from the same delivery characteristics. a good spinner can turn the ball a foot on any surface though, look at Shane Warne. its all about consistent presentation of the seam, and consistent revs on the ball. Warne was the king of consistency, which is why he was so successful.

relating that back to swing bowling... if you present the ball in the correct manner then it will swing. there isnt some magical cutoff condition where the ball will simply stop swinging. it will always swing to some degree. lets play devils advocate and say that humid high pressure air is the ideal for swinging (regardless of whether it is or not). on a day when you have such conditions you would expect more swing, the same as a 5 day old cricket pitch with footholes will give a spinner more turn. likewise if you are playing in dry low pressure conditions you would then expect less swing. but you wouldnt expect no swing at all. somewhere in the middle would be your average amount of swing.

when you watch a bowler like James Anderson, he can swing the ball round corners in England, in both directions. you watch slow motion footage and he presents a very consistent seam position, and he is fairly unique in that ability. there arent many bowlers that have achieved that same consistency on every delivery, and those that have are the great swing bowlers of all time. and he has the same ability for both inswing and outswing. it seems obvious, just keep the seam where it needs to be, but actually controlling the ball such that the seam is stabilised throughout its flight is extremely difficult. otherwise everyone would be doing it!

however, Jimmy has a reputation for being incapable of swinging a ball outside of England, and thats where the confusion comes into play, and most people just put it down to the Dukes ball and ideal English weather (the only northern hemisphere test nation). however i think slow motion footage would solve the mystery, as i reckon the difference in balls is probably the cause, but not because of swing, and simply because its harder to bowl with if youre not used to the size, shape, roughness, etc of it. give me a kookaburra gold crown and il bowl leg spin all day long with it and get good turn, i practice with them all the time. give me something different, slightly different in size, surface, seam, etc and il struggle with it for a while (i had a cheap Readers ball with a really hard shiny surface and it felt really big in the hand. it took me forever before i could get that ball turning, but eventually i did!). i reckon its probably similar for a seam bowler, although overlooked because people assume you just grip the ball and bowl it (there is no significant spin involved, but presenting the seam upright is an equally difficult task). its not that easy!! also, Australian swing bowlers swing the ball in Australia, so its obviously possible. the fact that one bowler who everyone thinks of as being probably the most technically consistent swing bowler in world cricket struggles in certain countries throws people off the real reasons why, and feeds the often absurd theories of the scientifically unqualified retired swing bowler lol.

onto club swing bowlers, and how they are able to swing a ball big even if they arent as consistent in their seam positioning, and again, ive got some speculation as to maybe why. at slower speeds youll still get the same aerodynamic effects, but things tend to increase in a non-linear fashion. for example, if a spin bowler bowls at 40mph theyll probably also spin the ball less because there is less energy in the action, but it will grip and turn a LOT more than at 55mph. the revs decrease non-linearly, and the grip increases non-linearly. for a club swing bowler, they present a decent enough seam, at slower speed (which is easier as well), and the slower speed will result in less swing. however its non-linear again, so they get more swing relative to the fast bowler, and because the ball is in the air for longer it will still swing big. if you increased their speed to 90mph but with the exact same action, theyd probably struggle a lot more. it may also account for early versus late swing. late swing is probably an illusion based on human perception and reaction time. when the ball is slower you can pick it up a lot earlier. it might also have something to do with the boundary layer forming around the ball, we assume in theoretical situations that everything happens instantly, when in reality that boundary layer probably takes time to stabilise as well. at 90mph the ball isnt in the air very long, so milliseconds make a huge difference!

most of the above is pure speculation, and based on no physical proof whatsoever lol. im no idiot when it comes to physics, im an engineer by profession and studied aerodynamics in quite a bit of detail at university (motorsport specific). i just cant be bothered to crunch numbers on something that scientists have struggled with for years to no avail lol. the only proof will come from real world experimentation anyway, and i reckon TV has the technology now to analyse it properly and actually come up with some clear ideas on everything if the right people looked at it. Ian Botham chatting about the tide coming in or out affecting swing bowling isnt helping anyone haha. if they formulated some solid ideas using hawkeye, slow motion camera, and atmospheric data at every match, found some correlations, and then handed those on to the scientists, i reckon it would all get solved.

but then swing bowlers want you to think its a huge mystery, if you understand how they do it then batsmen will play it better, and other bowlers will figure it out. in the same way as spinners do. if it wasnt for the advances in television camera technology we would still be pretty much clueless about most of the modern spinners armoury! Ajantha Mendis would be some kind of magician lol.
 
I wnt lie to you i've learned about the stuff above so i can sort of talk about it even at a low level, but im nt a geography expert/ geophys/ meterologist so i don't really know about pressure systems but doesnt when a high pressure system moves in it gets a bit warm and sticky? If in a high pressure system i think we can presume the same effect as above where theres a more compact thicker air level so greater density (density = mass/vol if more air in smaller volume greater density) so drag increases from same proportion Drag<>density.... higher pressure more swing.
Thats the best iv gt.

Normally high pressure weather is associated with clear skies, less wind, moisture laden starts and ends to the day. With the clear sky and sunshine, the days are warmer. So if as Doctoran has said - high pressure means higher denisity atmosphere. You're saying that higher density will mean more drag = more swing? But that doesn't match the anecdotal evidence that suggests that on a dry sunny fine day - High pressure 1025 milibars the ball would swing less? Swing occurs under muggy overcast conditions doesn't it (low pressure)?
 
Jim - "Australian swing bowlers swing the ball in Australia, so its obviously possible". Is this consistently and to a high degree even in hot sunny dry and cloudless weather?
 
Jim aren't there atmospherical pressure differences around the world? What is Australian bowlers are used to swinging it based on a specific atmospherical pressure whilst English bowlers are used to the humidity and presusure of britain? Would that explain why people struggle away and have advantage at home?
 
Swing bowling has to do entirely with how your arm comes down, relative to the seam position. Cutters are bowled, with how your wrist comes down, relative to the seam position. Cutters are more difficult to bowl, than swingers.
 
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