On
20/1/2004, Kevin Phyland posted:
The cricket (on ABC radio) has prompted a dormant question I've had for
years. Bill Lawry (probably not a physicist) mentioned that the
overcast humid conditions in Brisbane would be conducive to the new
ball swinging in the air.
Now I've heard this for thirty years or more and am no closer to
understanding why this might be so. I vaguely understand why a
rough-side/smooth-side ball might swing (Bernoulli effect) but if
anybody can point me to a site that explains why an equally smooth
sided cricket ball might swing due to an increased humidity (or
overcast conditions) I'd be grateful.
I'd say it was an urban myth except for the number of professional
bowlers who subscribe to the theory....self-fulfilling prophecies
perhaps?
Tennis may well be different as spin imparted from the racket may well
cause a similar Bernoulli effect...
Thoughts?
Peter
Macinnis replied:
There is a preponderance of
science types and cricket enthusiasts in our house (actually, it is
100% to nothing on two counts), and this question came up around the
dinner table a week or so back -- my 21-year-old suggested that the
humidity could well affect the leather, especially after it has touched
the grass a few times. We even toyed with placing a senstive electronic
balance and a cricket ball inside a controlled humidity environment, but lacked the equipment
necessary to see if leather is hygroscopic -- though that might be
negative, and damp grass may simply be to blame, being absorbed
differentially on the roughened side.
Zero Sum responded:
Cricket balls have a leather covering. leather absorbs moistue to
an extent and is inclined to expand when it does so. The feel of
a cricket ball is signiicantly different to me on hot dry days and
moist cool ones that I am disinclined to argue the case (either way).
Kevin
Phyland commented:
Hehehehe...
I meant to add to my original post
that I'm not totally unfamiliar with the feel of a cricket ball...
.except for fielding and the odd
times I felt in close without a protector...(that 'ouch' I can STILL
remember)...
Paul Williams answered:
The reason (on the surface) it appears to me to be a myth is the fact
that a *heavy* humid atmosphere is actually not so heavy at all. An
instructive example (if one lives in the country - in winter) is to see
how smoke from a chmney usually goes straight up on a clear dry still
day, yet on an overcast still humid day, the smoke will tend to
hang around nearer ground level.
Reason: "Heavier atmosphere" - humid - is actually less dense than dry
atmosphere!
Now why would a cricket ball swing more in a lighter moist atmosphere?
Damn good question.
I'll have a think upon this...
OK...Maybe the turbulence created by a spinning cricket ball through
water laden air is more pronounced.
Maybe the drag is increased by the molecular make-up of the atmosphere
- that is, that larger molecules create slower eddies which cling to
the ball for longer periods?
Umm... I'll have another think.. :-)
Angus
wrote:
to add to the fray - recent
commentary concerning Nathan Bracken et al., suggested that the
rough/smooth theory had gone out of favour and coaches were trying to
encourage a certain trajectory and tilting of the wrist during the delivery action as a means
of getting swing. I guess this is in combination with rough smooth
theory - they still shine the ball preferentially I think - but some
bowlers will still not swing in conditions where others do. Reverse
swing seems to be related to speeds above 140 kmph...??
Paul Williams reconsidered:
I'll have a second go at this now that I've done some more thinking
upon it.
The higher humidity thing appears to be incorrect. That is that the
ball will not swing more in humid conditions
Now overcast conditions are different. (see below)
Brand new ball:
The new ball swings! Both halves of the ball have the same
smoothness/polish.
Why does it swing?
For an outswinger, the seam of the ball is angled towads the slips. The
seam is rough and disturbs the airflow on one side more than the other.
This disturbed airflow clings to this side of the ball just a little
longer creating a slight drag which causes the ball to move slightly in
the direction of this drag.
When the ball is a little older, we see that all the bowlers only
polish one side preferentially.
Further, they polish the centre of the ball only - it being more
difficult to polish the surface of the ball nearest the seam.
I think that this *may be* slightly wrong. (Need to think about this
more though)
The basic bowling theory (as opposed to physics theory) appears to be
that the rough side creates drag (more turbulent flow) than the smooth
side.
Reverse swing I'd best leave for another time.
Anyway back to the question of the ball swinging more in humid
conditions?
The Brisbane Cricket Ground in summer often has humidity approaching
100%.
The ball does not swing more at this ground than cooler grounds with
lower humidity.
What appears to happen in cloudy weather is that the turbulent
interference resulting from rising air is reduced.
In overcast conditions the air turbulence usually arising from the sun
heated ground (this can be seen on any hot sunny day) is reduced to
such an extent that this rising air no longer interferes with the
subtly different separation of the airflow which will 'stick' to one
side of the ball a little longer than the other.
Umm... before I bury myself any deeper, I'd best think some more upon
this.
Peter
Macinnis enquired:
Wearing my work hat, I have just
had a request from India on where to go for live scores on the
Web. I used to sometimes use a small streamer that ran in the
task bar, but now i can't find it. I have given him a couple of
answers, but does anybody know where to get that score streamer/ticker,
or whatever it is called?
Robin answered:
There was one at baggygreen.com.au. Now it is a small popup.
http://aus.cricinfo.com/db/NATIONAL/AUS/
Podargus quipped:
I thought they were no longer a concern since the invention of the jock
strap.
Paul
Williams wrote:
Some very good physics here:
"The Motion of Balls in Sports"
- Matthew Turner
April 2002
http://www.mth.uea.ac.uk/~h229989/project.pdf
See page 45 and onwards for
cricket ball swing.
(Humid conditions do not help the
ball to swing but overcast conditions do).
Ray replied:
"The Motion of Balls in Sports"
With or without a jock-strap?
(someone had to)
Paul
Williams responded:
No - I imagine you are talking of
Coriolis Force?
This is so incredibly weak,
regarding our day to day experience, that it is best ignored.
If you mean what happens to the
(rapidly) spinning ball during flight? - this is a very interesting
thing to think about...
Tim Daly posted:
Kevin, your enquiry certainly has elicited a good deal of hesitant
comment; I am not able to shed any useful light on this mysterious
matter, though I can see other parallels.
The 'curved ball' phenomenon is not restricted to cricket, it
also effects other ball games eg: golf, base-ball, squash, tennis,
ping-pong, etc. those balls travel at similar or higher speed
than cricket balls and can certainly be made to follow a very curved
trajectory, though I guess only base ball can really be compared to
cricket (I know nothing about the construction of a base ball).
(From an engineering perspective, I find it difficult to believe either
RH or Barometric pressure would do much to influence the flight of the
ball over such a relatively short distance, even taking into
consideration extreme Bernoulli, Coanda or related effect, I feel there
may be some other devilish force to blame; consider the much more
overwhelming effects gravity has on the same projectile travelling at
say 140 Km/hr over 1 chain).
To add to your question, I am interested to know why tennis balls are
furry and what the scrolled pattern on the ball is meant to do ?
Cheers,
Kevin
Phyland responded:
I read that entire section of that
.pdf document you posted the URL to...
and strangely, if it has any
credibility at all, it seems to point to the fact that humidity DOES in
fact affect the swing of a cricket ball!!!
I point to the section on
micro-turbulence (more usually known simply as
micrometeorology)..."best to bowl after rain"...this implies that the
moister air at the very low boundary layers (less than 1-2m) DOES
inhibit the microturbulence that the author feels destroys the
propensity for a ball to swing!
I also found it odd that the
author felt that humid (although being 'close' or 'muggy') had little
to do with moisture content
in this micro boundary-layer..?
More grist for the mill however...
Angus replied:
I thought the article was interesting - do you know if boundary layers
and Benoulli's effect are the same thing?
Regarding the humidity thing, I got the impression humidity was
inconsequential; the issue is whether rising air is present. Thus,
where rain or dew is present the surface is cool enough to prevent
rising air/microturbulence, likewise on very hot and very still
conditions, as well as overcast days where direct sunlight and little
breeze (and
conincidentally tend to be muggy). I am not fully convinced about the
very hot days, but I guess I'll have to watch more cricket and see...
