# The Naked Scientists Forum

### Author Topic: Why does a cricket ball swing?  (Read 11067 times)

#### Chemistry4me

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##### Why does a cricket ball swing?
« on: 11/12/2008 23:12:55 »
If you have ever watched a cricket match you'll know what I mean. Why does the ball sometimes swing around? I understand the concept of reverse swing with ball having one rough side and a smoother side but why does a new ball swing???

#### lyner

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##### Why does a cricket ball swing?
« Reply #1 on: 11/12/2008 23:35:58 »
A smooth, spinning, ball will follow a curved path because of the Bernoulli effect.
Consider the side of the ball where the surface is moving forward. It is pushing air forwards due to friction, against the air it is moving through and the speed of this air is a bit less than the air traveling backwards on the other side, which is moving backwards.
Bernoulli says that the pressure is lower where air is moving faster. So the pressure will be lower on the side which is going backwards, pushing the ball to one side. This will give it a curved path.

#### Chemistry4me

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##### Why does a cricket ball swing?
« Reply #2 on: 11/12/2008 23:42:58 »
Cool! But why is the air travelling/moving forward a bit less/slower than the air travelling backwards?

#### Chemistry4me

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##### Why does a cricket ball swing?
« Reply #3 on: 11/12/2008 23:45:38 »
If both sides are of equal smoothness.

#### turnipsock

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##### Why does a cricket ball swing?
« Reply #4 on: 12/12/2008 00:37:29 »
This is all going to be greek to the americans

#### RD

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##### Why does a cricket ball swing?
« Reply #5 on: 12/12/2008 01:20:19 »
A smooth, spinning, ball will follow a curved path because of the Bernoulli effect.

Isn't this the Magnus effect ? ...

Quote
The Magnus effect is the phenomenon whereby a spinning object flying in a fluid creates a whirlpool of fluid around itself, and experiences a force perpendicular to the line of motion and away from the direction of spin. The overall behaviour is similar to that around an aerofoil (see lift force) with a circulation which is generated by the mechanical rotation, rather than by aerofoil action. In many ball sports, the Magnus effect is responsible for the curved motion of a spinning ball. The effect also affects spinning missiles, and is used in some flying machines.

German physicist Heinrich Magnus first described the effect in 1853, but according to James Gleick  Isaac Newton described it and correctly theorised the cause 180 years earlier, after observing tennis players in his Cambridge college.
http://en.wikipedia.org/wiki/Magnus_effect
« Last Edit: 12/12/2008 01:23:37 by RD »

#### lyner

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##### Why does a cricket ball swing?
« Reply #6 on: 12/12/2008 11:04:51 »
Cool! But why is the air travelling/moving forward a bit less/slower than the air travelling backwards?
Because the ball meets the oncoming air and has to slow it down before pushing it forwards, on the forward moving side. On the backward moving side, it only needs to push the air, even faster, backwards. Hence the difference in speed of the air.

Whatever you call the effect, that's what is going on. The Bernoulli effect is a bit more general, I think, and involves laminar flow, if you like. Does the Magnus effect involve turbulence? In any case, the Bernoulli effect explains the reduced pressure in turbulence too.

Having just read the wikkers link, I think magnus basically pinched Bernoulli's idea and applied it in a specific case.
Bernoulli is quite counter intuitive, I always think.
« Last Edit: 12/12/2008 11:14:52 by sophiecentaur »

#### syhprum

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##### Why does a cricket ball swing?
« Reply #7 on: 12/12/2008 11:20:20 »
The American throwers do the same thing with the ball in their version of rounders

#### dentstudent

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##### Why does a cricket ball swing?
« Reply #8 on: 12/12/2008 11:58:08 »
The American throwers do the same thing with the ball in their version of rounders

#### graham.d

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##### Why does a cricket ball swing?
« Reply #9 on: 12/12/2008 16:00:35 »
Not so simple I believe. Whilst a ball with spin applied about a vertical axis will indeed swing (as is used to good effect by good takers of free kicks in football (soccer) matches), if you watch any good new ball bowler he will endeavour to try to prevent any such spin. His aim is to fire the ball down with the seam fairly straight up and down but angled slightly in the direction he wishes the ball to swing. Although there may be some rotation of the ball about a horizontal axis, the swing probably results from the breakup of the airflow by the leading seam of the ball. This can lead to more lamina flow on one side and more turbulent flow on the other. A good bowler can exploit this so that the ball flies straight but curves late as it slows down due to air friction. The use of the seam in this way, and any slight rotation, is further exploited by allowing the ball to hit the ground with its seam and cut away, often in the other direction from the swing.

#### DoctorBeaver

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##### Why does a cricket ball swing?
« Reply #10 on: 12/12/2008 17:47:54 »
With regard a new ball, the angle of the seam in or out does not have that much effect so long as it is not too pronounced. I've got a ball to swing with the seam facing along the pitch and at around 45o to the ball's direction (albeit the latter very rarely when the conditions were perfect for swing bowling and I got my action just right). What is more important is that the axis of rotation is perpendicular to the seam so that it doesn't "wobble".

There are also optimum speeds for different weather conditions.

As the ball ages, other factors come into it.
« Last Edit: 12/12/2008 17:49:30 by DoctorBeaver »

#### lyner

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##### Why does a cricket ball swing?
« Reply #11 on: 12/12/2008 17:51:59 »
The seam on a cricket ball makes it a much more technical situation. You can control the axis of rotation better because of the seam aerodynamics and, when it hits the ground, the seam can provide traction for 'steering'.
Or so it seams to me.

#### DoctorBeaver

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##### Why does a cricket ball swing?
« Reply #12 on: 12/12/2008 17:59:41 »
I remember 1 particular match I played in when the conditions were absolutely perfect for swing bowling. I slowed my action down a bit and the ball swung beautifully. What's more, the swing was controllable by the speed of rotation I put on the ball. I was getting the ball to swing in then dart away off the seam. My 1st delivery went for 4 byes but I ended with figures of 7 overs, 6 maidens, 5 wickets for 4 runs; my best ever bowling figures.

#### lyner

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##### Why does a cricket ball swing?
« Reply #13 on: 13/12/2008 00:31:09 »
Only played Cricket up to the second form. Too bloody scary with that hard ball. Even hurts your fingers when you catch it.

#### Chemistry4me

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##### Why does a cricket ball swing?
« Reply #14 on: 13/12/2008 00:49:00 »

#### Chemistry4me

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##### Why does a cricket ball swing?
« Reply #15 on: 13/12/2008 00:49:57 »
By the way, good figures DB!

#### Chemistry4me

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##### Why does a cricket ball swing?
« Reply #16 on: 14/12/2008 07:41:35 »
Cool! But why is the air travelling/moving forward a bit less/slower than the air travelling backwards?
Because the ball meets the oncoming air and has to slow it down before pushing it forwards, on the forward moving side. On the backward moving side, it only needs to push the air, even faster, backwards. Hence the difference in speed of the air.
What if there was a tail wind, how about a head wind?

#### DoctorBeaver

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##### Why does a cricket ball swing?
« Reply #17 on: 14/12/2008 13:59:06 »
Cool! But why is the air travelling/moving forward a bit less/slower than the air travelling backwards?
Because the ball meets the oncoming air and has to slow it down before pushing it forwards, on the forward moving side. On the backward moving side, it only needs to push the air, even faster, backwards. Hence the difference in speed of the air.
What if there was a tail wind, how about a head wind?

There are quite a few factors that can affect it.

#### lyner

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##### Why does a cricket ball swing?
« Reply #18 on: 14/12/2008 21:09:51 »
Whatever the wind, there would still be a difference.

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##### Why does a cricket ball swing?
« Reply #19 on: 14/12/2008 22:24:40 »
Try this explanation and there is an experiment to go with it.
http://www.planet-scicast.com/experiment.cfm?cit_id=2678

In the second one you can click on a link to a nice lady explaining things.
« Last Edit: 14/12/2008 22:28:23 by Make it Lady »

#### Chemistry4me

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##### Why does a cricket ball swing?
« Reply #20 on: 05/01/2009 11:07:58 »
There are inswing and outswing bowlers, how does that work?

#### lyner

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##### Why does a cricket ball swing?
« Reply #21 on: 05/01/2009 12:04:53 »
I would guess that they spin the ball in opposite directions?

#### DoctorBeaver

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##### Why does a cricket ball swing?
« Reply #22 on: 05/01/2009 12:22:57 »
Inswing or outswing depends on which side you have the rough & the angle of the seam. Holding the ball with rough side in (pointing across the pitch) and the seam pointing slightly in will result in inswing; rough side & seam out for outswing (I won't go into reverse swing).

There is a difference in bowling action required for the 2 types which is why some bowlers are better at 1 than the other and some can only manage to get the ball to swing one way.

#### The Naked Scientists Forum

##### Why does a cricket ball swing?
« Reply #22 on: 05/01/2009 12:22:57 »