# Thread: Cricket - nobody understands it!

1. ## Cricket - nobody understands it!

Watching the ( <img src=/S/flags/SouthAfrica.gif border=0 alt=SouthAfrica width=30 height=18> vs. <img src=/S/flags/Australia.gif border=0 alt=Australia width=30 height=18>) cricket test match today, I saw a familiar sight, where a ball was hit along the ground, running out towards the boundary. A fielder would give chase, gaining on the runaway ball, seemingly about to stop it from reaching the boundary. But all of a sudden, the ball would pick up speed along the ground, and beat the fielder to the boundary. How is it possible, for a ball travelling along the ground, to suddenly "accelerate" away from the person chasing it?

BTW, the whole playing field can be assumed to be horizontal.

Alan

2. ## Re: Cricket - nobody understands it!

My guess would be that the ball had some backspin on it when it left the bat. As it rolled the backspin would be slowing it down. As the backspin wears off, the ball will then appear to roll faster.

3. ## Re: Cricket - nobody understands it!

That's getting close to the mark Steve. However, rather than just "appearing" to travel faster, it actually does. I've seen cases where a fielder, chasing the ball towards the boundary, slows down to gather it in, only to have it "run away" from him and beat him to the boundary rope.

Alan

4. ## Re: Cricket - nobody understands it!

Well, having thought about this overnight I have a theory. A bowler will deliver the ball and it will always have a spin on it. When the batsman hits the ball towards the boundary this spin will continue in one axis but the very fact it has been hit by the bat could cause it to spin on a secondary axis. This will effectively cause the ball to tumble through the air until it hits the ground but this will not always deaden the two axis spin evident in the ball until it starts rolling along the ground. As the ball rolls one of the axial spins will dampen and then that energy is transferred to the roll of the ball which effectively cause it to accelerate.

So in short, there is a two axis spin on the ball, one dampens and the drag is removed allowing the ball to continue on faster.

5. ## Re: Cricket - nobody understands it!

I can visualize the situation there Jezza - a very interesting hypothesis. The part that doesn't fit (I think) is that just because the spin on the "non-rolling" axis dampens and decays to nothing, this does not explain how the ball actually accelerates, a some point along its path. But you also are thinking along the appropriate lines.

Alan

6. ## Re: Cricket - nobody understands it!

Ok, had my lunch now <img src=/S/grin.gif border=0 alt=grin width=15 height=15>

There is a double spin of the ball, Spin A and Spin B

Spin A has X amount of kinetic energy decaying at a rate

Spin B has Y amount of kninetic energy decaying at a rate

As the ball lands it creates a minor Spin C with Z amount of kinetic energy. The spins A and B thene start to coincide with the "ground spin" of the ball. After a while the 3 spins coincide to create a collective kinetic energy of X+Y+Z. As the various spins decay and join the rotational spin of C the kinetic energy increases and as it does so the ball appears to accelerate, which will eventually deteriate with ground and air friction. Is that better <img src=/S/grin.gif border=0 alt=grin width=15 height=15>

7. ## Re: Cricket - nobody understands it!

Alan,

Spin theory aside, the most likely answer is that they sydney cricket ground is very much sloped, especially at the boundaries. You can really see this if you sit in the first row of seats. Any fielder on the other side of the wicket has no feet (well, he does have feet, but you cannot see them. Any fielder on the other side of the field is just an upper body.

8. ## Re: Cricket - nobody understands it!

<hr>How is it possible, for a ball travelling along the ground, to suddenly "accelerate" away from the person chasing it?<hr>

Sounds to me like someone walloped it again which would definitely make it go faster! <img src=/S/laugh.gif border=0 alt=laugh width=15 height=15>

9. ## Re: Cricket - nobody understands it!

I just went and got lunch and I was thinking about this while I was out. There are two other possible causes ...

1) As everyone knows, the centre of cricket action on the field is the wicket. This concentration on the middle of the ground causes cricket gravity. As the ball gets further away from the cricket gravity well, it is affected by this gravity less and less and thus speeds up.

2) You probably noticed this while the aussies were batting. If the SA guys were batting, the ball would have slowed down. This is a manifestation of crowd will. When the home team is batting, everyone is willing the ball into the fence. When the home team is fielding, everyone is willing the ball to stop. Naturally, as the ball only has very little will power, it is affected by this crowd attitude.

10. ## Re: Cricket - nobody understands it!

Jezza

I really like that answer, with two or more spins tending to coincide then reinforcing each other, manifesting in a single rolling spin which is larger than when the ball first hit the ground. It's still not the one I'm thinking of, but awfully close to the sort of principle involved.

Hint: When I wrote "accelerate" (in quotes) I did so because the effect observed is not a continuous or gradual one. The observed motion is:
- the ball travels across the infield, gradually slowing (as usual)
- it gets to some point (in or near the outfield) then the speed suddenly jumps
- it then continues, gradually slowing down (as would be expected)

Alan

11. ## Re: Cricket - nobody understands it!

Hi Tim

I can't argue with the ground sloping, since I don't know the SCG too well. I used what I saw as an example of what can (and sometimes does) happen on any horizontal ground. Nor can I argue with the "will of the crowd", since I only understand the physics of it - not the metaphysics. <img src=/S/grin.gif border=0 alt=grin width=15 height=15> For the same reason though, I'd have to dismiss the gravity well idea <img src=/S/laugh.gif border=0 alt=laugh width=15 height=15> as being too small an effect. It might also have the opposite effect - effectively presenting an "escape velocity" problem. This might help explain the difficulty of hitting the clock at the MCG. <img src=/S/grin.gif border=0 alt=grin width=15 height=15>

Alan

12. ## Re: Cricket - nobody understands it!

Ok, I am running out of ideas here so I am going add to my theory and mention friction.

"The pitch and outfield are both mown before play on every day of a match, if weather allows. The outfield may be watered, but the pitch may not be watered during a match."

If the outfield and surrounding area is watered will there be a reduction of friction due to the water or extra moisture in that area and the shortness of the grass compared to infield <img src=/S/shrug.gif border=0 alt=shrug width=39 height=15>.

<img src=/S/whisper.gif border=0 alt=whisper width=29 height=17>This is addition to my above answer.

Oh yes, as an extra, as the grass is mown daily does the direction of the grass blades have an influence <img src=/S/shrug.gif border=0 alt=shrug width=39 height=15> <img src=/S/shrug.gif border=0 alt=shrug width=39 height=15>

13. ## Re: Cricket - nobody understands it!

Ahh, well done Jezza "outfield may be watered". What they do sometimes have on cricket grounds is practice wickets near the boundary. These are rolled, hard and quick as lightning. So, when the ball hits them, it just zooms away.

14. ## Re: Cricket - nobody understands it!

Both you and Tim have pretty well hit on it... well, close enough to award [choccybars]. It's essentially a combination of effects - differences in frictional characteristics and the motion of the struck ball. Across the "high friction" infield, the ball is rolling along the ground, gradually slowing down as would be expected. Then all of a sudden, it encounters a change of conditions - the "low friction" outfield.

You may have seen/ heard that the SCG was rain-affected prior to the start of play. Once the infield was dry enough (the outfield is largely in shadow) then play commenced. The outfield was still wet! So when the rolling, decelerating ball meets the wet surface, there is no longer sufficient friction to sustain rolling, so the ball starts to slide. Initially, the total kinetic energy (or momentum) of the ball contributed to both its translational motion and its rotational motion. Once it can no longer roll, and begins to skid, all of that energy/ momentum is directed into translational motion. In other words, the ball must increase its speed in a step function fashion. It will always be decelerating of course, except for this sudden apparent jolt it receives.

I should give Skitter a [choccybar] too, since it is just as if the ball were given another wallop by an invisible batsman. Enjoyable thread, thanks folks.

Alan

15. ## Re: Cricket - nobody understands it!

Alan

This conundrum continues to be discussed in my office as there are a few cricket fans and players.

One of the guys came up with this The magnus effect and how a cricket ball swings... nice light reading for you <img src=/S/evilgrin.gif border=0 alt=evilgrin width=15 height=15>

Not an answer but quite interesting nonetheless

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