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"The Kick Riddle Continued - An Alternative View - By Robert Blayne"

The Kick

I could rattle on about the impact force between cue and object ball being over a tonne. Or that the acceleration of the object ball is in the order of 10,000 times that of gravity. I could introduce the concept that the cue ball in any full ball shot, whether it be a run-through, screw or stun shot, always stops momentarily before running through or screwing back down the table.

I could introduce the subject of linear momentum being converted into angular momentum in the vertical direction being a result of a kick. I could show how the friction forces between cue and object ball are so great that they lock together resulting in what is called gear effect. I could discuss why the face of a golf driver is curved, and how the same gear effect works in that case. But, I am sure that I would bore you quite quickly if I haven’t already.

An Experiment

So, I will just say this. A kick is a completely natural phenomenon. This can be easily proven as follows. Get a selection of balls ranging from tennis balls to footballs, ten-pin bowls balls to golf balls and get two of each. Using a variety of surfaces, table tops, carpets, wooden floors or even an ice rink, roll one ball so that it collides with its twin head on, as in a full-ball snooker shot.

Observe the reaction of the balls. In many cases you will see that the rolling ball jumps on impact just as the cue ball does in that type of kick. With footballs or compressed rubber balls you will always get a kick and with some ball / cloth combinations you may never get a kick. Exactly the same laws of physics apply in every case, footballs or snooker balls, or whatever balls.

Vertical Movement

Have you ever seen a kick during a stun shot? I suggest not. The reason being that in a stun shot, the cue ball has no angular momentum, therefore it is impossible for it to jump as all forces are in the horizontal plane, the plane of the cloth. Significant vertical forces large enough to lift the cue ball 10 millimetres off the table, can only be generated through the transfer of angular to linear momentum during the contact phase.

Ultra Slow Motion

I believe that the actual contact period between snooker balls at impact is approximately 10 microseconds or 0.00001 seconds. This may seem instantaneous to us humans, but to the microprocessor inside this computer, that is an age.

Its heart, or internal clock, beats 20,000 times in that same period of time. If one were to use a 1000 frame per second camera, like the one they used on at Wimbledon this year, I can’t remember its name, you would see some interesting effects. Obviously the balls would have to be marked such that their spin could be seen. The camera should be zoomed in so that the frame covered the width of say five snooker balls with the object ball centre frame. With the cue ball rolling from right of picture and heading for a full ball impact you will I believe see the following.

Normal Impact

The cue ball stops on the spot, but continues to rotate. Its rotational speed reduces to no less than half its original speed. At this point the object ball starts to the left while rotating with backspin!

As the object ball slides away to the left the backspin gradually changes to a forward roll. The cue ball meanwhile, which was spinning on the spot, picks up speed as the angular momentum is converted back into linear momentum due to the friction between ball and cloth. The cue ball then follows the object ball and exits frame left.

A Kick

During a kick however a couple of notable differences could be observed. Firstly, the object ball would, I think, have no backspin but would slide away from the impact as in a stun shot. Secondly, the cue ball would jump at impact with its rotational speed reducing. As it comes back down to the cloth its rotational speed will be much less than that in the first case. It will then pick up speed and roll to the left but nowhere nearly as fast as previously.

I find this subject fascinating and urge you to try the “two ball experiment”. I would love to have the facilities to do the high-speed photography or to see the results of such.

The Reason

All balls of whatever material, or size have a tendency to kick and some will kick more readily than others. Some balls will always kick as with footballs on a carpet. Some balls may never kick, perhaps bowling balls on a highly polished surface. It is the ball and surface combination, which dictates how readily a kick may occur.

Something seems to tip the balance making conditions just right for a kick in snooker. It may be chalk on the cue or object ball increasing the ball to ball coefficient of friction, allowing gear effect to take place. It may be that the object ball is in a hollow increasing the force required to get it moving which allows gear effect to occur. If a ball stops on a sweaty patch where a player has bridged, it may increase the ball to cloth coefficient of friction.

There are many different ‘types’ of kick but I believe they can all be explained using the above type of analysis. If I had the time, money and equipment there are a number of experiments and calculations that I would like to do or see done.

Frank Callan Suite - 282 Ribbleton Lane, Ribbleton, Preston, Lancashire, England - PR1 5EB - tel. + 44 (0) 1772 702211 - info@fcsnooker.co.uk

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