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.
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
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
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.
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.
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.
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.
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!
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
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.
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.
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
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.
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.