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While watching snooker on TV I saw the following incident: When the cue ball hit the object ball a "kick" occurred which caused the object ball to lift off the bed of the table. The commentator remarked that this had taken a lot of the pace out of the cue ball, and in fact this is what seemed to have happened, although the cue ball did not jump.

My thinking is as follows: the kinetic energy transferred from the cue ball to the object ball is governed by the mass and velocity of the cue ball. How can the cue ball lose more of its kinetic energy as a result of something that the object ball does after the contact?

It didn't. That would be putting the cart before the horse. It lost the energy at the moment of impact, when that energy was transferred to the object ball which, as a result, then expended some of it by defying gravity (oh no, not that again).

I understand when the energy is lost, but why should a different amount of energy be transferred at that point because of what happens to the object ball?

Conservation of energy says they have to end up with the same total energy as before, so if one jumps, the other can slow down to compensate.

How much energy each one gets depends on the details of the collision, it can be divided in all different ways.

If you understand when the energy was lost, then you'll notice that your 'because' is inapplicable. The action of each ball upon the other ended at the moment of impact. The timeline is strictly one-way.

I'm not looking for retro-causality, it's just one of those things I've accepted without question for years, but suddenly find myself asking "why?"

Call the energy of the cue ball at the point of impact “x” and say that “y” energy is transferred to the object ball. To keep things simple, say that x = 2y. Without the kick, both balls leave the impact with KE = y. With the kick, say that the object ball loses “z” energy when it jumps. After the impact the object ball has KE = y – z, but why should the cue ball have less than KE = y?

Bill, what you described could be accounted for if the player applied bottom-spin to the cue ball. That would account for both its loss of pace and the lift given to the object ball.

True, but that's not quite the same as getting a kick. A kick always seems to take the pace out of the cue ball, however it is struck.

If we had a slow motion close-up replay, we would at least have more information upon which to base an analysis. As it is, we can take a shot at it (!) based on what we think happened, and maybe feel sure we're right - but it only amounts a guess. Pity we're so far apart, else we could spend a few hours doing experiments.

It doesn't have to. The object ball could lose all its kinetic energy at a stationary point in its jump, and it wouldn't affect the cue ball.

However if we know the object ball will have y kinetic energy, and then i jumps as well, it has to be given y+z energy to do both those things. Therefore the cue ball has to lose z to conserve energy.

What you described is the jumping energy taken from the object ball's kinetic energy. What I described is it coming come from the cue ball's kinetic energy.

This makes perfect sense, it also suggests that the kick is caused by the fact that the cue ball has, for some unexplained reason, imparted extra kinetic energy to the object ball. This brings us back to the original question: if the kinetic energy of the cue ball is determined by its mass and speed, why would it impart a greater percentage of that energy to the object ball some times, rather than others, at any given speed?

You are forgetting rotation, which carries its own energy in addition to Ek=0.5*mv2

I would suspect that the "kick" you describe is most likely a result of the cueball having a high degree of rotation, in the same direction as its direction of travel. In both snooker and pool its common to give the cueball back or forward spin, in order to alter the behavior of the cueball and the ball being struck. By "spin" I mean rotation aside from the normal rolling motion of a ball.

Bryan

Generally, the dreaded kick seems to be something that is independent of either spin or the normal rotation of the cue ball. Experts have devoted much time and experimentation to trying to diagnose the cause of the kick, so far with little or no success. However, it was interesting to have people with scientific knowledge comment on the possible transfer and distribution of kinetic energy. Thanks.

Why shouldn't it? How much is transferred depends on lots of little details. A glancing blow would transfer less, if it was spinning, it might transfer quite a lot (a ImagingGeek described), and even then it'd depend on the friction between the two balls. The friction force probably depends on how hard they hit. Perhaps in one case the point of contact is slightly above the center of the object ball, and in another case it's slightly below - after all the table's surface is soft.

I found an interesting comment by Steve Davis:

"People talk about static electricity but in my opinion it is all related to chalk dust that sticks to the cue ball. That can be because of static or humidity, and it can also be embedded into the cue ball with the striking shot. If chalk is in between the contact, you get an explosion to varying degrees."

I am aware that this is by no means the only opinion about kicks, but it would certainly answer my original question.

Chalk explodes? I suppose it must to because I always grind heaps of chalk into the cue, and usually the cue ball jumps, sometimes onto the floor So it's pretty powerful.

Never underestimate the power of chalk; or Steve Davis!