At the 2:30 mark of this video of a snooker match, Judd Trump acieves wonderful action on his cue ball following its contact with the red ball. (His shot is also shown in slow motion immediately afterwards.)
Unlike the youthful commentator at 2:50, I’m not asserting that the cue ball “defies the laws of physics”. Obviously not.
What I am wondering about, though, is what mechanism accounts for the cue ball speeding up a moment or two after striking the red. In other words, immediately after hitting the red ball, the cue ball is moving very slowly. Then, a moment later, it’s moving much faster. What mechanism accounts for that acceleration - acceleration that seems to be occurring in the absence of any external force.
Thanks!
(I should say that this phenomenon is not at all rare for the cue ball in snooker. It’s just that this video shows a particularly nice example of it.)
If I’m watching the right one, he’s put backspin on the cue ball. Although it’s traveling forward it is not rolling smoothly on the table rather it is spinning/rolling as if it were moving towards him. Once the cue hits the red ball, its forward momentum is transferred to the red ball. It stops but it is still spinning as if it were rolling towards him. The friction with the table slows the spin a bit so that it slips less and this lets the spin carry the ball back towards him.
The key is that a ball spinning quickly will slip against the felt as the spin slows a bit, the slipping lessens.
You’re both basically saying, then, that as the cue ball’s spin slows/stops, it gains more traction (and so speeds up in terms of ‘translational’ motion)?
Oh that I knew. My uncertainty is what explains the gap - between the cue ball’s initial very slow backwards motion and the subsequent speeding up of that backward motion. Is it really due to the fact that when it’s spinning rapidly it doesn’t get ‘traction’? I’m not disputing that, it just doesn’t seem self evident to me that a rapidly spinning ball doesn’t ‘grip’ as well.
The lack of friction may not be caused by the rapid spin. Maybe it just happened to be at a spot on the felt where the oil from someone’s hand made the felt a little slicker, and then once it got past that spot, it had more traction. Do similar shots always show the same kind of acceleration, or do some of them have the cue ball move rapidly from the moment of impact?
I don’t think it’s to do with gaining more traction as the spin slows, it’s the fact that the cue ball is initially struck forward with significant force. When it hits the red, its forward motion is impeded. Without any backspin, the momentum would normally carry it forward anyway, at a lower speed. With backspin, this causes it to move backwards. With most shots of this type (known as ‘stun’ shots if the intention is to more or less stop the cue ball dead, or ‘screw’ shots if you want to move the cue ball backwards a fair amount - ‘English’ is generally the term for sidespin rather than backspin), the cue ball screws back immediately, but when played with this much force the forward momentum takes a little longer to counteract, which is why there appears is a delay in the backspin taking effect. I bet if you filmed this using high speed video from directly overhead, you would see the cue ball continuing to travel forward a short distance after hitting the red, before retracing its path backwards. Whereas in a gentler screw shot, the elasticity of the contact between the balls is sufficient to start the cue ball on its backward journey almost immediately, without the delay.