Physicists will tell you that a cut shot in billiards cannot be made in which the cut angle is less than 90 degrees. The reason is simple. In order to cut the ball at less than a 90 degree angle, some component of force would have to be applied in the direction opposite the direction that the cue ball is travelling on impact. Another way to look at this is that the cue ball would have to contact the object ball at a point on the back of the ball – this cannot occur because other parts of the ball are in the way.
However, I have witnessed the shot shown below executed successfully many times. It’s exceedingly difficult even for a top-notch player and it requires such precision that it takes repeated attempts. I have verified with great care that the angle is actually slightly less than 90 degrees in many cases when the ball is pocketed!
The above shot was demonstrated for me by a friend I’ve known for many years who is highly capable player, has good judgement, and who I consider trustworthy. The equipment was a 9ft Gold Crown III with mid-wear Simonis 860 cloth, double-shimmed, and using Brunswick Centennial balls. He learned the shot from Jose Parica who could make it at least one time in ten tries. I’ve had him demonstrate it for myself and some of the best players in town who were occasionally able to duplicate the results themselves. All believe sincerely that they are cutting the ball more than 90 degrees. Despite the hustles and tall tails that commonly circulate in pool rooms, this is definitely a real shot. In fact, it can even be overcut (missing on the outside).
An explanation is required, but unfortunately without expensive slow motion cameras I won’t be able to offer anything conclusive.
First, here are some simple attempts at explanations that were easy to rule out by observation or careful attention to the set-up and execution of the shot:
- My first impression was that an optical illusion was involved. It looks over 90 degrees but is not. There is certainly an optical illusion at play, but I know that neither my friend nor any other decent player would fall for that. The optical illusion occurs because the exact angle of the cut is not based on the angle formed by the center of the two balls in their initial positions and the point where the cue ball ends up (hopefully in the pocket). Rather, the true angle is based on the intersection of the straight line formed by the path of the edge of the cueball closest to the 9 ball and the line between the center of the 9 ball and the point of contact of the two balls.
- My next thought (before seeing the shot) was that the object ball was being hit at slightly less than 90 degrees, hitting the rail but at enough of an angle to rebound into the outside jaws of the pocket. If this were the whole trick, it could only fool a novice or a rube and would hardly rank as anything more than a cheap parlor trick. In fact, on the table where the shot was demonstrated, even the off-the-rail shot must be hit at 90 degrees to sink the ball. And I saw the shot sunk cleanly at least once, and overcut once (missing outside!) My friend insists he’s witnessed Parica make it clean many times.
- My next thought was that the shot was being set up slightly different for each try and when it was made, the initial positions of the balls were off just enough to make it work. I eliminated this possibility by setting the shot up carefully myself hundreds of times while the shot was attempted and occasionally made.
- Another thought I had was that the object ball (9) rolled so slowly that the nap of the cloth (the trench that forms along the rails of a worn cloth) guided it towards the pocket, curving it subtly as it went. The same curve could also be attributed to warped slate or an unlevel table. When I witnessed the shot, however, the 9 ball was never moving slowly enough for any of those explanations to pan out. It actually rolls at about “pocket speed” so that any table or cloth effect would be very obvious to a seasoned player.
I made precision measurements of the table and concluded that several of the shots I had witnessed really were less than 90 degrees. It was repeatable, though not easily. And it seemed to withstand careful scrutiny.
Here are some other possible explanations that I can rule out, but which require a little more pool knowledge:
- Throw. An object ball can easily be thrown off the line of contact by english imparted on the cue ball. For example, you can easily hit a ball dead straight on and have the object ball shoot off to the side by a very noticeably amount. In fact, this is a very common reason shots are missed. However, there are several reasons throw can be excluded from any explanation of this shot. First, throw is most effective when the balls collide softly. At the enormous speed of this shot, no spin could have any effect. Also, throw is most effective when the balls collide head-on. In this shot, the contact is as thin as possible giving no chance for cue ball spin to be transferred to the object ball, even at slow speed. Also, the shot is made with no english by players who can actually execute such a hard shot without putting much or any english on the cue ball, evidenced by the rebound angle the cueball follows off the rail. Players who cannot make the shot as described have tried using extreme outside english but it does not work any better, and it makes the shot much more difficult. Throw is not involved.
- Curve. Good players can shoot the cueball so that it curves along an arc. It’s theoretically possible for the cue ball to curve and then nick the back edge of the object ball enough to shorten the angle as observed. An obvious way to do this would be a masse shot with the cue jacked up (not level). A talented player with enough chances could certainly masse the ball into the pocket, at any angle. However, the shot is performed with a level cue, with no english, and at a very high speed. All of the above, eliminate the chance that masse was used in any shot I witnessed. Not to mention that no one would find the shot particularly interesting if it was simply a difficult masse shot.
- Air pressure. It’s been suggested that if the cue ball actually missed the object ball but was hit with such speed that the negative air pressure ‘sucked’ the object ball into contact and it bounced off, it could behave as shown. I think this is an extreme stretch. For one thing pool balls are heavy and anyhting more than a slight rock due to air pressure is not realistic. Also, it still wouldn’t explain a back cut since the shockwave from the air pressure would be directed forward.
- Recoil off the cloth. Suppose that the object ball were resting in a divot in the cloth the size of a dinner plate and a half inch deep. You could imagine that a hit at the right speed would cause the ball to roll up the far side of the divot and be deflected backwards appearing as a backcut. Could a very minor effect of the sort be attributed to the ball resting on a indentation in the cloth due to the weight of the ball, or perhaps made by the number of tries at the same location? Interesting idea, but again, the speed of the observed impact and the mass of the balls eliminates this as a possible explanation.
- Jump. Some players contend that the cueball jumps imperceptibly and bounds across the cloth contacting the object ball in mid-flight just enough to hit the back of the ball and back-cut the ball. This is actually a plausible idea, but still highly suspect. In defense of the idea, when the cue ball is hit this hard, it will almost certainly jump and bounce by a very small amount sometimes if not most of the time. It’s been shown with slow motion photography that almost all break shots bounce to some degree when pros break a rack of 9 ball. Despite the instructions that this shot be hit with a level cue, that’s usually not entirely possible. However, IF the cueball were closer to the object ball and a good player was asked to hit the back side of the object ball without using rails or masse shots, he would certainly elect to jump the cue ball a few inches off the table, clear the edge of the object ball and nick it from behind. It would require the cue to be elevated and struck down into the cueball. And yet an illegal jump shot could conceivably be made with a level cue, miscuing and doing a very subtle scoop of the cueball. That would at least get the cueball bouncing. The main reason that I have doubts that an inadvertent jump shot tells the whole story is that I did not see any shot where the cue ball jumped perceptibly (except after missing the ball and rebounding off the rail). Also, if you look at the geometry, though such a shot is possible, even with and apparently level hit, the cueball would have to be at an extraordinarily precise distance off the cloth and on a precise downward trajectory to leap over the edge of the object ball and then glance the back side of the ball. But then again we’re only trying to account for a degree or two less than 90.
- Random direction of incident force. This explanation is probably the most radical, and unlikely. Suppose the collision was so utterly slight and tangentially glancing that the force vector was entirely perpendicular to the motion, that there was no forward component of motion remaining in the force at all. Such a result seems mathematically impossible, but not beyond consideration in the real world. In fact, a different but well known and accepted effect occurs when a cut shot is thrown with english (see diagram below).When the shot below is hit with no english, the object ball does not exactly follow the perpendicular angle as would be expected, but instead follows another path, an elongated angle caused by the collision-induced throw due to friction. This is a very real effect, so much so that even intermediate pool players adjust for it on most shots. However when the same shot is hit with running english, the english-induced throw counter-acts (seemingly exactly) the friction-induced throw from the collision. In other words, the object ball follows the path perpendicular to the hit, exactly opposite the point of contact. By the way, this is why most players prefer to add running english on long precise shots.The analogy between the shot above and the proposed 90 glancing collision is striking because in both cases the forward component of motion is cancelled. The only difficulty (and not a minor one by any means!) is explaining how a simple cut at 90 degrees cancels the throw of forward motion without english coming into play – we already discussed how english is not on the cueball and how its effect decreases and approachs zero at 90 degree angles. But lets assume this is a real physical effect that happens with such diminishingly thin cuts. Now we can imagine that the collision could be simulated by a tiny needle striking the contact point on the object ball from a perpendicular direction. In that case, the vast majority of the force vector would be directly forward (in the 90 degree cut direction). However, due to minute flaws in the surfaces of the two balls, dust particles, residual chalk, and even to a very minor extent random chance, some portion of the collision force would resonate in all directions. Any portion of the collision force that’s not in the direction of impact would be slight, but undeniably not zero. Think of poking a volleyball. The ball initially deforms then re-expands and bounces off your finger. Tiny differences in the consistency on the surface of the ball or point of contact, could cause slight variations in rebound angle. On certain occasions, perhaps this quasi-random variation in the force vector would just happen to push the ball ever so slightly in the opposite direction of the collision. We only need to account for a degree or two to justify most of the observations. I suppose (but cannot assert definitively) that such an effect may really occur on a small enough scale. Unfortunately, even if this conjecture has any validity, I cannot seriously imagine the effect having an observable result at real-world scales involving highly rigid and uniform-surfaced billiard balls
Any finally a few that I find credible:
- Another plausible explanation is simply that the cue ball misses the 9 ball hits the rail, and rebounds into the back side of the 9 so fast that it is never noticed. At the thinness of the shot this might even be possible if it glanced the nine on the way in. Observations seem to contradict this idea, but not conclusively. A good experiment would be to try it without the rail, or further off the rail. That would make placement of the balls much more difficult (and subject to error), but it might confirm or deny whether a simple kick is all that’s happening.
- My favorite explanation tends to be the last one involving contact after the bank, but perhaps a little hop over the contact point on the way in lets a straight no-english kick brush the edge of the object ball on the way out. A full day with a high speed camcorder might be the only way to know.
Personally, I do not believe a cut of less than 90 degrees is possible and have confirmed this with a physicist and a retired physicist. Nor do I believe there is some hidden force or variable that we haven’t yet considered, again confirmed by the physics experts. I am certain that at least some shots I witnessed were less than 90 degrees, but I am by no means certain that the cueball did not hop or glance the object ball after hitting the rail. The latter explanations (1 and/or 2 above) must be true, but I have not proven it, and cannot with the equipment I have access to. Players who make the shot (better players than myself), seem quite certain they can cut a ball a bit less than 90 degrees.
There’s a variation of this shot in which the cueball is placed in the jaws of the corner pocket and the object ball is placed on the spot and shot into the other corner pocket (the one nearest the one that the cue ball was shot from). I haven’t measured this one, but on a bar box with large pockets, it might simply be an optical illusion that the angle is less than 90 degrees. But that’s another issue.