Dr. Dave's answers to frequently-asked questions (FAQs), mostly from the AZB discussion forum
for more information, see Section 4.04 in The Illustrated Principles of Pool and Billiards
and Vol. II of the Video Encyclopedia of Pool Shots
What factors affect how much the cue ball swerves?
See "Squirt - Part VIII: squerve effects" (BD, March, 2008). It summarizes and illustrates all of the important factors and effects. A complete summary of all squirt, swerve, and throw effects can be found here. See also: squirt, swerve, and throw confusion. And for demonstrations of how squirt and swerve combine on different types of shots, see squerve.
Why doesn't the CB curve when struck with a perfectly horizontal cue?
The videos, explanations, and links on the OB turn resource page help explain this.
You can actually make the CB curve in the opposite direction than expected by shooting up on the CB (with negative cue elevation), for example, by shooting with sidespin up at a CB sitting on a piece of chalk on the rail. So if the CB curves one way when striking downward, and curves the other way when striking upward, it must not curve at all when striking horizontal.
OB "swerve" and "turn"
Can an object ball swerve or turn due to sidespin?
First of all, OB "swerve" or "turn" should not be confused with throw, which is an entirely different effect. A ball can swerve (curve) only while it is sliding with a massé component of spin. An object ball rolling with sidespin has no reason to turn, just like a CB rolling straight with sidespin. For an explanation and visualization of this, see "spin," "slide," and "roll."
Here are some videos that demonstrate the lack of significant turn, which also apply to a CB rolling with sidespin (e.g., after any swerve has taken place)
A slight amount of "OB swerve" is possible with massé spin transferred to the OB from follow or draw on the CB (see the end of TP A.24), but the effect is very small and of little practical importance (unless there is a large amount of throw due to cling/skid/kick). With the videos above, I am just looking at the effect of pure sidespin as a ball is rolling. I call this effect "OB turn."
There is a special cue twisting and stroke swooping technique that can be used to swerve or turn an object ball, but it is quite controversial. Here's a demonstration:
For more information, see the stroke swoop/swipe resource page.
Bob Jewett's June '92 BD article has an experiment you can use to test OB bending (or lack there of). Here's the test from the article:
TP B.2 provides a physics and math-based approach to the question of whether or not a ball rolling with sidespin can turn. The analysis looks at the interaction of rolling resistance and spin resistance. Another theoretically possible effect is Magnus Effect curving (as with a baseball pitch), but because pool balls are so smooth (unlike a baseball with raised stitches) and because they travel relatively slowly over short distances (compared to a baseball) and because the magnitude of aerodynamic forces are so small compared to the weight of the ball (unlike with a baseball), the effect is negligible on a pool table. Here are the conclusions of the TP B.2 theoretical analysis: A ball rolling with sidespin goes straight for all practical purposes, and cloth irregularities or dirt (or slightly out-of-round or unbalanced balls) can create a much bigger effect, especially at slow speeds. Sometimes the ball will go straight as it slows with right spin, sometimes it will curve right as it slows with right spin, and sometimes it will curve left as it slows with right spin. Having said that, the physics does seem to suggest that there might be a small tendency for the ball to curve in the spin direction (e.g., right curve for right spin), but due to the assumptions in the analysis and the results of the video demonstrations, the value of this information is questionable.
The CB can be caused to curve only with an elevated cue. For explanations and demonstrations, see squerve and massé shot aiming. The CB almost always swerves on sidespin shots because the cue is rarely level, since it needs to clear the rails (see cue elevation effects). Once a CB is done swerving (after any massé spin wears off), it rolls in a straight line with sidespin.
A ball with a massé spin component will change direction by the same amount regardless of how slick or "grabby" the cloth. But a more grabby cloth will cause that change of direction to happen sooner, so the curve will be tighter. In your example, a slicker cloth might be of some help since you want the 1-ball to execute a more open curve in order to clear the 2-ball.
However, an OB will not curve to any significant extent without being struck by an airborne cueball. That is, in order to induce a massé spin component, the cueball has to contact it above the horizontal equator. ... this will also cause it to jump. The combination of jump and subsequent curve might be enough to clear the 2-ball and then "straighten out" after landing/bouncing and head for the pocket.
In principle, you can get the OB to curve just be using draw or follow at any non-zero cut angle. But calculations indicate that, at most, the change in direction is miniscule (in fact, very hard to measure and probably overridden by random buffeting by the cloth's weave pattern).
dr_dave reply to Jal:
I agree 100%. OB swerve is not a significant-enough effect to be of any practical use. The effect is larger in clingy conditions, but still not significant enough to be useful or even noticeable.The best chance to seemingly make an OB curve around an obstacle is by jumping the CB into the OB, creating OB hop over the obstacle ball, or by relying on table roll-off, a lopsided (non-round OB), or well-placed dirt or irregularities on the cloth. The rail groove can also influence an OB close to the cushion. Even the magical cue twist and swoop technique won't work (without a weighted ball).
Can swerve be eliminated with an above-center hit?
The closer the cue is to level, the less a shot with english will swerve. Unfortunately, because of the height of the rails, the diameter of the cue (above the rail), and clearance between the cue and rail, the cue will not be level on most pool shots. Also, if you hit above center with english, the squirt direction is down a little; so even with a level cue, an above-center hit still results in slight swerve action (because the effective impact direction is still down a little).
Having said all of this, an above center hit can reduce cue elevation and reduce the effects of shot speed and ball/cloth conditions (i.e., there will be less swerve).
What is squerve?
Squerve, sometimes called "effective squirt" or "net cue ball deflection," is the combined effect of SQUirt and swERVE. For more information, see "Squirt - Part VIII: squerve effects" (BD, March, 2008) and "Squirt - Part III: follow/draw squirt and swerve" (BD, October, 2007). For a summary of all of the factors affecting squerve, see: squirt, swerve, and throw effects. Also, here's a video excerpt from Vol. II of the Video Encyclopedia of Pool Shots that explains and demonstrates things:
For more information, see squirt/swerve cue elevation effects and draw/follow effects.
One way to compensate for squerve when aiming a shot is to use back-hand english (BHE), front-hand english (FHE), or a combination of the two, as described and demonstrated on the BHE/FHE resource page. For the following discussion, see the diagram below from Patrick Johnson's AZB post, which shows how the effective pivot point for aim compensation (the "squerve or squirve pivot point") is different from the natural pivot point (the "squirt pivot point") for the shaft.
Based on the diagram, to use pure FHE, you would first align the cue center-ball along the desired shot line, with your grip at the squerve pivot point. Then you would shift your bridge hand sideways (while keeping your grip hand fixed at the squerve pivot point) to place the tip for the desired amount of sidespin. Then you would stroke straight along this new alignment. This will correctly compensate for squirt and swerve and send the CB in the desired shot-line direction.
To use pure BHE, you would first align the cue center-ball along the desired shot line, with your bridge at the squerve pivot point. Then you would shift your grip hand sideways (while keeping the bridge position fixed at the squerve pivot point) to pivot the cue to the desired amount of sidespin. Then you would stroke straight along this new alignment. This will correctly compensate for squirt and swerve and send the CB in the desired shot-line direction. An alternative to a pre-stroke BHE pivot is to use a swoop stroke (with the pivot occurring during the stroke), which would still send the CB in the same direction (unless the swoop motion is really exaggerated).
For this particular shot, with the effective pivot point ("squerve pivot point") as diagrammed, it might not be easy to do either of these. That's where a combination of BHE and FHE becomes useful, as demonstrated in the video and articles on the BHE/FHE resource page.
An alternative to these approaches is to do what most pros and top players do: Just place the cue along the necessary direction, away from the desired shot line, to intuitively compensate for squirt and swerve, guided by countless hours of practice and successful experience, and then stroke straight to send the CB in the desired shot-line direction.
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