Back arm perpendicular- why?

[Jal]

Peak acceleration is at the point where your arm is perpendicular to table.

From what you said after after this, you obviously mean peak velocity, not peak acceleration. It's a nitpick, but there is an important difference.

...As you go forward past that point, the tip of the cue goes down, not up as others have suggested, and the forward movement slows.

While the part in bold (my edit) is probably substantially correct, it's not clear how much volitional control you have over this. You may be able to change the timing significantly even with a pendulum stroke. Do you have any info that supports this, either way?

That is the best result with the least effort.

It does require the least effort and is a very good way to shoot. It's debatable as to whether it's necessarily the best way. Final cue speed is less sensitive to some stroking errors when "accelerating through", in theory.

Jim

 

[Jal]

I worded that poorly by evenly I mean a gradual constant increase that peaks at impact

I think I get the gist of what you're saying, which is to have a smooth stroke without any jerkiness. Probably no one would disagree. Technically though, if you were to graph the cue's rate of change of speed against time, ie, its acceleration against time, it would be anything but constant. This has been done with accelerometers and examples were linked to earlier. It goes from zero to some peak value, then back down to zero, or thereabouts.

But more in line with what you're saying are plots of speed against distance, as shown in this video by Dr. Dave, Bob J. and Dave Gross.

http://billiards.colostate.edu/high_speed_videos/new/HSV_B-40_stroke_analysis.wmv

If you draw a line tangent to one of the curves at any point, that is the rate of change of speed against distance at that point of the curve. While not constant (the one for Dave Gross comes pretty close), the tangents show a fairly smooth and gradual decrease over distance.

Jim

 

[ShootingArts]

interesting

I just watched the video a few times. As I suspected, acceleration seems to peak early in the stroke for most people and falls off as we approach the cue ball. Velocity increases but at a much slower rate than it did at peak acceleration. This has little to do with the mechanical properties of the forearm angle to the cue stick and everything to do with the nature of our muscles.

Dave Grossman is the exception to the rule. Although his acceleration doesn't increase it is very near constant throughout the stroke with a slight blip at the end, due to wrist or shoulder movement I suspect. This is a distinctly different way of shooting and I believe perhaps a better way. Sometimes I focus on a slow start of my forward stroke and what I describe as a gathering of power. I deliberately gradually increase speed all the way to the cue ball. This isn't my natural stroke as my natural stroke more resembles Bob's and Dr. Dave's, a fairly early ascent to peak acceleration with it falling off as I approach the cue. I have worked to make my start a bit more gradual than if I was swinging a hammer for example but I am usually far from Dave Grossman's approach.

I do think that constant or increasing acceleration might be the better stroke for maximum control of accuracy hitting the cue ball. It is also obvious that speed control would be easier with the strokes of Bob Jewett and Dr. Dave. As always, we live with trade-offs, there isn't a perfect solution to most issues we deal with in the real world and stroke just seems to be one more place that we compromise regardless of which stroke we choose.

Hu

I think I get the gist of what you're saying, which is to have a smooth stroke without any jerkiness. Probably no one would disagree. Technically though, if you were to graph the cue's rate of change of speed against time, ie, its acceleration against time, it would be anything but constant. This has been done with accelerometers and examples were linked to earlier. It goes from zero to some peak value, then back down to zero, or thereabouts.

But more in line with what you're saying are plots of speed against distance, as shown in this video by Dr. Dave, Bob J. and Dave Gross.

http://billiards.colostate.edu/high_speed_videos/new/HSV_B-40_stroke_analysis.wmv

If you draw a line tangent to one of the curves at any point, that is the rate of change of speed against distance at that point of the curve. While not constant (the one for Dave Gross comes pretty close), the tangents show a fairly smooth and gradual decrease over distance.

Jim

 

[Jal]

I just watched the video a few times. As I suspected, acceleration seems to peak early in the stroke for most people and falls off as we approach the cue ball. Velocity increases but at a much slower rate than it did at peak acceleration....

Agreed.

Dave Grossman is the exception to the rule. Although his acceleration doesn't increase it is very near constant throughout the stroke with a slight blip at the end, due to wrist or shoulder movement I suspect.

His acceleration is not constant, or even close to being constant. Acceleration is the change in speed divided by the time it took for that change to occur. Tangent lines to the curves on those graphs represent the change of speed divided by the distance it took for the change to occur. Since these are curves and not straight lines, the tangent lines represent the 'instantaneous' change of speed divided by distance traveled while that change took place, when that distance is very small (infinitesimal). This happens to be equal to the acceleration, as just defined, divided by the cue's speed at that moment, or equivalently, the force acting on it divided by its momentum.

Sorry to harp, but it's difficult to communicate when people have different definitions. The one just offered is the standard one.

Jim

 

[ShootingArts]

ramp shape

I gotta look again. He is talking about one thing and graphing something else.

Hu

Agreed.

His acceleration is not constant, or even close to being constant. Acceleration is the change in speed divided by the time it took for that change to occur. Tangent lines to the curves on those graphs represent the change of speed divided by the distance it took for the change to occur. Since these are curves and not straight lines, the tangent lines represent the 'instantaneous' change of speed divided by distance traveled while that change took place, when that distance is very small (infinitesimal). This happens to be equal to the acceleration, as just defined, divided by the cue's speed at that moment, or equivalently, the force acting on it divided by its momentum.

Sorry to harp, but it's difficult to communicate when people have different definitions. The one just offered is the standard one.

Jim

 

[ShootingArts]

still looks the same to me

Jim,

I went and reviewed the video several more times and I still see a much more consistent acceleration throughout the part of the stroke graphed considering Dave G's stroke. Breaking down the percentage of maximum speed over time or distance gives a similar result. Dr. Dave and Bob Jewett achieve most of their contact speed early in their stroke and have largely stabilized velocity, acceleration is nearing zero, when they hit the cue ball. Dave Grossman's velocity increase is spread far more evenly over the length of the stroke that is graphed. In simple layman's terms, the acceleration appears near constant over the length of the stroke.

Looking at the way the human body works, it appears that Dave Grossman's muscles are still fully engaged at cue ball impact while there is little if any muscular effort in Dr. Dave and Bob Jewett's strokes at that point, at least in the strokes graphed.

At the end of the day I am interested in the stroke that most consistently pockets balls and gives me shape on the next one. If speed control were the primary criteria, and it often is, I would prefer Bob J's and Dr. Dave's stroke. However if the greatest possible accuracy hitting the cue ball and object ball were most important I would favor Dave G's stroke.

My reasoning is simple. Our muscles in our arms are not well balanced, some are stronger than others. Also they are not perfectly synchronized. That is why we have to put so much effort into gaining and maintaining a straight stroke. These muscles in a steady state, contracting comparatively gently under light load will deliver the cue more accurately than if they are in transition, starting to relax.

Our bodies aren't designed to work like machinery and trying to compare their actions too closely to the workings of machinery leads to bad science. Physics isn't a strong point of mine. However I studied how our muscles work and how to make them work better for about a decade.

Hu

 

[Deadon]

From what you said after after this, you obviously mean peak velocity, not peak acceleration. It's a nitpick, but there is an important difference.

You are correct, didn't want to get too techie, cue forward speed would be the appropriate term as I wanted to avoid the vector discussion in using the term velocity. As you did when you explained acceleration without including the vector component ie speed or direction can be acceleration.

[/QUOTE]While the part in bold (my edit) is probably substantially correct, it's not clear how much volitional control you have over this. You may be able to change the timing significantly even with a pendulum stroke. Do you have any info that supports this, either way?[/QUOTE]

Because of the arc of the arm as it travels past the lowest point, as it goes up, the forward vector of the cue would have to slow to some degree.

[/QUOTE]It does require the least effort and is a very good way to shoot. It's debatable as to whether it's necessarily the best way. Final cue speed is less sensitive to some stroking errors when "accelerating through", in theory.[/QUOTE]

This was an answer to spoons question on the best way with the least effort. Best way overall is what works best for the player.

 

[dr_dave]

more force with "accelrate into the ball"

... I am interested in the stroke that most consistently pockets balls and gives me shape on the next one. If speed control were the primary criteria, and it often is, I would prefer Bob J's and Dr. Dave's stroke. However if the greatest possible accuracy hitting the cue ball and object ball were most important I would favor Dave G's stroke.

My reasoning is simple. Our muscles in our arms are not well balanced, some are stronger than others. Also they are not perfectly synchronized. That is why we have to put so much effort into gaining and maintaining a straight stroke. These muscles in a steady state, contracting comparatively gently under light load will deliver the cue more accurately than if they are in transition, starting to relax.

Dave Gross, by "accelerating into the ball," is contracting his muscles more than he needs to achieve the desired cue speed. Bob's and my strokes in HSV B.40 are using less muscle effort to create the same cue speed. Dave Gross builds up the speed more slowly at first, so he needs to work harder to build up the rest of the speed later in the stroke. I would think this would lead to less accuracy, in general (although, Dave Gross is a great player).

Regards,
Dave

PS: The muscle transitions during the stroke are more complicated than many people think. For more info, see the 2nd Q&A here:

http://billiards.colostate.edu/threads/stroke.html#SPFF​

 

[Jal]

Dave Gross, by "accelerating into the ball," is contracting his muscles more than he needs to achieve the desired cue speed. Bob's and my strokes in HSV B.40 are using less muscle effort to create the same cue speed.

Dr. Dave,

I'm not sure what you mean by "less muscle effort." According to the force-time curves I've looked at, it takes less peak force to get the cue up to some particular speed when "accelerating through." This should be true of sinusoidal-like curves in general, but I haven't really looked for exceptions.

Dave Gross builds up the speed more slowly at first, so he needs to work harder to build up the rest of the speed later in the stroke.

In terms of energy expenditure throughout the entire stroke, this is likely true.

It's hard to say anything with absolute confidence since I don't think anyone here has studied how the force-time curve varies with different stroke lengths. It sounds like Hu (ShootingArts) does know a lot about the muscles themselves.

Jim

 

[dr_dave]

Dr. Dave,

I'm not sure what you mean by "less muscle effort." According to the force-time curves I've looked at, it takes less peak force to get the cue up to some particular speed when "accelerating through." This should be true of sinusoidal-like curves in general, but I haven't really looked for exceptions.

Jim,

Thank you for pointing out my error. I was a little careless with my "intuition." I prepared a TP with analysis and plots ... probably similar to what you have done already. Here it is:

TP B.4 - Stroke speed and acceleration vs. distance​

I'd be curious whether or not you (and/or others) agree with my conclusions at the end. Also, let me know if you spot any errors.

In terms of energy expenditure throughout the entire stroke, this is likely true.

I agree. The energy delivered to the cue is obviously the same for any stroke if the cue speed (just before CB impact) is the same. Work done = energy = 1/2mv^2. However, the energy expended by the complex muscle physiology might vary significantly from one stroke type to another.

Regards,
Dave

 

[softshot]

ENT]

I'd be curious whether or not you (and/or others) agree with my conclusions at the end. Also, let me know if you spot any errors.

Regards,
Dave

compare a muscle and a rocket....both are pure acceleration

which one maintains a more consistent trajectory over distance... overcoming other implied forces...

does an accelerating rocket resist gravity better than a non accelerating rocket... yes because it influences the forces involved to a greater extent.. than it would if it didn't apply it's controllable force.

on a pool table gravity is not the concern ... friction is...

an accelerating stroke overcomes friction.. in the same way a rocket overcomes gravity..

and in both instances you gain a greater control of trajectory.. and you do it by utilizing the only force under your control.. in the most effective way...

if a rocket is on the trajectory you want it on.. you should fire the main engine and accelerate straight through the other forces...

rather than

trying to coast through using your booster rockets to maintain trajectory...

acceleration is your freind.... and the bonus is you have complete direct control over it...

 

[Patrick Johnson]

[snip comparison of rockets and arms]

The cue ball doesn't know or care whether your stick is accelerating, coasting or decelerating - it only knows how fast your stick is going at the moment of impact. Your stick's speed at impact is all that determines how far the cue ball goes and how spin (especially follow and draw) works on it.

Since your stick's velocity at impact is the only thing that matters to the cue ball, it's the only thing that should matter to you. You should adopt the method of accelerating your stick that ensures the stick will be moving at the speed you want when it hits the cue ball.

If you "accelerate through" the cue ball, you ensure that your stick's speed is changing (accelerating) at the time it hits the cue ball and is traveling at the speed you want for only an instant - and that instant is likely to occur before or after you hit the cue ball.

But when your stick reaches its peak speed it moves at nearly the same speed (while it transitions from accelerating to decelerating) for a few moments rather than one instant, giving you a larger "window of opportunity" to hit the cue ball at that speed. In other words, peak speed ("coasting") is an easier speed target to hit because it's not so much of a moving target.

The obvious conclusion is that, while it's important to control your stick's acceleration, you should control it so it ends (reaches peak speed) as your stick reaches the cue ball, not so it continues through the cue ball.

Conveniently, peak speed (the end of "speeding up" acceleration) occurs naturally at about the "bottom" of your stroke - where your tip is also traveling in the straightest line for the longest time. So it makes sense (and is probably easiest) to try to make all three things - reach peak speed, reach bottom center of your stroke, and hit the cue ball - happen simultaneously.

pj
chgo

 

[randyg]

Nice post Patrick!....SPF=randyg

 

[whitewolf]

Here's my dilemma:

I've been told by players much more accomplished than I that I should work on getting my back arm perpendicular at contact. Nearly every book, article, AZB post, etc. that discusses fundamentals seems to agree.

When I play, my arm is usually forward of perpendicular at contact. I wouldn't say dramatically, but very clearly visible to anyone who's looking. I've done this for years, because it's more comfortable, it feels more natural to me, and I seem to get significantly more consistent results.

I've tried several times to "fix" my stroke, by moving my grip hand back, or addressing the ball differently- for months at a time, even - with no appreciable result other than feeling awkward at the table, and playing significantly worse than I know I'm capable. I feel like I have enough "stroke" playing my usual way, to do anything I need to do at the table, but I can't help but wonder if there's a compelling reason to try fixing my mechanics again.

Ultimately, I don't want my game to be limited by something like this, so I thought I'd put the question to the board. What are the benefits, scientific or otherwise, of addressing the cue ball with your back arm perpendicular to the ground?

Thanks!

Well I had a chance to go to the US Open this year and I pointed out to my friend that most of the pros were NOT perpendicular. Their arms were slighty forward from the vertical plane.

And, for the pros who really get down over the shot, it is impossible for the forearm to be perpendicular if the elbow is up in the sky because the anlge at the elbow is NOT 90%. Nonetheless, those pros who were down low had their foreamrs tilted slightly forward.

I have proclaimed in the past that this is similar to the 'forward press' in golf, whereby just before making the swing the hands move slightly forward on the plane of the backswing. What this does is to cue your mind on the PLANE.

It is my theory that this is what pool players do naturally. If you don't believe me, try tilting the forearm slightly back and see how awkward this is.

Regards, WW

 

[Jal]

...If you "accelerate through" the cue ball, you ensure that your stick's speed is changing (accelerating) at the time it hits the cue ball and is traveling at the speed you want for only an instant - and that instant is likely to occur before or after you hit the cue ball.

But when your stick reaches its peak speed it moves at nearly the same speed (while it transitions from accelerating to decelerating) for a few moments rather than one instant, giving you a larger "window of opportunity" to hit the cue ball at that speed. In other words, peak speed ("coasting") is an easier speed target to hit because it's not so much of a moving target.

The obvious conclusion is that, while it's important to control your stick's acceleration, you should control it so it ends (reaches peak speed) as your stick reaches the cue ball, not so it continues through the cue ball.

Patrick,

The cue's speed at impact is the result of what has taken place during the entire stroke. The argument you just presented focuses in on the last few moments and is valid as far as it goes. But when you look at the stroke from start to finish, it's at least plausible that accelerating through can result in better speed control. The physics of why this could be true is fairly straightforward. Naturally, it gets muddled up when the human element is added. However, these unknowns apply just as much to the "coasting through" argument as to the "accelerating through" argument. You don't gain much accuracy if you're coasting at the wrong speed.

All in all, it may be that coasting is the best way since it happens to coincide, more or less, with zero movement of the tip in the vertical plane. It's also likely a little more energy efficient. But the "best speed control" part of the argument is not so clear cut, imo.

Jim

 

[softshot]

snip Patrick making the same argument he made several pages ago

and now we have come full circle..

human muscles can maintain acceleration much more consistently than they can maintain velocity..

I want consistent control. so I will let my muscles do what they naturally are good at...

 

[Patrick Johnson]

JAL:
...it's at least plausible that accelerating through can result in better speed control.

"Better speed control" means creating a better chance for the correct stick velocity and impact with the cue ball to occur simultaneously. How could "accelerating through" (accelerating continuously up to and past impact) do this?

pj
chgo

 

[Patrick Johnson]

human muscles can maintain acceleration much more consistently than they can maintain velocity...

Human muscles don't "maintain" peak velocity - it's "maintained" naturally by the fact that your muscles have stopped accelerating your arm.

I will let my muscles do what they naturally are good at...

They're naturally good at doing nothing during "peak velocity" - it's what they naturally do.

pj
chgo

 

[Bob Jewett]

... human muscles can maintain acceleration much more consistently than they can maintain velocity. ...

That's not implausible, but what sort of study determined this? Do you have a reference?

As for what we can sense of the motion, I think we can sense at least three factors in motion and controlling motion: position, velocity and acceleration. It's not clear to me that separating them out somehow gives either a better feeling of what's going on or more control. Also, I suspect that we sense force far more than acceleration, but if you use the same cue stick all the time they are directly proportional.

I think we may also be able to sense -- have a feel for -- jerk, which is the rate of change of acceleration. Minimizing jerk also seems to be important in developing a consistent stroke.

 

[Jal]

"Better speed control" means creating a better chance for the correct stick velocity and impact with the cue ball to occur simultaneously. How could "accelerating through" (accelerating continuously up to and past impact) do this?

Good question.

During the forward stroke the cue is being accelerated up to some speed (duh). Take the time from the start of the stroke up to impact and divide it into small intervals (ideally infinitesimal). During each of these intervals the magnitude of the acceleration will have a particular value. Multiply the magnitude of the acceleration by the length of the time interval. The cue's final speed at the commencement of impact will be the sum of all these products, one for each interval. So its final speed depends on its acceleration at each interval throughout the entire stroke, not just at the end. In that sense, none (or any group) of the time intervals is more important than any other.

When accelerating through to obtain some particular speed, the curve of acceleration plotted against time is generally flatter than when not accelerating or decelerating at impact. This makes it less sensitive to timing errors, i.e., shifting it, or some portion of it, right or left on the time scale. It's kind of hard to explain why this is true without a diagram, which I can't offer at this time.

It has to be said, however, that small variations in bridge length favor coasting through if no adjustments are made for them. It then becomes a question of how well these variations are noticed by the player and what adjustments are made. This element adds considerable fuzziness to the whole issue.

Jim