Why is Acceleration Important?

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PaulM

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I’ve searched this forum for posts on acceleration to find out why acceleration is important, and what characteristics about it are important. This sparked the following questions I hope you will answer.

Here's one that's directed to Dr. Dave or Bob Jewett. Do we stroke with a constant acceleration or do we increase the rate acceleration? Does it make a difference?

How does acceleration affect stroke timing? What exactly is the desired stroke timing? Contact at maximum acceleration?

Many said that we should contact the cue ball at maximum acceleration. With an accelerating cue, don’t we, by default, contact the cue at maximum acceleration?

Some said that acceleration is the key to consistency. Why is acceleration key to consistency when there are many other things that can affect consistency?

Some say that the follow through promotes accelerating through the cue ball. This would make sense for helping someone who decelerates their cue during contact. If you are accelerating through the cue ball, does follow through matter?

There isn’t much of a difference in contact time between an accelerating cue and a decelerating cue. Is there a difference is cue ball speed or spin?

Controlling speed can be accomplished through acceleration and/or with the length of the back swing. Are there advantages of one over the other?

How does timing and acceleration affect each other and what should our goal for timing be?

Do we need to accelerate through the CB for short delicate shots?

Should the grip change if we change the acceleration from shot to shot? Does the grip affect acceleration?

How does the pause affect acceleration?

Since acceleration seems to be an important factor in our stroke, are there any drills or exercises that improve your ability to consistently accelerate through your stroke?
 
N

Neil

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PaulM said:
I’ve searched this forum for posts on acceleration to find out why acceleration is important, and what characteristics about it are important. This sparked the following questions I hope you will answer.

Here's one that's directed to Dr. Dave or Bob Jewett. Do we stroke with a constant acceleration or do we increase the rate acceleration? Does it make a difference?
I'm not them, but it depends on what type of stroke you use, and to the second part-not necessarily, (see later parts)

How does acceleration affect stroke timing? What exactly is the desired stroke timing? Contact at maximum acceleration?
Here's the thing- everything you do goes into your subconscious. That is why you are able to repeat certain actions. Over time, with experience to paying attention to what you are doing, you learn that at this speed, with this angle, the cb will go this far. (speed control)Or, this far off center of cb gives me this much spin and deflection.

Now, your mind is geared towards the cue going forward. And striking the cb at a certain speed. When your stroke decelerates on the way to the cb ( an number of things can cause this), you will not get the action or speed on the cb that you had programmed in your mind. What happens then is you increase your speed or offset for the next shot to make up for what you saw happen. Now, on the next shot, you struck the cb with the correct speed that you initially had programmed, but now hit farther out on the cb getting much more spin than you thought you would get. No consistency there.

Many said that we should contact the cue ball at maximum acceleration. With an accelerating cue, don’t we, by default, contact the cue at maximum acceleration?
Not necessarily. If you have your mind set for this does this at this speed with this bridge length, and you now have to shorten your bridge for a shot, you will not get the same reaction you thought you would because your speed will be off due to a shorter bridge length.

Some said that acceleration is the key to consistency. Why is acceleration key to consistency when there are many other things that can affect consistency?
It's one of the big keys, not the only one. (See above)
Some say that the follow through promotes accelerating through the cue ball. This would make sense for helping someone who decelerates their cue during contact. If you are accelerating through the cue ball, does follow through matter?
No. Technically, you only need 1/8" of followthrough. That is the distance your tip is normally on the cb.

There isn’t much of a difference in contact time between an accelerating cue and a decelerating cue. Is there a difference is cue ball speed or spin?
Yes. If you think you are hitting at a certain speed, but decelerate, you are hitting at a lesser speed and will not get the same reaction with the cb.

Controlling speed can be accomplished through acceleration and/or with the length of the back swing. Are there advantages of one over the other?
Largely depends on the person, and what you normally do. Again, you want consistency. Trying to do the same thing the same way every time possible.

How does timing and acceleration affect each other and what should our goal for timing be?
Timing is nothing more than hitting the cb where you want to at the speed that you want to, and getting the resulting action that you expected. Acceleration makes that timing much easier and consistent.

Do we need to accelerate through the CB for short delicate shots?
Technically, you never need to accelerate. You can play just as well with a consistent decelerating stroke. That sounds like heresy, buy it's not. The problem is, it is extremely difficult to decelerate consistently while making the cue go forward. Much, much, easier to strive for an accelerating or constant cue speed at contact with the cb. Now, for short shots, as much as is humanly possible, you want to strive for consistency. For example- you have a short, close shot. It behooves you to also know what is likely to cause the ob to skid in that situation? Knowing that, you may want to make sure you do accelerate, or else know how to compensate so the skid does not occur.
Should the grip change if we change the acceleration from shot to shot? Does the grip affect acceleration?

How does the pause affect acceleration?
It can help set your mind to the cue now going forward smoothly as a complete action. Not pausing can result in the cue going forward in a decelerating motion due to your brain thinking that the action has already happened and it tries to stop the cue.

Since acceleration seems to be an important factor in our stroke, are there any drills or exercises that improve your ability to consistently accelerate through your stroke?
Click to expand...

The easiest is to just stop trying to hit the cb with your tip. That is not the goal, that should be the end result. The cb should just get in the way of your actual stroke. Just work on finishing your stroke. Don't stop your stroke, let it come to it's natural finish position. For a pendulum stroke, that would be when your forearm gets stopped by your bicep, or when your hand hits your chest. For other stroke styles, that would be when your arm naturally stops going forward.

Hope that helped you some.
 
Bob Jewett

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Actual measurements of acceleration (or of velocity) of the cue stick are hard to do accurately. High speed video can be used to get velocity from change of position in each frame of the camera and then you can get acceleration from its definition of rate of change of velocity. The original video has to be done with great care or you end up useless noise at the end. An example of the measurement of velocity from video is at http://www.sfbilliards.com/articles/1999-06.pdf

Accelerometers measure acceleration more directly, but are somewhat harder to set up. I have seen a few measurements from them for billiard shots but I don't have a link right now. In general, they suffer from the very wide dynamic range of acceleration during the shot -- that is, you have low acceleration during the backswing and very high acceleration during the time of tip-to-ball contact. You would like to capture both regimes accurately.

The result from measurements is that players seem to have at least two (and likely more) acceleration profiles in their strokes. In the example above, the forward stroke acceleration looks to be roughly a bell curve or raised-cosine with the impact somewhat before acceleration goes to zero.

In a different measurement by a different player at a different speed, the acceleration seems to be more constant during the entire forward stroke.

As for whether it makes a difference, I think the answer is that no one knows. Maybe one acceleration profile is more natural and easier for beginners to use. Maybe another is more accurate in the long run. Maybe the best profile depends on the shot and the shooter.

I think that contact at maximum acceleration is a bad idea assuming you want to minimize the requirement of accurate timing. A hand-waving argument says that contact at zero acceleration gives maximum speed accuracy.

Many said that we should contact the cue ball at maximum acceleration.
Click to expand...

Most people who discuss pool physics cannot distinguish between acceleration, speed, velocity, etc. Perhaps what they really mean was maximum velocity which is zero acceleration.

Some said that acceleration is the key to consistency.
Click to expand...

The result of the shot depends on the speed of the cue stick. The speed of the cue stick depends on the acceleration you have applied up to the time of contact. If you have consistent acceleration, it will be easier to have consistent speed. To get a particular speed, you need to have a particular total acceleration.

There isn’t much of a difference in contact time between an accelerating cue and a decelerating cue. Is there a difference is cue ball speed or spin?
Click to expand...

No. The spin/speed ratio is determined by how far off-center you hit the cue ball. The speed of the cue ball for a given amount of off-center contact is proportional to the speed of the cue stick.
 
Ralph Kramden

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PaulM said:
I’ve searched this forum for posts on acceleration to find out why acceleration is important, and what characteristics about it are important. This sparked the following questions I hope you will answer.

Here's one that's directed to Dr. Dave or Bob Jewett. Do we stroke with a constant acceleration or do we increase the rate acceleration? Does it make a difference?

How does acceleration affect stroke timing? What exactly is the desired stroke timing? Contact at maximum acceleration?

Many said that we should contact the cue ball at maximum acceleration. With an accelerating cue, don’t we, by default, contact the cue at maximum acceleration?

Some said that acceleration is the key to consistency. Why is acceleration key to consistency when there are many other things that can affect consistency?

Some say that the follow through promotes accelerating through the cue ball. This would make sense for helping someone who decelerates their cue during contact. If you are accelerating through the cue ball, does follow through matter?
There isn’t much of a difference in contact time between an accelerating cue and a decelerating cue. Is there a difference is cue ball speed or spin?

Controlling speed can be accomplished through acceleration and/or with the length of the back swing. Are there advantages of one over the other?

How does timing and acceleration affect each other and what should our goal for timing be?

Do we need to accelerate through the CB for short delicate shots?

Should the grip change if we change the acceleration from shot to shot? Does the grip affect acceleration?

How does the pause affect acceleration?

Since acceleration seems to be an important factor in our stroke, are there any drills or exercises that improve your ability to consistently accelerate through your stroke?
Click to expand...

IMHO you are correct by saying cue acceleration seems to be an important factor.
Deceleration of the cue has many affects... none that would seemingly be of value.
Cue ball control depends the on acceleration of the cue for any stroke consistency.
No, I'm not an instructor... but I have tried that decelerating cue thing in the past.
 
Mark Avlon

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I'm been exploring a relatively new feature of StrokeAnalyzer that plots acceleration of the three axis. This a representative plot of the stroke axis during the back swing and forward stroke.

Acceleration.jpg



This plot appears to be in agreement with the plot in Bob's article. Acceleration is at or near peak acceleration. At contact, the cue decelerates, then accelerates briefly before decelerating to a stop. Since the deceleration at contact lasts for 7ms (for this stroke), it appears to reflect the compression of the tip and movement of the flesh of the hand.
 
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Bob Jewett

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Mark Avlon said:
I'm been exploring a relatively new feature of StrokeAnalyzer that plots acceleration of the three axis. This a representative plot of the stroke axis during the back swing and forward stroke.

Acceleration.jpg



This plot appears to be in agreement with the plot in Bob's article. Acceleration is at or near peak acceleration. At contact, the cue decelerates, then accelerates briefly before decelerating to a stop. Since the deceleration at contact lasts for 7ms (for this stroke), it appears to reflect the compression of the tip and movement of the flesh of the hand.
Click to expand...
I think the software cannot handle the speed of the collision. In my article cited above, the time of deceleration at tip-ball contact is consistent with it only happening during tip-ball contact, which is what the physics predicts. That means that all of that deceleration happens in about 1ms. The reacceleration from the hand compressing and pressing forward on the cue lasts for about 20ms.

If StrokeAnalyzer uses video to analyze the cue motion, it cannot resolve times/actions that are shorter than the time between frame captures. What was the frame capture rate for your setup?
 
Mark Avlon

Mark Avlon

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Bob Jewett said:
If StrokeAnalyzer uses video to analyze the cue motion, it cannot resolve times/actions that are shorter than the time between frame captures. What was the frame capture rate for your setup?
Click to expand...

StrokeAnalyzer does use video for motion analysis, but there is also an optional accelerometer for use with it that I used. The sample rate was 1ms for this plot.
 
Bob Jewett

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Mark Avlon said:
StrokeAnalyzer does use video for motion analysis, but there is also an optional accelerometer for use with it that I used. The sample rate was 1ms for this plot.
Click to expand...
Then I think there must be additional filtering in the software or the sensor itself. It is more or less impossible for the deceleration to take 7ms.
 
Bob Jewett

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Thanks for the plot.

There are several things to note from this plot. The first is that the system seems to miss the deceleration from the tip-ball contact almost completely. That is the blunt-tipped negative spike that occurs at about 194 on the X-axis. The indicated maximum deceleration at that time is only about 3/4 of a vertical division. This is probably due to the sensor being unable to handle the very, very large deceleration during that contact time. A rough calculation shows that it is one hundred or a few hundred gs. (1m/s decrease in stick velocity in 1ms is 1000 m/s/s average deceleration during the contact. That is about 100 gs. The peak deceleration during that time is expected to be twice that, or 200 gs.

Looking at the time before the tip-ball collision, you can see the a decrease in the forward acceleration from about 10 to 50 on the time axis. That is probably the time during the back stroke that the cue stick is coming to a stop and reversing direction.

There is a minimum acceleration at about 50 but it does not go to zero. This means that either the zero is not set correctly on the sensor or that the stick does not come to a dead stop for a non-zero length of time at the back of the stroke. It would be good to be able to correlate acceleration with stick position.

Between 50 and 150 the acceleration builds smoothly then it decreases to close to or just barely below zero at contact. This means that the cue stick is at peak velocity at impact. Some feel that is the best time to make contact as it doesn't waste any energy in the stroke.

Between 195 and 220 you can see rapid oscillations in the curve. These are probably due to longitudinal vibrations in the cue stick. The frequency of these vibrations is expected to be about 1000/second. In a recent post of ultra-high-speed video from a Russian cue maker, these vibrations are actually visible. They are only a fraction of millimeter of tip movement, but since they are at a very high frequency, the acceleration involved is large. Since the sensor is marginally capable of capturing data at that rate (technical terms: undersampled, beyond first Nyquist, aliased) that part of the plot is very difficult to interpret. In this case "longitudinal" means that the compression/expansion of the stick is along the length of the stick and the stick is getting longer/shorter 1000 times per second.

Some average positive acceleration is visible from 195 to 115. This is probably the hand and stick coming into equilibrium as in the article linked to above. After 115 the stick is slowed to a stop which seems to occur around 270.
 
Mark Avlon

Mark Avlon

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Bob,

Thanks for the analysis.

Bob Jewett said:
It would be good to be able to correlate acceleration with stick position.
Click to expand...

I've tried to plot velocity and position, but the system noise and integration to calculate them make it tough to get a reasonable estimate of them. The math (Kalman Filters and Quanterions) are beyond me. If anyone could help with this, it would be appreciated.
 
Bob Jewett

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Mark Avlon said:
... I've tried to plot velocity and position, but the system noise and integration to calculate them make it tough to get a reasonable estimate of them. The math (Kalman Filters and Quanterions) are beyond me. If anyone could help with this, it would be appreciated.
Click to expand...
I don't think any complicated math is required to get a reasonable answer, provided that you can extract the position versus time (roughly) from the video. Matching up the time scales is just a matter of sliding one against the other, I'm pretty sure.
 
P

PaulM

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What I take away from this is that -

Correct timing ensures that the cue ball is struck with the desired speed of the cue.

Acceleration doesn't effect spin. Speed and offset do.

We want contact to happen at maximum velocity, not peak acceleration.

The give in the flesh of the hand reduces or eliminates the effects between a loose and tight grip.

Many thanks to all who responded.
 
F

FranCrimi

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This is what I got from the thread:

Quote: Bob Jewett:

"In a different measurement by a different player at a different speed, the acceleration seems to be more constant during the entire forward stroke.

As for whether it makes a difference, I think the answer is that no one knows. Maybe one acceleration profile is more natural and easier for beginners to use. Maybe another is more accurate in the long run. Maybe the best profile depends on the shot and the shooter."


This is what I was talking about earlier regarding planning an increased follow through. No one knows for sure the effect on each individual regarding acceleration, or what if any is the correlation between the two, but I can say that for myself, the results are interesting, indeed.
 
Bob Jewett

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FranCrimi said:
This is what I got from the thread:

Quote: Bob Jewett:

"In a different measurement by a different player at a different speed, the acceleration seems to be more constant during the entire forward stroke.

As for whether it makes a difference, I think the answer is that no one knows. Maybe one acceleration profile is more natural and easier for beginners to use. Maybe another is more accurate in the long run. Maybe the best profile depends on the shot and the shooter."


This is what I was talking about earlier regarding planning an increased follow through. No one knows for sure the effect on each individual regarding acceleration, or what if any is the correlation between the two, but I can say that for myself, the results are interesting, indeed.
Click to expand...
As instructors, the problem for us becomes how to try to modify the timing/acceleration/sequence/mechanics of a student who seems to be struggling with an ineffective stroke. One of the most common problems I see with stroke timing is that the stroke starts to stop before contact. Impact happens after the peak in speed and when the hand is slowing the cue down.

A recent student had this problem which was particularly evident in his draw shots. (You have to hit draw shots quite a bit harder than other shots, in general.) In his case the problem was solved by moving the grip back as he had adopted a grip well forward of "perpendicular at contact". The usual dictum "accelerate through the ball" would not have worked well for him without moving his back hand -- asking for more performance from bad mechanics leads to frustration. Also, that phrase "accelerate through the ball" is technically problematic, and I'm relatively technical.
 
F

FranCrimi

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Bob Jewett said:
As instructors, the problem for us becomes how to try to modify the timing/acceleration/sequence/mechanics of a student who seems to be struggling with an ineffective stroke. One of the most common problems I see with stroke timing is that the stroke starts to stop before contact. Impact happens after the peak in speed and when the hand is slowing the cue down.

A recent student had this problem which was particularly evident in his draw shots. (You have to hit draw shots quite a bit harder than other shots, in general.) In his case the problem was solved by moving the grip back as he had adopted a grip well forward of "perpendicular at contact". The usual dictum "accelerate through the ball" would not have worked well for him without moving his back hand -- asking for more performance from bad mechanics leads to frustration. Also, that phrase "accelerate through the ball" is technically problematic, and I'm relatively technical.
Click to expand...

I agree. I'm not fond of the statement "accelerate through the ball." I also find it to be problematic.

However, the phrase, 'increase your follow-through,' and even at times, "exaggerate your follow-through," seems to resonate in a positive way with some players I've worked with, including my own experimentation with my game.
 
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