Back arm perpendicular- why?

[dr_dave]

Patrick,

Sorry ... apparently I misunderstood again. I should have known you would know about the universal law of gravitation. You would be surprised how many engineering students (after many years of college-level physics and engineering classes) fail to understand where W=mg comes from.

Without getting into space-time distortion, I think the best explanation is:

For every action, there is an equal and opposite reaction. Regards of the sources of the "forces," if there are two bodies exerting force(s) on each other, the resultant force must be equal and opposite between the bodies.

Sorry if I sounded condescending before. That was not my intention. I know yours physics knowledge and understanding are solid.

Regards,
Dave

Thanks, but I'm interested specifically in a conceptual explanation of the notion that the gravitational attraction between two masses is a unitary force that's not the sum of discrete gravitational forces* generated by each.

pj
chgo

*I'm familiar with the General Relativity concept of gravity as spacetime distortion, so I may be taking liberties with the word "force" here.

 

[Patrick Johnson]

Patrick,

Sorry ... apparently I misunderstood again. I should have known you would know about the universal law of gravitation. You would be surprised how many engineering students (after many years of college level physics and engineering classes) fail to understand where W=mg comes from.

Without getting into space-time distortion, I think the best explanation is:

For every action, there is an equal and opposite reaction. Regards of the sources of the "forces," if there are two bodies exerting force(s) on each other, the resultant force must be equal and opposite between the bodies.

OK, I think I'm getting you now. Thanks.

Sorry if I sounded condescending before. That was not my intention.

I'd never suspect you of that. Sorry if I gave that impression.

I know yours physics knowledge and understanding is solid.

Maybe in a colloquial sort of way...

pj
chgo

 

[Bob Jewett]

... if you could somehow pinpoint acceleration and track it through the stroke... and graph it... ...

This has already been done twice with ultra-high-speed video. The first time was in the Jacksonville Project in 1998 and the second time was in Dr. Dave's basement this summer.

People have also made accelerometer measurements of pool strokes, and some of them have been posted in previous threads.

A graph of velocity versus position from the Jacksonville Project is available in http://www.sfbilliards.com/articles/1999-06.pdf While that's not acceleration versus time, from the simple relation of velocity and acceleration, it possible to conclude that the acceleration for this particular stroke was zero at impact and the velocity was at its peak.

I'll see if I can post some similar results from the Ft. Collins Project.

 

[Jal]

....one graph is a 45 degree angle ....up until impact..

This would be a very inefficient way of generating cue speed and doesn't really resemble players' actual strokes...at least the ones for which we have recorded data (see Dr. Dave's website). Better to have it shaped like a hill. Bob J (and probably other instructors) recommend the stroke be such that impact takes place at around the point where the force is going from positive to negative, ie, having the cue essentially coast into the cueball at constant speed. This corresponds to having the peak at roughly the halfway mark of the graph, timewise. There are several advantages to this and it's certainly a good way to shoot. There are also advantages to having that peak of acceleration occur later into the stroke and then continue to have some positive acceleration right up to impact. But to my knowledge, there's not much to be said for having the peak happen at the very end of the stroke, ie, at impact.

if you graph the player who reaches peak acceleration.. and then attempts to control velocity... you will find a graph that looks more like an EKG graph of a heartbeat.. accelerate stop.. accelerate stop...as the muscle attempts to maintain a velocity.... you are asking a muscle to do something its not designed to do and getting inconsistent results...

Yes, that would be pretty bad.

Jim

 

[spoons]

Spoons is sitting somewhere with a bemused and befuddled look on his face, "I just asked a simple question about pool, that is really all I did . . . '

Hu

Ha ha... The physics is interesting to me, but it's all far above and beyond the basics that I understand. I suppose that's what you get when you take Physics 101 and your book for the course features a cartoon character named Ringo.

Oddly enough, I got several more direct answers out of this thread than the other thread I created the same day. (although, I am convinced that it's my fault for not being able to articulate my question very well). We did get to the answer eventually, but it took some doing!

 

[ShootingArts]

Nature of this forum

No fault of yours. Just the nature of the forum members, me included, we are easily sidetracked.

Hu

Ha ha... The physics is interesting to me, but it's all far above and beyond the basics that I understand. I suppose that's what you get when you take Physics 101 and your book for the course features a cartoon character named Ringo.

Oddly enough, I got several more direct answers out of this thread than the other thread I created the same day. (although, I am convinced that it's my fault for not being able to articulate my question very well). We did get to the answer eventually, but it took some doing!

 

[smoooothstroke]

arm angle

I have heard that it is best to be straight up and down at contact also.From observing other players I have noticed many top players who's arms are forward at contact.I think as long as your arm angle is not more then 90 degrees you are doing good.

 

[spoons]

No fault of yours. Just the nature of the forum members, me included, we are easily sidetracked.

Hu

Eh... no worries. Like I said, the physics is interesting to me, too. And I'm paying a whole lot less for these lessons than I paid to watch Ringo run around acting like a fool!

It's strange to me, though. For as much as I hear about keeping your back arm perpendicular to the cue, the floor, or any other reference point for that matter.... consensus seems to be that it's a great starting point, but that it's really a matter of personal experience and results.

I guess you have to start somewhere when you teach. I just wish more of the instructional folk and authors in this game would acknowledge that individual bodies aren't built the same; and, that finding a way that works for you is really the end goal.

Predictable, repeatable results are paramount. Sure, you've got to be able to stroke the ball a little bit, especially in rotation games. But, it seems like very few matches are ultimately won or lost on stroke shots.

I guess that's why I started this thread- to see if anyone knew of a major difference between perpendicular and past perpendicular that would be holding me back in the years to come. Anyway, thanks for all the input everyone. Please resume your physics discussion

<------ stepping off soap box.

 

[JoeyA]

This would be a very inefficient way of generating cue speed and doesn't really resemble players' actual strokes...at least the ones for which we have recorded data (see Dr. Dave's website). Better to have it shaped like a hill. Bob J (and probably other instructors) recommend the stroke be such that impact takes place at around the point where the force is going from positive to negative, ie, having the cue essentially coast into the cueball at constant speed. This corresponds to having the peak at roughly the halfway mark of the graph, timewise. There are several advantages to this and it's certainly a good way to shoot. There are also advantages to having that peak of acceleration occur later into the stroke and then continue to have some positive acceleration right up to impact. But to my knowledge, there's not much to be said for having the peak happen at the very end of the stroke, ie, at impact.


Jim

Jim, would you mind listing the advantages of having the cue coasting into the cueball at constant speed?

What are the advantages of having the peak acceleration occur later into the stroke and continuijng to having some positive acceleration right up to impact?

Thanks,
JoeyA

 

[Patrick Johnson]

Jim, would you mind listing the advantages of having the cue coasting into the cueball at constant speed?

The one I always hear (and that makes obvious sense to me) is that it gives the greatest chance of hitting the CB at the speed you want because you don't have to time the hit so precisely (more margin for timing error). Maybe Jim knows more.

What are the advantages of having the peak acceleration occur later into the stroke and continuijng to having some positive acceleration right up to impact?

A disadvantage would be the converse of my point above: when the stick is constantly changing speed you have to hit the CB at a very precise moment to get the speed you want.

Sorry to butt in. As you were.

pj
chgo

 

[softshot]

This has already been done twice with ultra-high-speed video. The first time was in the Jacksonville Project in 1998 and the second time was in Dr. Dave's basement this summer.

People have also made accelerometer measurements of pool strokes, and some of them have been posted in previous threads.

A graph of velocity versus position from the Jacksonville Project is available in http://www.sfbilliards.com/articles/1999-06.pdf While that's not acceleration versus time, from the simple relation of velocity and acceleration, it possible to conclude that the acceleration for this particular stroke was zero at impact and the velocity was at its peak.

I'll see if I can post some similar results from the Ft. Collins Project.

that is some excellent information... but even your own graph shows acceleration after impact... the speed increases after contact..


what are we measuring? speed of the cue? friction between the grip hand and the wrap?... weight of the cue vs the ball? or acceleration of the bicep?? was the experiment done with a perfect pendulum stroke...??

Randy turned me on to the Jacksonville experiments.... I am not arguing that you guys got good data... I am not questioning it...

you are measuring ... from what I see... acceleration and velocity as it relates to a cue stick... valid data...

I am coming from a biometrics position.. what the shooter is doing while those effects are occurring..?

they are accelerating their bicep twards its own center.. that is the only thing a muscle is capable of doing....

and at the end of the day that is the ONLY thing they can control... so they should focus on that...

control what is under your control.. and let the results be what they are....

I still believe that if you accelerate through the cueball.. you will attain the most consistent results... ...

JMO

 

[Jal]

Jim, would you mind listing the advantages of having the cue coasting into the cueball at constant speed?

What are the advantages of having the peak acceleration occur later into the stroke and continuijng to having some positive acceleration right up to impact?

Thanks,
JoeyA

Joey, I should note that the following is based on computer modeling of the physics of pushing an object forward and not with any insight into the mechanics of the human arm. I did try to generate a force curve that looked similar to one supplied by John Pizutto here at Dr. Dave's website:

http://billiards.colostate.edu/technical_proofs/new/TP_A-9.pdf

What the program did was to compare different length pendulum style strokes for a fixed bridge distance. By stroke length, I mean the distance traveled by the tip from the beginning to the end of the stroke sans any intervening collision with a cueball. The cue's speed was calculated as it reached the cueball for different stroke lengths and various injected errors. The errors produced deviations in stroke length via timing, grip position and length of backstroke (starting position of the tip). The effects of deviations in bridge length, and peak force were also calculated.

Coasting at impact (having the acceleration pass from positive to negative) was taken as a baseline stroke length and set the "standard" grip position. It was assumed that for every inch forward of this grip position, stroke length would be decreased by two inches, and for every inch rearward, stroke length would be increased by two inches. The rationale and assumption behind this was that the angle of the forearm where the transition from positive to negative acceleration occurs is fixed (barring a timing error). This seemed consistent with trials done with my cue.

According to this then, the advantages of coasting are:

- It uses the least amount of energy to get the cue up to some speed.

- Cue speed is less sensitive to the various imposed errors compared to having the cue decelerate before impact. Particularly noteworthy is its reduced sensitivity to small deviations in bridge length of around 5-10%.

The advantages of stretching the stroke out and continuing to accelerate at impact are:

- It can generate more cue speed given the same peak force and/or it requires less peak force to reach some particular speed.

- Cue speed is less sensitive to all of the deviations than either coasting or decelerating at impact, except for small variations in bridge length and peak force. With all three modes, cue speed is equally affected by deviations in peak force.

A major assumption was that the basic shape of the force-time curve is the same regardless of stroke length (grip position), only being stretched or contracted along the time scale. Lacking more information, this was necessary and the idea was just to compare different stroke lengths anyway, keeping other variables the same. Given that we're dealing with the human arm and human volition, many things can change. But if you step back from the numbers and mull over the general physics, I think the conclusions have a pretty fair chance of being applicable despite those changes. I have found that moving my grip hand back of vertical by about 3 inches did produce more draw distance by roughly what was predicted (very informal test). I would guess that you could effect something similar, or more so, with elbow drop instead.

Not recommending anything, just trying to compare them from a purely physics point of view. I do think it argues strongly against peaking early and then decelerating, from having your grip hand well-forward.

Jim

 

[JoeyA]

Thanks Jim.

JoeyA

 

[Bob Jewett]

... what are we measuring? speed of the cue? ...

The quantity that is plotted on the vertical axis is the speed of the cue stick versus time. The sudden drop just after the first peak is caused by the tip hitting the ball -- about 50% of the stick speed is removed in a thousandth of a second. The return of the stick speed to about 80% of its maximum value occurs after the ball has left the tip. This smaller second peak is from the hand, which does not slow down during the nearly instantaneous collision, pushing the stick partly back up to speed.

In order to determine the acceleration of the stick, you need to plot the slope of the graph for each time. Since the slope of the curve is more or less zero when the tip hits the ball, the acceleration for the stroke shown is more or less zero when the tip hits the ball.

 

[Jaden]

perpendicular

As many people have pointed out, it's a good starting point and there is valid reasons for having your elbow at a 90 degree angle at contact. It does come down to personal preference, but at the same time, it is important to understand why it can be helpful to developing consistency.

I just found this thread and it is LONG so forgive me if this has already been stated.

By having the forearm pointing straight down at the point of impact, you have gravity pulling straight down on the arm, this limits the amount of tangential force and makes it easier to have a closer to perfectly straight point of contact. This is not to say that it is not possible to do without being perfectly perpendicular, but it is easier for the vast majority of people this way. There are too many top players with unorthodox strokes for this to be a perfect representation of what is necessary, but it does make it easier for the average player and will have the highest percentage for the highest percentage of players.

Jaden.

 

[softshot]

This smaller second peak is from the hand, which does not slow down during the nearly instantaneous collision, pushing the stick partly back up to speed.

Exactly!!

.. the hand is still accelerating .. because the shooter is accelerating through the ball..

I don't care what the physics are... ( I can't change the laws of nature)

I care about how I can control the physics... and I have the best control of impact velocity.. when I accelerate through the cueball.. I am allowing my muscle..to do what it does naturally..

all the physics a pool SHOOTER needs to know is to accelerate evenly through the cueball...and to be able to do that at different rates... you accelerate through a lag the same way you accelerate through a break shot...

 

[Deadon]

Thanks for the thoughtful replies everyone, there are some great tidbits in here.

I've long been a proponent of the importance of predictable results over raw ability, but obviously if I could combine the two, I'd like to do that.

I'm not sure what I'll do in the end, but I appreciate the perspectives of folks on here.

I think if there's one thing that would push me over the edge, it would be somehow knowing that I could achieve the same results with less effort. From what I'm reading here, it sounds like making contact peak acceleration is generally regarded as more important than the actual position along the pendulum swing. Right?

Spoons;

Peak acceleration is at the point where your arm is perpendicular to table. That is why it is recommended, has little to do with having the cue level also. 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. Think of a piston in a car engine, at what point is the piston moving up the fastest?

That is the best result with the least effort. Doesn't mean that its best for you, as you may feel more comfortable doing otherwise. Its just the best technical place to start.

 

[Jal]

...I don't care what the physics are... ( I can't change the laws of nature)

Of course you don't need to know physics in a formal way, but you do need to understand it at gut level. Since you can't change it, it's generally a good idea to not try to violate its laws.

I care about how I can control the physics... and I have the best control of impact velocity.. when I accelerate through the cueball.. I am allowing my muscle..to do what it does naturally..

What you say about accelerating through could very well be right - physics supports this.

all the physics a pool SHOOTER needs to know is to accelerate evenly through the cueball...

If by "evenly", you mean to accelerate the stick forward during the stroke at a more or less constant rate, this just doesn't happen. It's not "natural" for the muscles to do this.

Jim

 

[Patrick Johnson]

Peak acceleration is at the point where your arm is perpendicular to table. That is why it is recommended, has little to do with having the cue level also.

It's recommended for both reasons, and in fact level cue* may be more important than peak acceleration.

pj
chgo

*What's really meant by this is "the part of the stroke's arc where the grip hand is traveling parallel with the stick". The stick is never really level.

 

[softshot]

If by "evenly", you mean to accelerate the stick forward during the stroke at a more or less constant rate, this just doesn't happen. It's not "natural" for the muscles to do this.

Jim

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