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DeuceBigalo
09-26-2006, 09:19 PM
I've never really put any real thought in to the kick shot, especially when you're going for a 'kick and pot' but I recently watched a guy playing 9 Ball, using his fingers to measure distances between cushion and OB and then mirroring that same distance on the edge of the table beyond the cushion. My question here is, what are the tricks of the trade to calculate accurate kick shots when you're trying to kick and pot?

Simple, to the point answers would be highly appreciated.

pooltchr
09-27-2006, 04:17 AM
Actually, you measure the distance from where the center of the "ghost ball" would be when it makes contact with the object ball to the point in front of the rail where you can see a line on the bed. This line is made because there is a point where every ball touches the table when it is on a rail. Mirror that distance up on to the rail, and find the line from that point back to the center of the cue ball. That is the aiming line to make the kick. You will need to adjust slightly depending on how the rails are playing, and for whatever spin you plan on putting on the cue ball. Each table will play differently, but this is a very good method to give you a reasonable opportunity to make the shot. Just remember to take into account the other variables. It's just like a bank shot...angle in + angle out works in theory...but speed, spin and table conditions must be considered as well.
Steve

Chopstick
09-27-2006, 08:47 AM
I have always found the explanation of mirror shots hard to understand. Here's something I did to help me get it. Go get a small ring or bottle top or something like that. Put it on the rail and put a ball on top of it. Put it right beside the ball you are kicking at. Measure from the edge of the ball you are kicking at to the rail, then measure from the same point on the rail to the edge of the ball sitting on top of the rail. Make the distance the same. I used chalk to measure when I started. Now take the cue ball and shoot straight at the ball sitting on top of the rail.

I always had trouble visualizing my ghost ball correctly. This gives you a real target to train your eyes with. Shoot this a few times and you won't need the ball on the rail anymore.

For other kick shots I recommend Grady Matthew's tape "Only Kicks".

Jal
09-27-2006, 10:11 AM
One problem with the mirror method is locating the "surface" of the mirror. Pooltchr described it at the rail gutter, which is almost half a ball out from the nose of the cushion. Chopstick has it at the nose. But the fact is that it can be somewhere in-between. For softer shots at shallow incident angles, it's nearer the gutter. For harder shots at steeper angles, it's nearer the cushion's edge, I believe.

I say "believe" because this is based on a cushion contact time of about 0.013 seconds, which was taken off the Austrian high speed video by me. This is for a carom table and I can only assume the frame rate for this portion of the video was 2000/sec. It still applies if the actual time for a pool table is around 0.01 seconds or so, but not so much if it's substantially less than this. I haven't found any source which gives this number. Maybe someone here knows a more reliable figure.

The error in estimating the position of the mirror gets applied twice: first in measuring the distance of the real object ball from the mirror plane; then again from the mirror plane to the virtual object ball. In other words, it gets doubled. So even with angle in = angle out (and there is a spin state which comes very close to achieving this) there remains the problem of figuring out just where the imaginary ball should be located. If the above contact time is approximately correct, you could easily be off by nearly a ball's width.

Jim

Bob_Jewett
09-27-2006, 01:46 PM
<blockquote><font class="small">Quote Jal:</font><hr>... I say "believe" because this is based on a cushion contact time of about 0.013 seconds, which was taken off the Austrian high speed video by me. This is for a carom table and I can only assume the frame rate for this portion of the video was 2000/sec. It still applies if the actual time for a pool table is around 0.01 seconds or so, but not so much if it's substantially less than this. I haven't found any source which gives this number. Maybe someone here knows a more reliable figure.... <hr /></blockquote>
The time is shown on the Jacksonville Project video, which is now available as a DVD. I didn't measure the time in the cushion, but the distance into the cushion for a fairly hard shot was easily measurable at 10 mm. If I remember, I'll look at that section again. The whole tape needs to be re-edited for DVD -- maybe when I have some spare time.

Jal
09-27-2006, 03:46 PM
<blockquote><font class="small">Quote Bob_Jewett:</font><hr>The time is shown on the Jacksonville Project video, which is now available as a DVD. I didn't measure the time in the cushion, but the distance into the cushion for a fairly hard shot was easily measurable at 10 mm. If I remember, I'll look at that section again. The whole tape needs to be re-edited for DVD -- maybe when I have some spare time. <hr /></blockquote>That would be great. It's surprising that this number has apparently never come up in all of the discussions.

I don't mean to snub Dr. Dave. I have many of his videos downloaded for various reasons. But I haven't come across one where I thought I could get this particular measurement. It's probably there, somewhere.

Jim

dr_dave
09-28-2006, 07:38 AM
<blockquote><font class="small">Quote Jal:</font><hr>I don't mean to snub Dr. Dave. I have many of his videos downloaded for various reasons. But I haven't come across one where I thought I could get this particular measurement. It's probably there, somewhere.<hr /></blockquote>
Jim,

FYI, HSV 6.1 (http://www.engr.colostate.edu/~dga/pool/high_speed_videos/HSV6-1.htm) shows an example of maximum cushion deformation for a very high speed kick shot.

Regards,
Dave

dr_dave
09-28-2006, 07:43 AM
FYI, "Banking with the Beard" by Freddy Bentivegna is an excellent resource for kick and bank aiming techniques and effects. I humbly suggest that Chapter 6 in "The Illustrated Principles of Pool and Billiards (http://www.engr.colostate.edu/~dga/pool/book/book_description.html)" is also fairly good.

Regards,
Dave

<blockquote><font class="small">Quote DeuceBigalo:</font><hr> I've never really put any real thought in to the kick shot, especially when you're going for a 'kick and pot' but I recently watched a guy playing 9 Ball, using his fingers to measure distances between cushion and OB and then mirroring that same distance on the edge of the table beyond the cushion. My question here is, what are the tricks of the trade to calculate accurate kick shots when you're trying to kick and pot?

Simple, to the point answers would be highly appreciated. <hr /></blockquote>

wolfdancer
09-28-2006, 08:01 AM
The book was money well spent.

dr_dave
09-28-2006, 08:09 AM
<blockquote><font class="small">Quote wolfdancer:</font><hr> The book was money well spent.<hr /></blockquote>
I assume you meant my book /ccboard/images/graemlins/tongue.gif.

Actually, I know you were probably referring to Freddy's book. I agree! It is money well spent.

Dave

wolfdancer
09-28-2006, 08:44 AM
I didn't read your book, but do have your DVD....also money well spent !!!
I haven't read my way all through Freddy's book, but just the part about compensating for acquired english paid for acquiring the book

dr_dave
09-28-2006, 08:51 AM
<blockquote><font class="small">Quote wolfdancer:</font><hr>I didn't read your book<hr /></blockquote>Shame on you. /ccboard/images/graemlins/confused.gif

<blockquote><font class="small">Quote wolfdancer:</font><hr>but do have your DVD....also money well spent !!!<hr /></blockquote>Thank you. But the book is so much better than the DVD.

<blockquote><font class="small">Quote wolfdancer:</font><hr>I haven't read my way all through Freddy's book<hr /></blockquote>Shame on you. /ccboard/images/graemlins/confused.gif

<blockquote><font class="small">Quote wolfdancer:</font><hr>but just the part about compensating for acquired english paid for acquiring the book<hr /></blockquote>Read the rest of the book to really get your money's worth.

Regards,
Dave

Jal
09-28-2006, 02:21 PM
<blockquote><font class="small">Quote dr_dave:</font><hr>...FYI, HSV 6.1 (http://www.engr.colostate.edu/~dga/pool/high_speed_videos/HSV6-1.htm) shows an example of maximum cushion deformation for a very high speed kick shot.<hr /></blockquote>Hi Dr. Dave,

As a matter of fact I did come across it and measured the duration of contact at 6 frames (starting between 12,13 and ending between 18,19). If the rate is 500/sec, that would correspond nicely with the number arrived at from the Austrian video of about .013 sec.

In your video there appears to be a duplicate frame following each "normal" one (where movement is indicated). However, right in the middle of impact some movement is shown between three successive frames. I wonder if we can assume a missing duplicate frame? (Not that it makes much difference.)

It's been a while since you recorded these but can you provide some info on the timing?

Jim

dr_dave
09-28-2006, 02:53 PM
<blockquote><font class="small">Quote Jal:</font><hr> <blockquote><font class="small">Quote dr_dave:</font><hr>...FYI, HSV 6.1 (http://www.engr.colostate.edu/~dga/pool/high_speed_videos/HSV6-1.htm) shows an example of maximum cushion deformation for a very high speed kick shot.<hr /></blockquote>Hi Dr. Dave,

As a matter of fact I did come across it and measured the duration of contact at 6 frames (starting between 12,13 and ending between 18,19). If the rate is 500/sec, that would correspond nicely with the number arrived at from the Austrian video of about .013 sec.

In your video there appears to be a duplicate frame following each "normal" one (where movement is indicated). However, right in the middle of impact some movement is shown between three successive frames. I wonder if we can assume a missing duplicate frame? (Not that it makes much difference.)

It's been a while since you recorded these but can you provide some info on the timing?<hr /></blockquote>
HSV 6.1 (http://www.engr.colostate.edu/~dga/pool/high_speed_videos/HSV6-1.htm) is more useful for visualizing and measuring the amount of cushion deformation. However, concerning timing, I'm pretty sure the clip was shot at 1000 frames/sec (that's the rate I use for almost all of the HSV clips) and played back at some random speed (not necessarily an integer multiple of actual speed). For every frame where there is motion (or change), 0.001 seconds have elapsed since the previous frame with motion (or change). You can throw out any repeated frames resulting from the playback speed.

I hope that helps,
Dave

Jal
09-28-2006, 05:21 PM
<blockquote><font class="small">Quote dr_dave:</font><hr>HSV 6.1 (http://www.engr.colostate.edu/~dga/pool/high_speed_videos/HSV6-1.htm) is more useful for visualizing and measuring the amount of cushion deformation. However, concerning timing, I'm pretty sure the clip was shot at 1000 frames/sec (that's the rate I use for almost all of the HSV clips) and played back at some random speed (not necessarily an integer multiple of actual speed). For every frame where there is motion (or change), 0.001 seconds have elapsed since the previous frame with motion (or change). You can throw out any repeated frames resulting from the playback speed.

I hope that helps,
Dave <hr /></blockquote>Yes it does. Thank you. It yields a contact period of about 4 msec since there are 3 full frame intervals and 2 half intervals (between 12,13 and 18,19). If you assume 1000 frames/sec, with a cueball speed of 6 (movement) frames per ball diameter, you get 21 mph, which conforms well with the purpose of the video.

The Austrian video doesn't have filler frames and I took the measurements from two different segments at the beginning and end of the film. The latter is with the ball sliding on the cloth as opposed to flying in the air. Both show contact over 26 frames, within a couple of frames. If the rate was in fact 2000/sec, then the cushions on a carom table must be very different. Or, the frame rate would have to have been around 6500/sec for a .004 sec duration.

A figure of 4 msec is not a bad number. For one thing, if you treat the cushion as a damped oscillator which compresses 1/2 inch from a high speed impact (20 mph or so), I think you do get 3-4 msec for the period. But more to the point of the thread, the location of the mirror plane will not be affected by the finite contact time nearly so much. It will remain much closer to the rail gutter, at least from this consideration alone.

Any thoughts on resolving the discrepancy?

Jim

DeuceBigalo
09-28-2006, 07:22 PM
-----------------------------------------------------------
Simple, to the point answers would be highly appreciated.
-----------------------------------------------------------

Thanks for the responses guys. Particular thanks must go out to you all for keeping things really simple and easy to understand......NOT!

My head hurts!!!!!

dr_dave
09-28-2006, 08:01 PM
<blockquote><font class="small">Quote Jal:</font><hr>the location of the mirror plane will not be affected by the finite contact time nearly so much. It will remain much closer to the rail gutter, at least from this consideration alone.<hr /></blockquote>
Agreed. The rail groove (gutter) is a sufficiently accurate enough for kick and bank aiming systems. The effects of cut angle, English, speed, and roll are all far more significant than the shift in the rebound point due to rail deformation.

Regards,
Dave

dr_dave
09-28-2006, 08:08 PM
<blockquote><font class="small">Quote DeuceBigalo:</font><hr> -----------------------------------------------------------
Simple, to the point answers would be highly appreciated.
-----------------------------------------------------------

Thanks for the responses guys. Particular thanks must go out to you all for keeping things really simple and easy to understand......NOT!

My head hurts!!!!! <hr /></blockquote>
Your point is well taken and appropriate. Sometimes the geeks on the forum (myself included) sometimes get into too much detail. Sometimes the details are important, but I agree with you that in this case, the details are not very interesting or useful to the typical pool player.

Thank you for your candid remarks,
Dave

rukiddingme
09-28-2006, 11:03 PM
Your point is well taken and appropriate. Sometimes the geeks on the forum (myself included) sometimes get into too much detail. Sometimes the details are important, but I agree with you that in this case, the details are not very interesting or useful to the typical pool player.

Thank you for your candid remarks,
Dave <hr /></blockquote>

That Dave is probably one of the best posts you have made on this forum ever...most often you get way over involved in details that are not very interesting or useful to most players...note that it is not limited to this subject matter...but quite a few subject matters. Not everyone is an engineer/mathemetician nor do they care to be.
A lot to be said for the term "keep it simple ____".
ruk

DeuceBigalo
09-29-2006, 12:36 AM
Keep up the good work Dr. Dave. You've probably forgotten more about the game of pool they I will ever know and for that you have my respect.

Thanks.

pooltchr
09-29-2006, 06:04 AM
Everyone learns in different ways, and many have different requirements of what it takes to understand new concepts. Dr. Dave provides detailed analysis for those who need that. Others provide simple explanations of concepts for those who think conceptually. There is a place for both.
Steve

dr_dave
09-29-2006, 06:21 AM
<blockquote><font class="small">Quote DeuceBigalo:</font><hr> Keep up the good work Dr. Dave. You've probably forgotten more about the game of pool they I will ever know and for that you have my respect.

Thanks. <hr /></blockquote>
At first, I wasn't sure if this was a compliment or not. I think I know what your mean now. Thank you.

Dave

Fran Crimi
09-29-2006, 06:22 AM
<blockquote><font class="small">Quote pooltchr:</font><hr> Everyone learns in different ways, and many have different requirements of what it takes to understand new concepts. Dr. Dave provides detailed analysis for those who need that. Others provide simple explanations of concepts for those who think conceptually. There is a place for both.
Steve <hr /></blockquote>


[ QUOTE ]
Quote Jim....A figure of 4 msec is not a bad number. For one thing, if you treat the cushion as a damped oscillator which compresses 1/2 inch from a high speed impact (20 mph or so), I think you do get 3-4 msec for the period..... <hr /></blockquote>

Steve, I've gotta tell you...I can't think of any of my students (and I bet I've taught a thousand) or any of the pros I know who has a reqirement to understand pool in the above quoted terms. I can think of one or two people I know who might find it of interest, but requirement? Nah. No way.

Fran

dr_dave
09-29-2006, 06:40 AM
<blockquote><font class="small">Quote Fran Crimi:</font><hr> <blockquote><font class="small">Quote pooltchr:</font><hr> Everyone learns in different ways, and many have different requirements of what it takes to understand new concepts. Dr. Dave provides detailed analysis for those who need that. Others provide simple explanations of concepts for those who think conceptually. There is a place for both.
Steve <hr /></blockquote>
<blockquote><font class="small">Quote Jal:</font><hr>A figure of 4 msec is not a bad number. For one thing, if you treat the cushion as a damped oscillator which compresses 1/2 inch from a high speed impact (20 mph or so), I think you do get 3-4 msec for the period..... <hr /></blockquote>

Steve, I've gotta tell you...I can't think of any of my students (and I bet I've taught a thousand) or any of the pros I know who has a reqirement to understand pool in the above quoted terms. I can think of one or two people I know who might find it of interest, but requirement? Nah. No way.<hr /></blockquote>

Fran,

I agree with you 100%. That quote from Jim (Jal) was not "Jim the pool player" talking, it was "Jim the physicist" trying to explain detailed physical phenomenon. The purpose wasn't to help people play pool better. It was to better understand the physics.

I also like to understand the physics at a detailed level, as demonstrated in my online technical proofs (TPs) (http://www.engr.colostate.edu/~dga/pool/technical_proofs/index.html). This is usually only for physical understanding, but it sometimes provides insight useful at the table. In my monthly articles (http://www.engr.colostate.edu/~dga/pool/bd_articles/index.html), I try to limit my topics to things useful at the table (although, they are often backed up by theoretical and experimental results ... but again, the TPs are only for the people with interest in that sort of thing). I hope people don't think that I'm just a physics nerd. I pride myself in being able to illustrate and explain things in a way anybody can understand (when I am writing for a general audience). I also take pride in the 30 degree rule and V-sign technique, the trisect method, complete explanations of throw, and other tools and insight I've provided to help at the table.

Regards,
Dave

Fran Crimi
09-29-2006, 06:47 AM
Well stated, Dave. And above all, coming from my perspective, this is the key:

[ QUOTE ]
Quote Dave: I pride myself in being able to illustrate and explain things in a way anybody can understand (when I am writing for a general audience). <hr /></blockquote>

Fran

pooltchr
09-29-2006, 01:24 PM
Fran,
I would agree with you as far as the degree of detail outlined in the post. My point is that students are individuals who learn in different ways. Some need to see it, some to hear it, some to read it, some to do it themselves, and some to understand every aspect of why something happens.

No, I have never had a student who needed to understand the physics to that degree. If I did, I would probably refer them to Dave since I know I could never explain it in the detail he is able to.

Ask Randy how long and what is the process Carl goes through before he understands any new concept presented. Carl is a great instructor, but you can't just tell him something...he has to disect it and put it back together before it clicks. That is neither good nor bad...it's just Carl.

Steve

Jal
09-29-2006, 04:26 PM
<blockquote><font class="small">Quote DeuceBigalo:</font><hr> -----------------------------------------------------------
Simple, to the point answers would be highly appreciated.
-----------------------------------------------------------

Thanks for the responses guys. Particular thanks must go out to you all for keeping things really simple and easy to understand......NOT!

My head hurts!!!!! <hr /></blockquote>You gave the pre-condition that the answer must be simple. What was the justification for that? Maybe you have some insight that you'd be willing to share with us.

I also assumed that another condition was that the answer be correct, or at least reasonably accurate for the purpose at hand. Maybe I was wrong, but it's a premise that I normally take for granted. Sometimes trying to meet both criteria is not in the cards and you have to pick and choose.

Since your query involves not merely hitting the object ball, but sinking it, a different level of precision is required. I was trying to point out an aspect of the standard mirror system that makes it problematical at this level. There's even a potentially more severe issue with it, which I dare not mention. It depends on what method you use to achieve angle in = angle out.

If you want to believe the simple method works, that is, using the rail track for the mirror location, fine with me. In fact, I think it will work under some circumstances. Just how broad these circumstances are, I don't know. It's a difficult problem which is well beyond my current understanding. I'm pretty sure that it won't work (not even close) under other circumstances and that these may be fairly broad.

But apparently this is not the place to discuss these matters in an attempt to sort some of them out. I guess the best thing is to buy the right cue with the right tip and everything else will take care of itself.

Jim

cheesemouse
09-29-2006, 04:35 PM
here's something simple.....go buy a freaking mirror...:)

Bob_Jewett
09-29-2006, 04:35 PM
<blockquote><font class="small">Quote Jal:</font><hr>... simple method works, that is, using the rail track for the mirror location, ... <hr /></blockquote>
To further muddy the waters, I'll observe that many people who think they are using some kind of mirror system use the diamonds as the "refection" point on kick shots. That's only a few inches from the real reflection point. The fact that they are so far off on this basic aspect doesn't keep them from making some shots from some positions, with practice. Remarkably, they don't seem to realize that there might be some error in using the diamonds this way.

Sorry, Deuce, while there are simple kicking systems and accurate kicking systems, I think their intersection is the null set.

dr_dave
09-29-2006, 05:43 PM
<blockquote><font class="small">Quote cheesemouse:</font><hr> here's something simple.....go buy a freaking mirror...:) <hr /></blockquote>
Just be sure to move the mirror before you shoot if it is in front of the cushion. /ccboard/images/graemlins/wink.gif

Jal
09-29-2006, 05:47 PM
<blockquote><font class="small">Quote dr_dave:</font><hr> <blockquote><font class="small">Quote Jal:</font><hr>the location of the mirror plane will not be affected by the finite contact time nearly so much. It will remain much closer to the rail gutter, at least from this consideration alone.<hr /></blockquote>
Agreed. The rail groove (gutter) is a sufficiently accurate enough for kick and bank aiming systems. The effects of cut angle, English, speed, and roll are all far more significant than the shift in the rebound point due to rail deformation.

Regards,
Dave <hr /></blockquote>I can't fully agree with you here. For starters, has the contact period been resolved? The two videos cited show very different values.

Secondly, I'm not sure if by "rail deformation", you're talking about the same thing that I am. The effect I'm refering to is due to the finite contact time. Without considering anything else, any nonzero value for it pushes the mirror plane away from the rail track and toward the cushion. If you're trying to pot a ball, not much error can be tolerated here.

There is a potentially even larger effect which stems from the subsequent masse action of the ball after it leaves the cushion. This may work against you or for you, depending on what kind of spin you use to produce the mirror-like rebound. If you make an assumption about how much of a certain spin component is removed by the cushion (which may not be justified) this can move the mirror plane well into the cushion itself.

But it's hard to be sure because of the problem of the distribution of forces as it partially envelops the ball. How much torque does it exert compared to bed friction? I really hope that someday you might give this your attention.

Jim

P.S. I don't consider myself a geek. Even if I wanted to be one, they wouldn't have me.

dr_dave
09-30-2006, 02:29 PM
<blockquote><font class="small">Quote Jal:</font><hr> <blockquote><font class="small">Quote dr_dave:</font><hr> <blockquote><font class="small">Quote Jal:</font><hr>the location of the mirror plane will not be affected by the finite contact time nearly so much. It will remain much closer to the rail gutter, at least from this consideration alone.<hr /></blockquote>
Agreed. The rail groove (gutter) is a sufficiently accurate enough for kick and bank aiming systems. The effects of cut angle, English, speed, and roll are all far more significant than the shift in the rebound point due to rail deformation.

Regards,
Dave <hr /></blockquote>I can't fully agree with you here. For starters, has the contact period been resolved? The two videos cited show very different values.<hr /></blockquote>The video I cited is not a good example to use because the ball was hit at a very high speed. I would think that at more typical kick shot speeds, the amount of deformation (and the amount of shift into the cushion) would be much, much smaller (although, I don't have video to show this ... I'll add it to my list). Also, I don't think contact time is as important as the amount of shift.

<blockquote><font class="small">Quote Jal:</font><hr>Secondly, I'm not sure if by "rail deformation", you're talking about the same thing that I am. The effect I'm refering to is due to the finite contact time. Without considering anything else, any nonzero value for it pushes the mirror plane away from the rail track and toward the cushion. If you're trying to pot a ball, not much error can be tolerated here.<hr /></blockquote>
I understand the mirror plane shift. I just think it is a very small effect compared to the effects I list above.

<blockquote><font class="small">Quote Jal:</font><hr>There is a potentially even larger effect which stems from the subsequent masse action of the ball after it leaves the cushion. This may work against you or for you, depending on what kind of spin you use to produce the mirror-like rebound. If you make an assumption about how much of a certain spin component is removed by the cushion (which may not be justified) this can move the mirror plane well into the cushion itself.<hr /></blockquote>I don't see this as "moving the mirror plane." I see it as ball curve after rebound. But we agree that an adjustment must be made in aiming.

<blockquote><font class="small">Quote Jal:</font><hr>But it's hard to be sure because of the problem of the distribution of forces as it partially envelops the ball. How much torque does it exert compared to bed friction? I really hope that someday you might give this your attention.<hr /></blockquote>Funny you should mention that, because I have been planning to do a series of experiments and develop equations to attempt to describe and model all of the effects. I was hoping to write a series of articles about the results after I finish my throw series.

<blockquote><font class="small">Quote Jal:</font><hr>P.S. I don't consider myself a geek. Even if I wanted to be one, they wouldn't have me. <hr /></blockquote>I don't consider myself a geek either, but I think the non-physics people out there probably see us that way some times.

Regards,
Dave

cushioncrawler
09-30-2006, 05:15 PM
<blockquote><font class="small">Quote dr_dave:</font><hr> ..... Funny you should mention that, because I have been planning to do a series of experiments and develop equations to attempt to describe and model all of the effects. I was hoping to write a series of articles about the results after I finish my throw series.... <hr /></blockquote>
Hi Dr Dave -- this (cushion rebound angle) iz going to be a big job -- i made a simple computer program modeling most of the stuff that happens, but it needz more work -- one thing that it needz to show iz that check-side actually decreases the rebound angle when the angle of attack iz less than say 40dg -- but a 9' table iz different to a 12' table -- but a good program shood be able to model both anyhow.

I thort that this check side stuff (decreased angle) could be explained by the frictional effects -- but i found that it couldnt -- that means that there iz a sort of additional hysteresis coming into play (ie for the e for the rubber) -- and it haznt got anything to do with that silly notion of the cloth&amp;rubber bunching-up ahead of the ball -- at this point this little project stalled -- but i will return to it in about 12 months.

I doubt that u will get any joy from big equations -- i kept well away -- all of my stuff involves iterations of simple stuff like -- F=ma and -- Ft=delta mv and -- Fd=delta mv^2/2. madMac.

dr_dave
10-01-2006, 07:48 AM
madMac,

Thanks for the input. I do expect the modeling to be a big challenge. There's a lot of physics going on there, and everything varies with speed.

I hope I can come up with something interesting and useful.

Catch you later,
Dave

<blockquote><font class="small">Quote cushioncrawler:</font><hr> <blockquote><font class="small">Quote dr_dave:</font><hr> ..... Funny you should mention that, because I have been planning to do a series of experiments and develop equations to attempt to describe and model all of the effects. I was hoping to write a series of articles about the results after I finish my throw series.... <hr /></blockquote>
Hi Dr Dave -- this (cushion rebound angle) iz going to be a big job -- i made a simple computer program modeling most of the stuff that happens, but it needz more work -- one thing that it needz to show iz that check-side actually decreases the rebound angle when the angle of attack iz less than say 40dg -- but a 9' table iz different to a 12' table -- but a good program shood be able to model both anyhow.

I thort that this check side stuff (decreased angle) could be explained by the frictional effects -- but i found that it couldnt -- that means that there iz a sort of additional hysteresis coming into play (ie for the e for the rubber) -- and it haznt got anything to do with that silly notion of the cloth&amp;rubber bunching-up ahead of the ball -- at this point this little project stalled -- but i will return to it in about 12 months.

I doubt that u will get any joy from big equations -- i kept well away -- all of my stuff involves iterations of simple stuff like -- F=ma and -- Ft=delta mv and -- Fd=delta mv^2/2. madMac. <hr /></blockquote>

Bob_Jewett
10-01-2006, 09:37 PM
<blockquote><font class="small">Quote Jal:</font><hr> ... has the contact period been resolved? The two videos cited show very different values.

Secondly, I'm not sure if by "rail deformation", you're talking about the same thing that I am. The effect I'm refering to is due to the finite contact time. Without considering anything else, any nonzero value for it pushes the mirror plane away from the rail track and toward the cushion. ... <hr /></blockquote>
I looked at the Jacksonville Project DVD this morning. For those who have a copy, the shot I looked at was on 11/04/98 at 19:34:56. It was shot at 2000 FPS. A ball was frozen on the cushion, and the cue ball was shot straight into it. The inbound cue ball speed was 7.3 meters/second. The time of the ball in the cushion was 6.5 milliseconds. The max penetration of the ball into the cushion was about 17mm. The object ball kissed the cue ball back as it came out of the cushion, and the speed of the cue ball was 4.9 meters/second as it moved away from the object ball (both speeds measured in about the closest 50 mm of travel against the 10mm grid that was positioned behind the balls.

There were not a lot of such shots on the tape/DVD, and there were no similar soft shots, but the time in the cushion is expected to be nearly independent of ball speed. There were some shots into the cushion from an oblique angle, but they didn't have the graph paper positioned to allow easy measurements.

dave
10-02-2006, 08:05 AM
"Some need to see it, some to hear it, some to read it, some to do it themselves, and some to understand every aspect of why something happens."

I wanted to compliment you on your thoughtful response. I'm an educator and one of the things we are taught is the importance of understanding that there are different types of learners: visual, auditory, kinesthetic, numeric etc. To assume that one learning style is "better" or that one type is "smarter" than another is a terrible mistake and a great disservice to a great many people. Your post recognizes this point and the fact that certain people respond more effectively to different ways of presenting information and experience.

dr_dave
10-02-2006, 08:37 AM
<blockquote><font class="small">Quote Bob_Jewett:</font><hr>There were some shots into the cushion from an oblique angle, but they didn't have the graph paper positioned to allow easy measurements.<hr /></blockquote>
Bob,

FYI, there are inexpensive motion analysis software packages available (e.g., VideoPoint) that allow you to take digital measurements off videos (i.e., the graph paper is not required). The packages can even do kinematics calculations for you.

Regards,
Dave

Bob_Jewett
10-02-2006, 10:31 AM
<blockquote><font class="small">Quote dr_dave:</font><hr> ... Bob,

FYI, there are inexpensive motion analysis software packages available (e.g., VideoPoint) that allow you to take digital measurements off videos (i.e., the graph paper is not required). The packages can even do kinematics calculations for you.... <hr /></blockquote>
Do they somehow establish a 3D model of the scene? I can imagine that a rotating black/red Elephant Ball object ball might confuse it.

dr_dave
10-02-2006, 10:38 AM
<blockquote><font class="small">Quote Bob_Jewett:</font><hr> <blockquote><font class="small">Quote dr_dave:</font><hr> ... Bob,

FYI, there are inexpensive motion analysis software packages available (e.g., VideoPoint) that allow you to take digital measurements off videos (i.e., the graph paper is not required). The packages can even do kinematics calculations for you.... <hr /></blockquote>
Do they somehow establish a 3D model of the scene? I can imagine that a rotating black/red Elephant Ball object ball might confuse it. <hr /></blockquote>
Bob,

The cheap packages aren't that sophisticated. They just assume the camera is far enough away from the scene (e.g., with a telephoto lens) so 3D perspective effects are small. Also, the user must manually click key points in each video frame (e.g., the center of the ball, and two points for an angle). Unfortunately, with only one camera view, you can measure spin only about one axis.

Dave

Jal
10-03-2006, 11:52 PM
<blockquote><font class="small">Quote dr_dave:</font><hr> <blockquote><font class="small">Quote Jal:</font><hr> <blockquote><font class="small">Quote dr_dave:</font><hr> <blockquote><font class="small">Quote Jal:</font><hr>the location of the mirror plane will not be affected by the finite contact time nearly so much. It will remain much closer to the rail gutter, at least from this consideration alone.<hr /></blockquote>
Agreed. The rail groove (gutter) is a sufficiently accurate enough for kick and bank aiming systems. The effects of cut angle, English, speed, and roll are all far more significant than the shift in the rebound point due to rail deformation.

Regards,
Dave <hr /></blockquote>I can't fully agree with you here. For starters, has the contact period been resolved? The two videos cited show very different values.<hr /></blockquote>The video I cited is not a good example to use because the ball was hit at a very high speed. I would think that at more typical kick shot speeds, the amount of deformation (and the amount of shift into the cushion) would be much, much smaller (although, I don't have video to show this ... I'll add it to my list). Also, I don't think contact time is as important as the amount of shift.<hr /></blockquote>Dr. Dave, what I mean by the finite contact time altering the location of the mirror plane is this:

http://ww2.netnitco.net/users/gtech/MirrorSurface.jpg

It's due to the speed component of the ball parallel to the cushion. The effect is exaggerated in the diagram to make the geometry less crowded.

<blockquote><font class="small">Quote dr_dave:</font><hr><blockquote><font class="small">Quote Jal:</font><hr>There is a potentially even larger effect which stems from the subsequent masse action of the ball after it leaves the cushion. This may work against you or for you, depending on what kind of spin you use to produce the mirror-like rebound. If you make an assumption about how much of a certain spin component is removed by the cushion (which may not be justified) this can move the mirror plane well into the cushion itself.<hr /></blockquote>I don't see this as "moving the mirror plane." I see it as ball curve after rebound. But we agree that an adjustment must be made in aiming.<hr /></blockquote>I think that seeing it as a shift of the mirror plane is a potentially very useful way to look at it. But it all depends, of course, on knowing where the new one is located. Here are expressions for finding it, at least for two limiting cases: one where the spin component Wb (see the diagram) is rubbed out during the collision, and the other where it survives intact.

http://ww2.netnitco.net/users/gtech/MirrorSurface3.jpg

The value for Xm can then be added to Xt from the previous diagram and the total shift is:

d = (1/2)(Xt + Xm)cot(theta)

Note that this is for the case where the mirror reflection is achieved by stunning into the cushion with some fraction of natural running english in order to slow the cueball down in the parallel direction by just the right amount. Here, the mirror shift adds positively to the shift from the finite contact time. A correction needs to made for bed friction, but given the gross uncertainty over the spin component in question, it's hardly worth it at this time. The idea was to get an approximate value for the range of expected shifts.

Putting the two effects together with the above equation yields the following numbers for a cueball traveling at 8 mph and with the assumed values for the indicated constants:

http://ww2.netnitco.net/users/gtech/MirrorSurface4.jpg

If the math is correct, and I'm reasonably sure it is, you get a major shift of the mirror plane at relatively steep approach angles regardless of what happens to Wb, as long as it isn't amplified by bed friction.

Partially rolling the cueball into the cushion to obtain the mirror reflection will probably produce less shift since the curve is in the other direction and the contribution from the finite contact time will partly, or maybe almost totally, cancel it. I think that maybe this is what you've had in mind, while I've been discussing (implicitly) the other situation.

<blockquote><font class="small">Quote dr_dave:</font><hr><blockquote><font class="small">Quote Jal:</font><hr>But it's hard to be sure because of the problem of the distribution of forces as it partially envelops the ball. How much torque does it exert compared to bed friction? I really hope that someday you might give this your attention.<hr /></blockquote>Funny you should mention that, because I have been planning to do a series of experiments and develop equations to attempt to describe and model all of the effects. I was hoping to write a series of articles about the results after I finish my throw series.<hr /></blockquote>This is what I've been hoping to hear. Another wish is that I'll understand it. Like Cushioncrawler, I also have a program which tries to model it. I'm sure you want to do this with the sophisticated mathematical tools you have at hand, but I do have (oh boy, here he goes) an analytical result for the evolution of the bed friction over a time interval in which the spin doesn't change too much. If you get hung up on this.../ccboard/images/graemlins/grin.gif/ccboard/images/graemlins/grin.gif/ccboard/images/graemlins/grin.gif

<blockquote><font class="small">Quote dr_dave:</font><hr><blockquote><font class="small">Quote Jal:</font><hr>P.S. I don't consider myself a geek. Even if I wanted to be one, they wouldn't have me. <hr /></blockquote>I don't consider myself a geek either, but I think the non-physics people out there probably see us that way some times.<hr /></blockquote>Another poster suggested that you get too technical at times. But it seems to me that you (and Bob Jewett) go to great lengths to describe things in the vernacular. I don't see the basis for this charge.

Jim

dr_dave
10-04-2006, 08:53 AM
<blockquote><font class="small">Quote Jal:</font><hr>Dr. Dave, what I mean by the finite contact time altering the location of the mirror plane is this:

http://ww2.netnitco.net/users/gtech/MirrorSurface.jpg

It's due to the speed component of the ball parallel to the cushion. The effect is exaggerated in the diagram to make the geometry less crowded.<hr /></blockquote>
That's a nice diagram. Where did you get the approximate relationship between contact time (T) and horizontal shift (Xt)? Again, to me, the contact time (T) isn't important. What's important is the amount of shift (Xt). Have you measured Xt for a range of typical kick and bank speeds and angles? I would expect it to be very small. Again, I think the effects of English, vertical plane spin, friction vs. speed and angle, COR vs. speed and angle, rail throwback vs. speed and angle, and post-rebound masse curve are much more significant than the small amount of horizontal shift during impact (but I'm not sure without experimental results).

Regards,
Dave

dr_dave
10-04-2006, 09:10 AM
<blockquote><font class="small">Quote Jal:</font><hr><blockquote><font class="small">Quote dr_dave:</font><hr><blockquote><font class="small">Quote Jal:</font><hr>P.S. I don't consider myself a geek. Even if I wanted to be one, they wouldn't have me.<hr /></blockquote>I don't consider myself a geek either, but I think the non-physics people out there probably see us that way some times.<hr /></blockquote>Another poster suggested that you get too technical at times. But it seems to me that you (and Bob Jewett) go to great lengths to describe things in the vernacular. I don't see the basis for this charge.<hr /></blockquote>
Beauty is in the eye of the beholder. Some people like my TP's (but others think they are worthless), some like my well-illustrated book and instructional articles, some like my online videos and DVD. Very few like or relate to all of them, and I fully expect that. As others have pointed out, different learning styles and expectations are best reached by a variety of presentation approaches and material. To me, all of the approaches and levels of understanding are valuable and interesting.

Regards,
Dave

wolfdancer
10-04-2006, 10:50 AM
Dr. Dave, forgetting all the math and physics involved in measuring accurate kick shots.......Jimmy Reid's latest DVD's
"Almost everything I know" has some of his own well diagramed methods for putting a number on it(a rail point using a numbered diamond system). It's an "awesome" system, and he has a unique way of proving his systems mathmetically.
web page (http://www.freepoollessons.com/)
There is a lot of free info....I paid the nominal fee and joined the diamond club

dr_dave
10-04-2006, 11:42 AM
wolfdancer,

Thanks for the info. I haven't seen Jimmy's new DVD yet, but I would be surprised if it is a better kick/banking resource than Freddy's "Banking with the Beard."

Regards,
Dave

<blockquote><font class="small">Quote wolfdancer:</font><hr> Dr. Dave, forgetting all the math and physics involved in measuring accurate kick shots.......Jimmy Reid's latest DVD's
"Almost everything I know" has some of his own well diagramed methods for putting a number on it(a rail point using a numbered diamond system). It's an "awesome" system, and he has a unique way of proving his systems mathmetically.
web page (http://www.freepoollessons.com/)
There is a lot of free info....I paid the nominal fee and joined the diamond club <hr /></blockquote>

wolfdancer
10-04-2006, 12:34 PM
Dr. Dave, Freddy's banking system is based on angles and english adjustments....and it's a great book.
Jimmy has assigned a numbering system to the rails....for multi-cushion kick shots. I'd say,,,use Freddy for banks, Jimmy for kicks....and I like the way he "proves" his system works.

dr_dave
10-04-2006, 01:00 PM
Is Jimmy using the standard Diamond system (see TP 7-2 (http://www.engr.colostate.edu/~dga/pool/technical_proofs/TP_7-2.pdf)), or has he developed a new method?

Dave
<blockquote><font class="small">Quote wolfdancer:</font><hr> Dr. Dave, Freddy's banking system is based on angles and english adjustments....and it's a great book.
Jimmy has assigned a numbering system to the rails....for multi-cushion kick shots. I'd say,,,use Freddy for banks, Jimmy for kicks....and I like the way he "proves" his system works. <hr /></blockquote>

pooltchr
10-04-2006, 01:07 PM
<blockquote><font class="small">Quote DeuceBigalo:</font><hr> I've never really put any real thought in to the kick shot, especially when you're going for a 'kick and pot' but I recently watched a guy playing 9 Ball, using his fingers to measure distances between cushion and OB and then mirroring that same distance on the edge of the table beyond the cushion. My question here is, what are the tricks of the trade to calculate accurate kick shots when you're trying to kick and pot?

<font color="red"> Simple, to the point answers would be highly appreciated. </font color> <hr /></blockquote>

Just thought I would bring the thread starter back up to see how well we met his request...simple and to the point??? All he wanted to know was what someone was doing measuring from the ball to the rail to make a kick shot. Now I consider myself a fairly intelligent person, but y'all lost me way back there! /ccboard/images/graemlins/confused.gif
Steve

Bob_Jewett
10-04-2006, 01:15 PM
<blockquote><font class="small">Quote dr_dave:</font><hr> ... the standard Diamond system ... <hr /></blockquote>
There are many diamond systems, each of which seems to use its own numbering system. The "Corner-5" system is one that is often used to play 3-cushion shots in which the first rail contacted is a long rail, the cushions are contacted in order (each is adjacent to the previous), and running english is used on the cue ball.

Sometimes other systems have more or less descriptive names, such as "Plus 2" and "Opposite 3" but sometimes they are named idiosyncratically, such as "System Sid." "Corner-5" gets its name from the assignment of value 5 to the cue ball location when it is coming from the corner.

Most mirror systems assign unit values to the rail you are hitting starting with 0 at the rail you want to reach, and the same values are assigned to the rail parallel to the one you are going to contact (with 0 starting at the same "rail to be reached").

Even within one diamond system, there are various choices at to where a particular number is, such as at the diamond or on the rail groove by the diamond.

dr_dave
10-04-2006, 01:40 PM
Thanks Bob.

The system I illustrate in TP 7.2 (http://www.engr.colostate.edu/~dga/pool/technical_proofs/TP_7-2.pdf) is the "Corner-5" system.

Dave

<blockquote><font class="small">Quote Bob_Jewett:</font><hr> <blockquote><font class="small">Quote dr_dave:</font><hr> ... the standard Diamond system ... <hr /></blockquote>
There are many diamond systems, each of which seems to use its own numbering system. The "Corner-5" system is one that is often used to play 3-cushion shots in which the first rail contacted is a long rail, the cushions are contacted in order (each is adjacent to the previous), and running english is used on the cue ball.

Sometimes other systems have more or less descriptive names, such as "Plus 2" and "Opposite 3" but sometimes they are named idiosyncratically, such as "System Sid." "Corner-5" gets its name from the assignment of value 5 to the cue ball location when it is coming from the corner.

Most mirror systems assign unit values to the rail you are hitting starting with 0 at the rail you want to reach, and the same values are assigned to the rail parallel to the one you are going to contact (with 0 starting at the same "rail to be reached").

Even within one diamond system, there are various choices at to where a particular number is, such as at the diamond or on the rail groove by the diamond. <hr /></blockquote>

dr_dave
10-04-2006, 02:05 PM
<blockquote><font class="small">Quote pooltchr:</font><hr> <blockquote><font class="small">Quote DeuceBigalo:</font><hr> I've never really put any real thought in to the kick shot, especially when you're going for a 'kick and pot' but I recently watched a guy playing 9 Ball, using his fingers to measure distances between cushion and OB and then mirroring that same distance on the edge of the table beyond the cushion. My question here is, what are the tricks of the trade to calculate accurate kick shots when you're trying to kick and pot?

<font color="red"> Simple, to the point answers would be highly appreciated. </font color> <hr /></blockquote>

Just thought I would bring the thread starter back up to see how well we met his request...simple and to the point??? All he wanted to know was what someone was doing measuring from the ball to the rail to make a kick shot. Now I consider myself a fairly intelligent person, but y'all lost me way back there! /ccboard/images/graemlins/confused.gif
Steve <hr /></blockquote>

Good point. If he or others just want the basics, they can view the video demonstrations under "Chapter 6" here (http://www.engr.colostate.edu/~dga/pool/normal_videos/index.html).

Regards,
Dave

Jal
10-04-2006, 04:59 PM
<blockquote><font class="small">Quote dr_dave:</font><hr> <blockquote><font class="small">Quote Jal:</font><hr>Dr. Dave, what I mean by the finite contact time altering the location of the mirror plane is this:

http://ww2.netnitco.net/users/gtech/MirrorSurface.jpg

It's due to the speed component of the ball parallel to the cushion. The effect is exaggerated in the diagram to make the geometry less crowded.<hr /></blockquote>
That's a nice diagram. Where did you get the approximate relationship between contact time (T) and horizontal shift (Xt)?<hr /></blockquote>I took Xt to be the average parallel component of the ball's velocity times the contact period. The ball enters the cushion at some point and exits a little farther down. Nothing else need be considered, in my opinion, as far as this portion of the shift.

Without considering the small correction for the subsequent curve of the ball (and ignoring bed friction), in order to achieve a mirror reflection the ball's parallel speed component as it exits must be eVsin(theta), where e is the coefficient of restitution, which I took to be 0.8 (maybe you think it should be 0.7). So its average parallel speed component during impact is roughly 0.9Vsin(theta), 0.9 being the average of .8 and 1. Since the ball reaches the rolling state across the cushion at some point, this number should be biased more toward 0.8, on this consideration alone. However, the correction for the subsequent masse action biases it more toward 1, so all-in-all, I think 0.9 can't be very far off the actual value.

<blockquote><font class="small">Quote dr_dave:</font><hr>Again, to me, the contact time (T) isn't important. What's important is the amount of shift (Xt). Have you measured Xt for a range of typical kick and bank speeds and angles?<hr /></blockquote>I'm not sure what you mean here? That is, it's not clear why you don't agree that the amount of movement down the cushion isn't proportional to T? I haven't measured anything but see the discussion of throwback below.

<blockquote><font class="small">Quote dr_dave:</font><hr>I would expect it to be very small. Again, I think the effects of English, vertical plane spin, friction vs. speed and angle, COR vs. speed and angle, rail throwback vs. speed and angle, and post-rebound masse curve are much more significant than the small amount of horizontal shift during impact (but I'm not sure without experimental results).<hr /></blockquote>The equation for Xm in the second diagram does take several of these things into account. But it does assume a constant value for COR (0.8) and ignores bed friction. It's also, admittedly, a bit idealized in that the incoming spin axis is perfectly vertical, as opposed to the 3-5 degree tilt that you would normally expect. But it does account for the amount of incoming english necessary so that the ball, once it reaches natural roll after the collision, will be traveling at the mirrored angle. It also accounts for the changes to it during impact and the amount of masse action as a result, as well as the position of the ball where rolling begins.

As you say, the curve of the ball does contribute much more to the shift. For the four cases in the third diagram, the finite contact time contribution is 0.25"cos(theta). The masse contribution is, respectively, 1.4", 2.0", 0.6", and 0.75" times cos(theta). (Xt and Xm each have factor of sin(theta), which when multiplied by cot(theta) gives cos(theta).)

I'm assuming that no significant cushion throwback occurs, if by cushion throwback you mean a non-symmetric distribution of cushion compression forces. This is a bit iffy, but using Bob Jewett's 70% system as an indicator of the amount of shortening of the rebound angle of a ball stunned into the cushion (no english), I think the shortening can be almost totally, if not totally, accounted for by friction alone (bed and cushion), as per Mac's assertion. At least that's what my calculations show. If you already know that throwback occurs, or mean something else by it, please explain.

The assumption that COR is constant is based on measurements taken off of your videos. These were for balls kicked perpendicularly into the cushion at various speeds. The data was scattered a bit, but when averaged, seemed to show that it was the same for both slow and fast shots.

I also assume that there is no significant change in the coefficient of friction with speed, as per an article linked to at your site (although the article does indicate a significant drop at very slow surface speeds).

I don't have 100% confidence in any of the above stuff. They await your expertise and testing. If you have any more questions or criticisms, please fire away. The derivation of the equation for Xm is fairly longish and so I didn't include it in the diagram. If you're interested and want to save time rather than deriving it yourself, let me know. (As a professor, the equations generally go in the opposite direction. I hope you're not offended - just saying I'll send it along if desired.)

Jim

Jal
10-04-2006, 05:12 PM
<blockquote><font class="small">Quote Bob_Jewett:</font><hr> ... The inbound cue ball speed was 7.3 meters/second. The time of the ball in the cushion was 6.5 milliseconds. The max penetration of the ball into the cushion was about 17mm. The object ball kissed the cue ball back as it came out of the cushion, and the speed of the cue ball was 4.9 meters/second as it moved away from the object ball (both speeds measured in about the closest 50 mm of travel against the 10mm grid that was positioned behind the balls....<hr /></blockquote>Thanks for taking the time to figure this out - very much appreciated. In looking up something else, I came across a series of videos by Dr. Dave that also can be used to measure it. Here are the results (the 6-x are the video numbers):

6-1 4 msec
6-5 1
6-6 3.3
6-7 3.5
6-8 2
6-9 1.5
6-10 1.2
6-11 1
6-12 4

I only crudely estimated the intervals where the ball enters or exits the cushion between frames. At this point it seems that not much can be concluded. I suppose there must be dropped frames in some or all of the videos? If so, perhaps the 6.5 milliseconds is the right one?

Jim

Bob_Jewett
10-04-2006, 05:59 PM
<blockquote><font class="small">Quote Jal:</font><hr> ... Here are the results (the 6-x are the video numbers):

6-1 4 msec
6-5 1
6-6 3.3
6-7 3.5
6-8 2
6-9 1.5
6-10 1.2
6-11 1
6-12 4
... <hr /></blockquote>
Some of those numbers are down in the 1-2 millisecond region. That says that the cushion is as stiff as a cue tip, roughly. I think that's not true. Also, as I mentioned before, I think the contact time is nearly independent of cue ball speed (perhaps even for smaller angles?). If there is a time dependence, I would expect the time to be shorter for deeper penetration.

Jal
10-04-2006, 11:36 PM
<blockquote><font class="small">Quote Bob_Jewett:</font><hr>Some of those numbers are down in the 1-2 millisecond region. That says that the cushion is as stiff as a cue tip, roughly. I think that's not true. Also, as I mentioned before, I think the contact time is nearly independent of cue ball speed (perhaps even for smaller angles?). If there is a time dependence, I would expect the time to be shorter for deeper penetration. <hr /></blockquote>For what it's worth, I completely agree. If someone had said they measured it at a tenth of a second, I would have believed them. It's surprising to me that it is this small, apparently less than a hundreth of a second, but 1-2 msec is going too far.

I think I see why you say that it should be, if anything, smaller at larger deformations. Given the triangular cross-section, the stiffness should go up with increasing penetration. So the average stiffness should be larger for a faster ball or steeper approach angle.

It's not so clear (to me) why you don't see a major reduction in contact time, although I also believe you don't since the situation is somewhat like Hertz Law for the collision of spheres.

Jim

wolfdancer
10-05-2006, 12:14 AM
Jimmy Reid has actually developed his own systems, but they are based on a mirror system.

dr_dave
10-05-2006, 06:40 AM
<blockquote><font class="small">Quote Jal:</font><hr>The derivation of the equation for Xm is fairly longish and so I didn't include it in the diagram. If you're interested and want to save time rather than deriving it yourself, let me know.<hr /></blockquote>

Thanks for all of the info and for the offer to send me more. First, I want to do a complete set of experiments (with regular and high-speed video) to capture/identify all of the important physics. Then I want to try to develop models (equations) that explain it all. After that, maybe we can compare everybody's results.

Catch you later,
Dave

Jal
10-05-2006, 04:08 PM
<blockquote><font class="small">Quote pooltchr:</font><hr> <blockquote><font class="small">Quote DeuceBigalo:</font><hr> I've never really put any real thought in to the kick shot, especially when you're going for a 'kick and pot' but I recently watched a guy playing 9 Ball, using his fingers to measure distances between cushion and OB and then mirroring that same distance on the edge of the table beyond the cushion. My question here is, what are the tricks of the trade to calculate accurate kick shots when you're trying to kick and pot?

<font color="red"> Simple, to the point answers would be highly appreciated. </font color> <hr /></blockquote>

Just thought I would bring the thread starter back up to see how well we met his request...simple and to the point??? All he wanted to know was what someone was doing measuring from the ball to the rail to make a kick shot. Now I consider myself a fairly intelligent person, but y'all lost me way back there! /ccboard/images/graemlins/confused.gif
Steve <hr /></blockquote>It's generous of you to put it as "how well we met his request", since you did give a straightforward answer: use the rail track for the mirror. And I think it's substantially correct at shallow approaches to the cushion. But, unfortunately, the gremlins begin playing games at steeper angles.

If you use pure sidespin to get the angle-out equals angle-in effect, then the mirror plane moves toward the cushion, as per Chopstick's post. Or, if you prefer to fix the mirror at the rail track, then the ghost object ball must be positioned farther away from it.

If you use pure topspin to get the angle-out equals angle-in effect, then the mirror plane tends to move in the other direction, toward the center of the table. Or, the ghost object ball must be moved closer to the plane fixed at the rail track. There's not as much adjustment using topspin because the finite contact time partially cancels the contribution from the masse action.

Just how much the mirror moves with either method is still up in the air as far as the physics here is concerned (maybe it's been worked out somewhere else). It's pretty obvious though that whatever number happens to be exactly right for one situation, will not be for another. There's a large dependency on shot speed and incoming angle, and any variation in cushion reaction really mucks things up.

At this point I have to agree with Wolfdancer (although I'm shocked that anyone would suggest forgetting about the math and physics), get the DVD.

Jim

pooltchr
10-05-2006, 05:03 PM
Jim,
You are absolutely correct. Every shot, every angle, every table, it's all going to be a little different. Different speeds, different balls, different humidity levels, cleanliness of the balls...it seems like all the math and physics still can't provide a definitive answer to the question when there are so many variables. It all comes down to making your best judgement and shooting the shot. I don't know of anyone who can look at a shot, and apply all the different formulas and theories, and know exactly how to shoot it. The top players just know from experience what to do, and they do it.
All the in-depth discussions are interesting to a point, but personally, I have more fun shooting the shot than I do analyzing it. I'm not saying all this discussion is bad or wrong...but I wonder how useful it really is in practical application.
Steve

dr_dave
10-06-2006, 06:56 AM
<blockquote><font class="small">Quote pooltchr:</font><hr>Every shot, every angle, every table, it's all going to be a little different. Different speeds, different balls, different humidity levels, cleanliness of the balls...it seems like all the math and physics still can't provide a definitive answer to the question when there are so many variables. It all comes down to making your best judgement and shooting the shot.<hr /></blockquote>
Agreed. However, the physics can help explain and reveal trends that are useful to know. For example:
- more speed shortens the rebound.
- less speed lengthens the rebound.
- less ball roll (due to faster speed or closeness to rail) means shorter rebound.
- running English lengthens rebound, reverse English shortens it.
- follow lengthes kick rebound, draw shortens it.
- English is less important at larger approach angles.
- an outside cut shortens a bank, an inside cut lengthens it.
- spin transfer can significantly affect banks (e.g., left English on the CB can transfer right spin to the OB which shifts the rebound to the right).
- cut and spin transfer effects are related to throw effects, so a complete understanding of throw helps too (hence my current series of articles in BD).

If one doesn't understand all of these factors, or have great intuition built up from years and years of mistakes, the mirror systems are not very useful.

<blockquote><font class="small">Quote pooltchr:</font><hr>I don't know of anyone who can look at a shot, and apply all the different formulas and theories, and know exactly how to shoot it. The top players just know from experience what to do, and they do it.<hr /></blockquote>
I have often seen "top players" miss kick or bank shots because they didn't have a complete understanding (or adequate intuition) of some of the factors listed above.

<blockquote><font class="small">Quote pooltchr:</font><hr>All the in-depth discussions are interesting to a point, but personally, I have more fun shooting the shot than I do analyzing it. I'm not saying all this discussion is bad or wrong...but I wonder how useful it really is in practical application.<hr /></blockquote>
I also have more fun shooting, but I have even more fun when I make the shots. With me, understanding helps me make more shots. And when I miss shots, I try to learn from the misses to improve my understanding (and intuition). To me, this is fun too (although, not as fun as making the shots).

Regards,
Dave

pooltchr
10-06-2006, 08:28 AM
Dave,
You are correct that physics explains all the trends you listed. As an instructor, I believe that an understanding of these trends is critical to a player developing the ability to make sound judgements as to how to play a shot.

Please don't get me wrong. I am not coming down on you or anyone else who gets involved in these discussions. You should know that I often turn to your web site and book when someone feels they need a more in-depth explanation of some of these principles. I just believe that sometimes, we can get too caught up in the textbook physics. I guess I am just a believer in the KISS philosophy. We all function at different levels, and the information you provide is the foundation for much of what we teach our students. But if I tried to get most of my students to that level of understanding, I would be afraid they would run out of the room screaming and pulling their hair out. /ccboard/images/graemlins/shocked.gif
Most of us will never have the technical understanding of some of these things that you have. I appreciate your sharing, but sometimes, as I stated earlier, you do lose me. /ccboard/images/graemlins/confused.gif
Keep up your good work.
Steve

dr_dave
10-06-2006, 08:52 AM
<blockquote><font class="small">Quote pooltchr:</font><hr>You are correct that physics explains all the trends you listed. As an instructor, I believe that an understanding of these trends is critical to a player developing the ability to make sound judgements as to how to play a shot.<hr /></blockquote>
Agreed.

<blockquote><font class="small">Quote pooltchr:</font><hr>Please don't get me wrong. I am not coming down on you or anyone else who gets involved in these discussions. You should know that I often turn to your web site and book when someone feels they need a more in-depth explanation of some of these principles. I just believe that sometimes, we can get too caught up in the textbook physics. I guess I am just a believer in the KISS philosophy.<hr /></blockquote>
I am also a big fan of KISS ("keep it simple, stupid") being an engineer and not a physicist. Engineers actually have to make stuff that works, and sometimes we don't care if we fully understand all of the physics or not.

<blockquote><font class="small">Quote pooltchr:</font><hr>We all function at different levels, and the information you provide is the foundation for much of what we teach our students. But if I tried to get most of my students to that level of understanding, I would be afraid they would run out of the room screaming and pulling their hair out.<hr /></blockquote>
I understand completely. Again, the physics stuff I post on my website (e.g., the TP's) and sometimes discuss here (with Bob, Jal, Fred, Spiderman, and others) is not for everybody. The intended audience for that stuff is people with strong physics/math/engineering backgrounds. This material is not appropriate for a general pool school audience.

<blockquote><font class="small">Quote pooltchr:</font><hr>Most of us will never have the technical understanding of some of these things that you have. I appreciate your sharing, but sometimes, as I stated earlier, you do lose me.<hr /></blockquote>
Again, I don't expect everybody to understand the physics stuff. What I do want all people to try to understand is the basic knowledge that is useful at the table. The audience for my BD articles (http://www.engr.colostate.edu/~dga/pool/bd_articles/index.html) (and my book) is everybody, and I try to illustrate and explain stuff there simply and clearly without getting bogged down with the detailed physics. If I ever lose you or anybody else with my articles (http://www.engr.colostate.edu/~dga/pool/bd_articles/index.html), please let me know because that would mean I'm not doing my job well (in which case, I would want to change my approach some).

Thanks for the input and feedback,
Dave

pooltchr
10-06-2006, 10:12 AM
Dave,
I think we agree on many fronts. And for the record, I have found your book to be very well thought out and quite easy to understand...even for me! /ccboard/images/graemlins/wink.gif
Steve

dr_dave
10-06-2006, 10:26 AM
<blockquote><font class="small">Quote pooltchr:</font><hr> Dave,
I think we agree on many fronts.<hr /></blockquote>
Agreed.

<blockquote><font class="small">Quote pooltchr:</font><hr>And for the record, I have found your book to be very well thought out and quite easy to understand...even for me! /ccboard/images/graemlins/wink.gif
Steve <hr /></blockquote>
Thanks Steve. I appreciate that. But please set me straight in the future if I'm not making sense (when I'm writing for a general audience).

Keep up the good work with your students,
Dave

Bob_Jewett
10-06-2006, 02:50 PM
<blockquote><font class="small">Quote dr_dave:</font><hr> ... trends that are useful to know. For example:
- more speed shortens the rebound.
- less speed lengthens the rebound.
... <hr /></blockquote>
I believe these don't occur directly (without follow/stun/draw on the cue ball).

cushioncrawler
10-06-2006, 07:09 PM
<blockquote><font class="small">Quote dr_dave:</font><hr> .... running English lengthens rebound, reverse English shortens it .... <hr /></blockquote>
On my 12' table (L shaped rubbers), at slowish speeds, i found that reverse-english (check-side) decreased the rebound angle when the attack angle was 10dg or 20dg or 30dg, up to perhaps 40dg at which point it made no difference. This "decrease" being when compared to a (slowish) ball (rolling) with no side-spin. Dont know about 9' tables but.

Funny thing, not related -- with bare rubber -- with lots of reverse-english -- a ball attacking the cushion at 45dg came straight back along the same line -- and, uzing fingerspin, i could do this (trick) at up to about 40dg. madMac.

dr_dave
10-07-2006, 10:24 AM
<blockquote><font class="small">Quote Bob_Jewett:</font><hr> <blockquote><font class="small">Quote dr_dave:</font><hr> ... trends that are useful to know. For example:
- more speed shortens the rebound.
- less speed lengthens the rebound.
... <hr /></blockquote>
I believe these don't occur directly (without follow/stun/draw on the cue ball). <hr /></blockquote>
Bob,

I've seen your experiment to show this with stun shots (and I must admit that I was a little surprised by the result), but I'm not sure it's true at all angles and with all types of vertical plane spin. There is rail throwback (AKA tangential COR) and I think it does vary with speed. Friction also varies with speed. Maybe these effects cancel under most conditions, but I'm still not sure until I run a complete set of experiments. Do you have some data you can share with me for comparison?

Regards,
Dave

dr_dave
10-07-2006, 10:26 AM
Thanks for the info. Sounds reasonable to me.

Regards,
Dave

<blockquote><font class="small">Quote cushioncrawler:</font><hr> <blockquote><font class="small">Quote dr_dave:</font><hr> .... running English lengthens rebound, reverse English shortens it .... <hr /></blockquote>
On my 12' table (L shaped rubbers), at slowish speeds, i found that reverse-english (check-side) decreased the rebound angle when the attack angle was 10dg or 20dg or 30dg, up to perhaps 40dg at which point it made no difference. This "decrease" being when compared to a (slowish) ball (rolling) with no side-spin. Dont know about 9' tables but.

Funny thing, not related -- with bare rubber -- with lots of reverse-english -- a ball attacking the cushion at 45dg came straight back along the same line -- and, uzing fingerspin, i could do this (trick) at up to about 40dg. madMac. <hr /></blockquote>

Bob_Jewett
10-09-2006, 05:41 PM
<blockquote><font class="small">Quote dr_dave:</font><hr> ... Do you have some data you can share with me for comparison?
.. <hr /></blockquote>
I believe that I described my experiment and results in a BD article. Yes, it was not exhaustive with all possible speeds and angles, but since it had a negative result (there was essentially no variation in ball exit angle versus speed if the ball was stunned into the cushion without spin), I saw no reason to do further tests. The test is pretty easy to set up since it is for no spin whatsoever on the ball striking the cushion. Perhaps we can get a volunteer to extend the results to other angles.

For those who didn't see the article, the most interesting result was that for snooker cushions (L-profile) more speed makes the ball go longer.

Bob_Jewett
10-09-2006, 05:58 PM
<blockquote><font class="small">Quote cushioncrawler:</font><hr> ... On my 12' table (L shaped rubbers), at slowish speeds, i found that reverse-english (check-side) decreased the rebound angle when the attack angle was 10dg or 20dg or 30dg, up to perhaps 40dg at which point it made no difference. ... <hr /></blockquote>
When you say "increased" or "decreased" the angle, it is not clear what you mean. In optics, the angles of incidence and reflection are measured from the normal, so that a ball that is shot straight into a cushion that comes straight back to you has an incident angle of zero degrees. Because there is this chance of confusion, you should say something like "more parallel" or "straighter out" or some such, if you want to avoid that confusion.

It is well known that a ball played slowly nearly parallel to a cushion has a reflected angle more parallel to the cushion than the incident angle, regardless of the side spin. See Koehler, for example, I believe. As I pointed out in an article, somewhere, sometime, Bud Harris estimated the reflected angle as 70% of the incident angle when measured from the surface. This assumes that the cue ball is rolling smoothly on the cloth.

Bob_Jewett
10-09-2006, 06:03 PM
<blockquote><font class="small">Quote dr_dave:</font><hr> ... I'm not sure it's true at all angles and with all types of vertical plane spin. ... <hr /></blockquote>
If you mean spin around the vertical axis, I don't know of any good way to do an experiment that can keep the appropriate factors constant or even known while varying speed. Maybe with a robot doing the shooting and a fast video monitor to check.

By contrast, it is fairly easy to send a ball into a cushion at a repeatable angle without spin and at various speeds.

dr_dave
10-10-2006, 06:55 AM
<blockquote><font class="small">Quote Bob_Jewett:</font><hr> <blockquote><font class="small">Quote cushioncrawler:</font><hr> ... On my 12' table (L shaped rubbers), at slowish speeds, i found that reverse-english (check-side) decreased the rebound angle when the attack angle was 10dg or 20dg or 30dg, up to perhaps 40dg at which point it made no difference. ... <hr /></blockquote>
When you say "increased" or "decreased" the angle, it is not clear what you mean. In optics, the angles of incidence and reflection are measured from the normal, so that a ball that is shot straight into a cushion that comes straight back to you has an incident angle of zero degrees. Because there is this chance of confusion, you should say something like "more parallel" or "straighter out" or some such, if you want to avoid that confusion.<hr /></blockquote>
I agree completely. Everybody seems to use different conventions here. I, like Bob, prefer the optics convention. If the ball is hit straight into the rail, the approach angle is zero and the rebound angle is zero. As the approach angle increases, the rebound angle increases.

I think the only terminology that seems to be used consistently is "longer" and "shorter." When the rebound angle increases, the ball "goes longer," and when the rebound angle is decreased, the ball "goes shorter."

Regards,
Dave

dr_dave
10-10-2006, 07:01 AM
<blockquote><font class="small">Quote Bob_Jewett:</font><hr>If you mean spin around the vertical axis ...<hr /></blockquote>
By vertical plane spin, I mean varying amounts of draw or follow. Spin about the vertical axis is "English." Spin in the vertical plane is about the horizontal axis.

<blockquote><font class="small">Quote Bob_Jewett:</font><hr>I don't know of any good way to do an experiment that can keep the appropriate factors constant or even known while varying speed. Maybe with a robot doing the shooting and a fast video monitor to check.<hr /></blockquote>
When I do the experiments, I plan to film with both my digital camera (to observe approach and rebound angles and post rebound curve) and my high-speed camera (to verify speeds and spins, and to observe cushion compression).

<blockquote><font class="small">Quote Bob_Jewett:</font><hr>By contrast, it is fairly easy to send a ball into a cushion at a repeatable angle without spin and at various speeds.<hr /></blockquote>
Agreed.

Regards,
Dave

dr_dave
10-10-2006, 07:23 AM
For those that want to see Bob's article, it can be found here (http://www.sfbilliards.com/articles/2004-08.pdf)

Dave

<blockquote><font class="small">Quote Bob_Jewett:</font><hr>I believe that I described my experiment and results in a BD article. Yes, it was not exhaustive with all possible speeds and angles, but since it had a negative result (there was essentially no variation in ball exit angle versus speed if the ball was stunned into the cushion without spin), I saw no reason to do further tests. The test is pretty easy to set up since it is for no spin whatsoever on the ball striking the cushion. Perhaps we can get a volunteer to extend the results to other angles.

For those who didn't see the article, the most interesting result was that for snooker cushions (L-profile) more speed makes the ball go longer. <hr /></blockquote>

Bob_Jewett
10-10-2006, 08:55 AM
<blockquote><font class="small">Quote dr_dave:</font><hr> ... By vertical plane spin, I mean varying amounts of draw or follow. Spin about the vertical axis is "English." Spin in the vertical plane is about the horizontal axis.... <hr /></blockquote>
I think that draw and follow (as well as masse) have their spin axes in the horizontal plane, and that if you separate the draw/follow component from the left/right masse component, the draw/follow axis is perpendicular to the path of the cue ball while the masse axis is parallel to the path of the cue ball. Is there some other spin convention?

dr_dave
10-10-2006, 02:36 PM
Bob,

We are in agreement. Spin direction can be described either by the direction of the axis of rotation or by the plane within which the spin occurs. The spin axis is perpendicular to the spin plane. So English occurs in the horizontal plane, and results from moving the cue left and right of center in the horizontal plane, but its spin axis is vertical. Spin from draw and follow are in the vertical plane, and result from moving the cue tip vertically away from the center, but the spin axis is horizontal.

It's probably best to just avoid all of this confusing terminology where possible by simply referring to "English," "draw and follow," and "masse" spin where appropriate.

Regards,
Dave
<blockquote><font class="small">Quote Bob_Jewett:</font><hr> <blockquote><font class="small">Quote dr_dave:</font><hr> ... By vertical plane spin, I mean varying amounts of draw or follow. Spin about the vertical axis is "English." Spin in the vertical plane is about the horizontal axis.... <hr /></blockquote>
I think that draw and follow (as well as masse) have their spin axes in the horizontal plane, and that if you separate the draw/follow component from the left/right masse component, the draw/follow axis is perpendicular to the path of the cue ball while the masse axis is parallel to the path of the cue ball. Is there some other spin convention? <hr /></blockquote>

Bob_Jewett
10-13-2006, 04:22 PM
<blockquote><font class="small">Quote Bob_Jewett:</font><hr> I looked at the Jacksonville Project DVD this morning. For those who have a copy, the shot I looked at was on 11/04/98 at 19:34:56. It was shot at 2000 FPS. A ball was frozen on the cushion, and the cue ball was shot straight into it. The inbound cue ball speed was 7.3 meters/second. The time of the ball in the cushion was 6.5 milliseconds. The max penetration of the ball into the cushion was about 17mm. ... <hr /></blockquote>
Depending on what model you use for the cushion, you can calculate one quantity from the other two and also determine the "spring constant" of the cushion. The numbers above are consistent with a Hooke-type spring, although something towards a Hertz-like spring would be expected for the shape of the cushion. The spring constant is about 40,000 newtons/meter or 227 pounds/inch, and the force on the ball at maximum penetration is about 135 pounds.

dr_dave
10-14-2006, 12:58 PM
<blockquote><font class="small">Quote Bob_Jewett:</font><hr> ... the force on the ball at maximum penetration is about 135 pounds.<hr /></blockquote>

/ccboard/images/graemlins/shocked.gif
Watch your language. This is a public forum.

Jal
10-15-2006, 01:09 AM
<blockquote><font class="small">Quote Bob_Jewett:</font><hr>Depending on what model you use for the cushion, you can calculate one quantity from the other two and also determine the "spring constant" of the cushion. The numbers above are consistent with a Hooke-type spring, although something towards a Hertz-like spring would be expected for the shape of the cushion. The spring constant is about 40,000 newtons/meter or 227 pounds/inch, and the force on the ball at maximum penetration is about 135 pounds. <hr /></blockquote>I also tried pulling some numbers off the data you supplied. For the coefficient of restitution, after adjusting for inelasticity using Cushioncrawler's data, and bed friction, I get a value of about 0.79.

As far as Hooke's law, the numbers (to me) suggest something a little different. If you switch to the ball's frame of reference so that the cushion is moving toward it, the ratio of the amount of ball travel distance at maximum compression to its travel distance at the end of impact provides some indication of how spikey the force function is. If the contact time is T, and the force is perfectly symmetric about T/2, then the ratio for a constant force is 0.25. For a sinusoidal function (Hookes), it's 0.18. For an infinitely thin pulse at T/2, it's zero. So we should expect something between zero and 0.18. I get a figure 0.14.

Internal damping will make the function a bit unsymmetric about T/2, and increase this ratio, I believe. This suggests that 0.14 is higher than it would be for the undamped symmetric form. I'm much less sure about the effect of bed friction. For a sinusoidal form, I think it reduces the ratio. But for the infinitely thin pulse at T/2, it clearly increases it.

Jim