The amount you need to hit below center depends on shot distance and speed, and ball/cloth conditions. For more info, including illustrations and video demonstrations, see:
Regards,
Dave
modifying the tangent line
- Thread starter berlowmj2
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Regards,
Dave
The problem is undefined unless you say how far the cue ball is from the object ball and how hard you're going to hit the ball. The goal is to have the cue ball arrive at the object ball without draw or follow.
As the cue ball goes towards the object ball, it continuously acquires follow. The longer the distance and the slower the shot, the more follow the cue ball will acquire until it is rolling smoothly. I assume you already knew that as it is very, very basic.
So, if you are playing a stop or stun shot at five-length speed on an object ball that is only 6 inches away, you will hit the cue in the middle. If you are playing a soft stun shot on a ball in foot pocket when shooting from the kitchen, you better use your best draw.
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Thanks.
To better understand, what is "five-length speed" and "in foot pocket"?
There are so many terms to be learned in this Forum....so little time.
I think Dr. Dave has a pretty good pool dictionary on his site, but by "five-length speed" I meant hard enough to travel five lengths of the table. Most speed scales are based how many lengths of the table the ball will travel if it is going straight up and down the long string.Thanks.
To better understand, what is "five-length speed" and "in foot pocket"?
There are so many terms to be learned in this Forum....so little time.
The foot of the table is the end where the balls are racked. The head of the table is where you break from. A foot pocket is one that is adjacent to the foot rail. A list of the parts of a table is in the official rules definitions, but the diagram is missing.
With dead cushions, it is impossible to get five lengths unless the cue ball is flying from rail to rail. It isn't hard to do this, you just have to jack up a little. And pay for the broken lights and windows. I've managed to send the cue ball over the far end cushion on the second trip.
Can anyone here offer a clear definition of "release point?" So far we don't have one.
![CropperCapture[7].png](https://forums.azbilliards.com/data/attachments/103/103791-f310a2372147d3cb892cbb745be0dcb0.jpg?hash=8xCiNyFH08 "CropperCapture[7].png")
Thanks,
I understand "five-length speed" would be around <30 mph when breaking a rack or clusters of balls.
I read "in foot pocket" and the visual image was an OB in the jaws, but I now understanc that it was in[to] a foot pocket.
Also thanks for the diagram of the table...we don't use many of the terms contained therein at the bowling alley that I shoot at.
But...where's the kitchen? Just kidding.
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HSV (high speed video) needed for different mass cue ball / object ball contact
Bob:
I'm wondering if we might need one more high-speed video on Dr. Dave's site -- that of stop/stun/follow/draw using a cue ball and object ball of different masses?
Humbly, I think we need one. I'd suggest using the following cue ball, the Aramith Red Dot Dynamo (the heavyweight):
http://poolndarts.com/p-3599-Red-Dot-Dynamo-Belgian-Aramith-Ball/
(Note: there are *two* Aramith Red Dot cue balls available; one is Dynamo, the other isn't. The Dynamo is the heavyweight ball; the other weighs the same and responds the same as a regular Red Circle ball. I show the link above for the heavyweight Dynamo, which, on a postal scale, weighs in at 6.875 ounces.)
I have a feeling that on high-speed video, we're going to see some *very* interesting things related to this contact/release point stuff, and the 30/90 degree rules.
What think ye?
-Sean
Bob:
I'm wondering if we might need one more high-speed video on Dr. Dave's site -- that of stop/stun/follow/draw using a cue ball and object ball of different masses?
Humbly, I think we need one. I'd suggest using the following cue ball, the Aramith Red Dot Dynamo (the heavyweight):
http://poolndarts.com/p-3599-Red-Dot-Dynamo-Belgian-Aramith-Ball/
(Note: there are *two* Aramith Red Dot cue balls available; one is Dynamo, the other isn't. The Dynamo is the heavyweight ball; the other weighs the same and responds the same as a regular Red Circle ball. I show the link above for the heavyweight Dynamo, which, on a postal scale, weighs in at 6.875 ounces.)
I have a feeling that on high-speed video, we're going to see some *very* interesting things related to this contact/release point stuff, and the 30/90 degree rules.
What think ye?
-Sean
I don't think we need high-speed video for this, but I'll add it to the "list."
I actually have plans to write a couple of artilces for BD summarizing and illustrating all of the ball-weight effects, but this won't happen for another 6 months or so because I need to finish my VEPS article-series first. For now, I do have a good summary of ball weight effects (with a good quote from you) here:
Regards,
Dave
I actually have plans to write a couple of artilces for BD summarizing and illustrating all of the ball-weight effects, but this won't happen for another 6 months or so because I need to finish my VEPS article-series first. For now, I do have a good summary of ball weight effects (with a good quote from you) here:
Regards,
Dave
a think a lot like contact point
Bob,
Randy initiated the use of the term here so only he can define it. However I think it is much like the contact point, a little vague. There is no such thing as a contact point, it is a contact patch or area. Likewise the release point is the last point or area that affects the direction of the balls. Since the effect is less and less as the true release point is reached the only real way to determine the release point is to work backwards from the exact paths of the balls.
However, like all of the things we simplify, there are complications to this. Both balls do compress and they do snap back. We know that they can compress to an extent that there is a quarter inch of contact patch, maybe more. Do they snap back before the balls clear each other? Does this spring back effect have any effect on the paths of the balls?
We spend a lot of time trying to figure out things that don't really matter in pool. When I was in R&D I could have spent a hundred thousand dollars or so trying to prove exactly what happens when two balls collide. After all of that what I determined would only perfectly apply to those two balls and the collisions I examined. There is a reason we go with rules of thumb most of the time.
Hu
Bob,
Randy initiated the use of the term here so only he can define it. However I think it is much like the contact point, a little vague. There is no such thing as a contact point, it is a contact patch or area. Likewise the release point is the last point or area that affects the direction of the balls. Since the effect is less and less as the true release point is reached the only real way to determine the release point is to work backwards from the exact paths of the balls.
However, like all of the things we simplify, there are complications to this. Both balls do compress and they do snap back. We know that they can compress to an extent that there is a quarter inch of contact patch, maybe more. Do they snap back before the balls clear each other? Does this spring back effect have any effect on the paths of the balls?
We spend a lot of time trying to figure out things that don't really matter in pool. When I was in R&D I could have spent a hundred thousand dollars or so trying to prove exactly what happens when two balls collide. After all of that what I determined would only perfectly apply to those two balls and the collisions I examined. There is a reason we go with rules of thumb most of the time.
Hu
My guess is we don't need one.
I think "release point" is a byproduct of an incorrect mental picture of the interaction.
The concept is perhaps more useful for the tip-ball collision.
Tip-Ball Collision:
Here, the tip and ball are moving together for the milisecond or so duration of the contact. During this time, the tip compresses and decompresses, and for a sidespin shot the ball starts rotating and the tip goes with it.
So a tip that first contacts the cueball at 10.0 mm off center (contact offset) might be at 12 mm off center when the contact is done (release offset). If this is the case, then most of the force, and most of the influence on the cueball is happening near the middle, at an offset near 11 mm. The cueball probably acts as though it recieved an instantaneous impulse at an offset of 11 mm.
Ball-Ball Collision:
Once a person considers that the ball-ball collision is not instantaneous, that instead it takes some time and involves compression of the balls, it might be tempting to imagine the same stuff is going on as in the tip-ball collision, only at a smaller scale. But this is wrong.
The balls don't move together like the tip and cueball do. Instead they rub across one another while they are compressing and decompressing. So the contact patch starts as a point, grows to whatever it grows to, and ends as a point. If the contact time is 0.2 ms and the sideways relative speed of the balls is 4 mph (like it would be for a 6 mph cueball doing a quarter-ball hit), then the sliding distance (distance the cueball moves while in contact with the object ball) is perhaps 0.5 mm, half the thickness of a dime.
I don't think this matters for anything. I don't think it affects the tangent line or anything else we need to think about to understand pool.
Maybe Bob J. or Dave A. disagree with me?
Mike,
I agree with the gist of your post. Ball-to-ball contact time and compression are very small. Practically speaking, the balls can be thought of as rigid with pure point contact.
My only small disagreement is with your statement: "The balls ... rub across one another while they are compressing and decompressing."
The rubbing does actually stop during contact sometimes. For more info, see page 2 of TP A.14. That's why many throw graphs have straight-line portions (e.g., see the plots on pages 6 and 9 of TP A.14).
Regards,
Dave
I agree with the gist of your post. Ball-to-ball contact time and compression are very small. Practically speaking, the balls can be thought of as rigid with pure point contact.
My only small disagreement is with your statement: "The balls ... rub across one another while they are compressing and decompressing."
The rubbing does actually stop during contact sometimes. For more info, see page 2 of TP A.14. That's why many throw graphs have straight-line portions (e.g., see the plots on pages 6 and 9 of TP A.14).
Regards,
Dave
This thread truly shows the character of us all, were picky people, poolplayers with passion. We're always dealing with such infamatesimal amounts because of the game we chose, that here we are again peckin' away like a herd of rail birds, personally I love birds.
people, poolplayers with passion. We're always dealing with such infamatesimal amounts because of the game we chose, that here we are again peckin' away like a herd of rail birds, personally I love birds.Mike,
I think I'm with you here. The tangent line may not require a circle, but by definition it definitely requires a curve. It is entirely possible to hit a perfect stop shot on a cut shot in which case the cue ball will travel along what for all intents and purposes is a straight line.
We might be saying the same thing: at the moment of contact between cue ball and object ball, the cue ball will initially begin travelling parallel to a line tangent to the object ball at the point of contact. How it moves after that is entirely dependent on the spin on the cue ball.
Are we saying the same thing? I think we are.
KMRUNOUT
Not sure why you grasped at a far out highly complex example. How about something straightforward. Take a falling ball. If you drop a ball in a vacuum, the speed of this ball will change (it will accelerate under the influence of gravity.) The velocity of the ball at a given time after dropping is described by a curve. This curve is very well known, and extremely accurate. If you know how much time has elapsed after dropping the ball, you can calculate the ball's velocity with extreme accuracy. Thus here is an example in which a curve provides a highly reliable method of predicting a particular outcome.
As for the comments you made on the stuff you quoted from me, I'm sorry bro but it is unintelligible to me. I really don't know what you are trying to say.
KMRUNOUT
Actually Bill, we both are correct. In most cases the contact is the release point. But you already know that....
SPF=randyg
when you put it that way...hard not to agree.
I can't claim to know the small physics involved in the collision between two objects of equal elasticity and different mass. I don't know if they actually are in contact for more time or not. If they are, then Randy's distinction between contact point and release point is meaningful and necessary. If they are not in contact for any longer, then hmm...tough to say, but it would seem that contact point and release point are two words for the same thing.
KMRUNOUT
yes, and yes.
The requirement is a sliding cueball. By this I mean that at the moment of contact, the cue ball has no spin in the vertical axis. Also, you need to have a cue ball of the same mass as the object ball.
The principle variables involved are the cloth, the speed you hit the ball, how low you hit it, and the distance between the two balls. You must balance these variables to achieve a cueball that has no topspin or backspin at the moment of contact. I'm sure you have hit a perfect stop shot in pool when the cueball and object ball are lined up dead straight? Well it would be the exact same thing. If you hit the ball very low, you can hit it softer but still get a stop shot. If the object ball is far away, you must hit it harder to get a stop shot (or lower, or both).
If you want to see it in action, I recommend trying this. Set up a spot shot. Take cueball in hand and line up a straight in shot, with the cue ball about 1.5 feet away. Hit however you want so that you use medium speed, but do a perfect stop shot. Now, move the cueball so that you have a moderate angle, say 30 degrees. However, make sure the cue ball is the exact same distance away from the object ball. Now go ahead and make the shot the exact same way as before...same speed, same cue tip position. You should now watch the cueball split the corner pocket, which is where the tangent line points on a spot shot.
Hope this helps,
KMRUNOUT
Sean,
Thank you for bringing up a horrible idea for me. I HATE this cueball more than any other. (ok, maybe the big one is worse). I have one guy in my league, who ironically is a good SL7 who knows how to shoot, who absolutely refuses to use a standard cueball. At his place, we use the heavy Dynamo ball you describe. Several years back, I purchased one of those balls just to practice with when I know I am going to his place. The funny thing is that he thinks it gives him a "home field" advantage. What is even funnier is that he *still* believes this after I have beaten him 5-0 on his table with his ball many times ha ha!! I wonder if he will ever learn any of the things we are discussing in this thread, so he can actually learn to play position better?
KMRUNOUT