modifying the tangent line
- Thread starter berlowmj2
- Start date
Randy, you weren't supposed to disclose this "force the cut angle" lingo into such understandable language.
Not all the time!....SPF=randyg
Sorry Bill, just the Instructor in me....SPF=randyg
Well get a hanky, cause I think it's right but not relevant.
Yes there's a force on the spinning cueball, and yes it is equal and opposite to the force that throws the object ball.
But that doesn't mean the cueball is jolted off the tangent line. That force is directed exactly ALONG the tangent line. So it's effect is either to speed up or slow down the cueball along the tangent line.
close but no cigar
Mike,
Where does the "gearing" between the two balls take place, and what line does the cue ball roll down? You may have to rethink "exactly ALONG the tangent line."
Hu
Mike,
Where does the "gearing" between the two balls take place, and what line does the cue ball roll down? You may have to rethink "exactly ALONG the tangent line."
Hu
Throw an overweight cueball into that equation, and watch what happens
This is true in all instances -- including on a barbox using an overweight/"slug" cue ball. (Many folks would throw the barbox/overweight-cueball into the mix as some sort of "monkey wrench in the works," but the explanation is simple.) The overweight ball pushes the object ball out of the way (to some degree, depending on how much heavier that cue ball is over the object ball), and the tangent line then becomes from where the two balls "came apart," at the instant they came apart. That's why judging the tangent line on a barbox is difficult, until you hit some caroms on it to get an idea (we're obviously talking Valley/Dynamo tables here, not the Diamond SmartTables which use a normal cueball).
Hope this is helpful,
S.B.D.=sfleinen
This is true in all instances -- including on a barbox using an overweight/"slug" cue ball. (Many folks would throw the barbox/overweight-cueball into the mix as some sort of "monkey wrench in the works," but the explanation is simple.) The overweight ball pushes the object ball out of the way (to some degree, depending on how much heavier that cue ball is over the object ball), and the tangent line then becomes from where the two balls "came apart," at the instant they came apart. That's why judging the tangent line on a barbox is difficult, until you hit some caroms on it to get an idea (we're obviously talking Valley/Dynamo tables here, not the Diamond SmartTables which use a normal cueball).
Hope this is helpful,
S.B.D.=sfleinen
This is true in all instances -- including on a barbox using an overweight/"slug" cue ball. (Many folks would throw the barbox/overweight-cueball into the mix as some sort of "monkey wrench in the works," but the explanation is simple.) The overweight ball pushes the object ball out of the way (to some degree, depending on how much heavier that cue ball is over the object ball), and the tangent line then becomes from where the two balls "came apart," at the instant they came apart. That's why judging the tangent line on a barbox is difficult, until you hit some caroms on it to get an idea (we're obviously talking Valley/Dynamo tables here, not the Diamond SmartTables which use a normal cueball).
Hope this is helpful,
S.B.D.=sfleinen
You don't really believe this, do you? The balls don't stick together and then come apart, the collision is practically instantaneous. The tangent line is a different line when the balls are not of the same mass.
To explain in detail: The vector component of the CB's momentum in the direction the OB ends up going is key. If the balls have the same mass, this component is transferred entirely from CB to OB, meaning it ends up being 0 in the CB after contact. A 0 magnitude in the vector component in that direction means 100% of the momentum remaining is perpendicular to that direction, hence the 90-degree rule. With a heavier CB, the amount of that component transferred to the OB is not 100%, it's a little less. You can see this by hitting a sliding-ball dead-straight shot. With balls of the same mass, the CB stops dead. If the CB is heavier, the CB will follow a little, even when sliding at contact. This is the momentum in the OB's final direction that the CB retains after contact, and it's the same momentum that contributes to the change in tangent angle. Non-zero component in this direction means the resolved vector direction is no longer perpendicular to the OB direction.
-Andrew
Andrew:
Did you see me say/write anywhere about the balls "sticking" together? Just because I used the "come apart" phraseology? I think you read into this. I was just trying to use something different than Randy's "release point" phraseology -- something I'd hoped was for clarity.
And actually, in your description above, you describe exactly what I was referring to, with the non-100% kinetic energy transference (i.e. the cue ball sliding through the object ball to some degree). I think we just explained it in different terms, 's all.
-Sean
Additional factor-thanks
I was going to ask-re caroms- If predicting the OB path off the second OB was made more predictable if the original CB strike was with center stun-hopefully transferring similar sliding action (nearly straight shots) to the 1st OB-such that the path down the second collision tangent line is more reliable.
This 'release point' refinement sounds like more to think about. Is that then- a modified 90 degree starting point affected by either force/weight?
Yikes.
Always something.
Thanks guys.
This is true in all instances -- including on a barbox using an overweight/"slug" cue ball. (Many folks would throw the barbox/overweight-cueball into the mix as some sort of "monkey wrench in the works," but the explanation is simple.) The overweight ball pushes the object ball out of the way (to some degree, depending on how much heavier that cue ball is over the object ball), and the tangent line then becomes from where the two balls "came apart," at the instant they came apart. That's why judging the tangent line on a barbox is difficult, until you hit some caroms on it to get an idea (we're obviously talking Valley/Dynamo tables here, not the Diamond SmartTables which use a normal cueball).
Hope this is helpful,
S.B.D.=sfleinen
I was going to ask-re caroms- If predicting the OB path off the second OB was made more predictable if the original CB strike was with center stun-hopefully transferring similar sliding action (nearly straight shots) to the 1st OB-such that the path down the second collision tangent line is more reliable.
This 'release point' refinement sounds like more to think about. Is that then- a modified 90 degree starting point affected by either force/weight?
Yikes.
Always something.
Thanks guys.
I'm unclear on how they would "come apart" differently with the heavier ball if they don't stick together differently. I actually still think we disagree (I promise I'm not being intentionally contentious here), because what I'm saying is that if the balls are not equal mass, then the tangent line is never 90 degrees from the object ball direction. Also I'm pretty sure that "contact point" and "release point" are the same point.
-Andrew
I'm not following your questions. But here is my thinking.
Unless you catch a chalk mark or something, there is no "gearing" between the balls. There is a sliding (not static) friction when the two balls rub against one another.
If at contact the two balls are pointing exactly north/south, then the cueball will go east or west.
The friction causes the object ball to go maybe a degree or two east or west of straight north, and that same friction causes the cueball to go a little faster or a little slower in the east/west direction.
If you have just the right amount of "running" english such that the two surfaces don't rub, then the object ball will go exactly north.
Do you disagree with any of this?
Andrew:
First of all, we're talking about PLASTIC billiard balls here, not stone, not metal, not clay. Today's billiard balls are a bit elastic. So yes, there *is* a bit of "dwell time" when the two balls impact each other, and the surfaces between each ball is slightly compressed. You want proof? Lightly stand a piece of carbon paper on edge on the bed of the table, leaning against the contact point of the object ball -- carbon side facing against that contact point. Then, take cue ball in hand, and shoot into that contact point (from behind the carbon paper) at various speeds. Obviously, between each shot, turn the object ball to a clean area to lean a new piece of carbon paper onto.
I'll bet you dinner if we ever meet, that the carbon spot on the object ball gets progressively bigger the harder you hit the cue ball into it.
Now, I can't steal Freddy the Beard's thunder on this one. This is detailed in his Banking with the Beard book, so attributes and accolades go to Freddy on this one. I personally was dumbfounded when I tried this, because, like you, I thought the contact/release time between the two balls was instantaneous. It isn't. It's even more pronounced with a heavier cue ball, as the heavier cue ball is sort of a "bulldozer" in pushing the object ball out of the way, instead of the two balls coming apart at what you'd think would be the tangent line.
-Sean
To summarize so far and add a point or two ...
The tangent line is the line tangent to both balls through their contact point at the instant of contact. For all practical purposes that contact is instantaneous.
The cue ball will leave the collision (if it is a cut shot) parallel to that line.
If the cue ball has draw or follow, it will bend forward or back. That bend begins immediately for all speeds, but higher speeds make it appear delayed due to the wider curve.
If the surface of the cue ball is moving sideways relative to the object ball, the object ball will be thrown off-line by up to five degrees but the cue ball's initial direction will not be changed, although its side-ways speed is changed a little. This means that the cue ball and object ball will not have 90 degrees between their paths at the start.
If the cue ball is heavier or lighter than the object ball, it will start off forward or behind the tangent line. (forward from the shooter's view)
If the cue ball does not have perfect elasticity, it will start out ahead of the tangent line. This is very noticeable with ivory balls.
If the cue ball is in the air when it hits the object ball, it will still start off along the tangent line, but that line is pointing up and forward of the normal tangent line -- the problem is in three dimensions.
If the cue ball is bouncing off the object ball, its path will not be a curve as viewed from above but will be a sequence of straight segments with the joints at the touch-down spots. (This last detail is visible in the amazing Austrian slo-mo infrared videos from Leitner and Efler.)
Ball-ball contact time had been discussed on RSB in the late-80s/early-90s. It can be deduced from the size of the contact patch and a little calculation. The contact patch can be measured with either carbon paper or mist or wax. In Wayland Marlow's 1995 book, he describes measuring the contact time with a simple electronic circuit, and gets about the same result as the contact-patch method. The contact time, which is about 200 microseconds, is short enough that its exact duration has very little effect on what the balls do, and the collision can be treated as instantaneous forces for nearly all aspects.
Excellent summary!
For more information, articles, and video demos dealing with the "effective tangent line" for various types of shots, see:
Also, here are some articles (with lots of good illustrations) and analyses (with lots of boring physics) dealing with effects of inelasticity and English on the tangent line:
Also, here's a good summary of ball weight effects:
Regards,
Dave
For more information, articles, and video demos dealing with the "effective tangent line" for various types of shots, see:
Also, here are some articles (with lots of good illustrations) and analyses (with lots of boring physics) dealing with effects of inelasticity and English on the tangent line:
Also, here's a good summary of ball weight effects:
Regards,
Dave
Mike,
A reminder what I said to begin with, mostly pouring a little oil on troubled waters. "Sidespin shouldn't have any effect on the tangent line before it hits the rail. However that isn't quite true because in reality we almost never shoot with a level cue. I can and do change the tangent line a little with a lot of sidespin on a slow shot. Dirty balls help for this as does high humidity, two things I often deal with."
I still stand by that statement. Your statement I highlighted in red is basically true but it is an over simplification of what happens. The issue is where the contact and release points are and where the center axis of the two balls are. Something I consider that you are overlooking with the pure sliding contact without gearing is the amount the balls compress on impact. The cue tip imparts a lot of spin as well as forward speed in roughly 1/1000 of a second. The friction between the two balls obviously isn't as great but it is wrong to say that the dwell time of contact is meaningless.
Remember I said to begin with that I change the tangent line a little. This isn't something that is often of significance. A One Pocket shot in very tight quarters where I am using sidespin for the effect off of the rail anyway is the only application I can think of. It might make the difference between grazing a ball and missing it.
This effect isn't great but it's there. It is a lot like the 90 degree rule being talked about here. I think all of us know it isn't 90 degrees but it is close enough for most cases. I don't waste much time telling people it isn't quite 90 degrees in casual conversation.
Hu
What is the definition of "contact and release points?"
If the balls did "gear" for large amounts of spin on the cue ball, you would see skid/cling far more often. From this I think it is fair to conclude that "gearing" -- that is, the relative tangential motions of the two surfaces going to zero -- is fairly rare for significant amounts of spin on the cue ball.