The Miff of Topspin (Simpson chimes in)

[klockdoc]

Tom or Bob,

This is probably an over exaggeration of the shot, but, isn't over-spin happening here?

Or is the ball just returning to its original "roll" after impact?

CueTable Help

 

[Bob Jewett]

Is the distance that the CB will travel forward, the circumference or otherwise?

All the distances I was talking about are measured at the centers of the balls. If the object ball is driven 12 inches, the cue ball will roll forward 2 inches from its point of impact, which would be pretty much where the object ball was to start with. If the object ball is driven forward 60 inches, the cue ball will follow 10 inches after contact. I don't think that the circumference enters directly into the problem.

 

[Bob Jewett]

... This is probably an over exaggeration of the shot, but, isn't over-spin happening here?

Or is the ball just returning to its original "roll" after impact? ...

When the cue ball hits the object ball, nearly all the forward motion is removed. Then you have a cue ball with a little speed and tremendous spin. The tremendous spin is there because you hit the shot pretty hard. The cue ball is accelerated to the first rail contact, and bounces off the rail. The rail does not remove the spin. Well, it rubs some off, but not most. The ball bounces away from the cushion and the spins is still acting and reverses the cue ball's path and drives it back to the cushion, or in the case you have drawn, the nine ball. Depending on how much spin the ball has and how sticky the rail is, the cue ball could return a third time to the cushion.

The cue ball may well be rolling smoothly on the cloth before it hits the object ball -- that is, its motion is matched to its spin. When it loses all its motion, the spin dominates and you get the "screwy" action.

But the discussion of "excess" spin is about what happens directly off the tip of the cue stick, not what happens after the balance of spin and speed is upset by running into an object ball. A main point Tom made in his article is that you are unlikely to get more than "smoothly rolling topspin" on any shot -- until, of course the cue ball runs into something.

 

[klockdoc]

All the distances I was talking about are measured at the centers of the balls. If the object ball is driven 12 inches, the cue ball will roll forward 2 inches from its point of impact, which would be pretty much where the object ball was to start with. If the object ball is driven forward 60 inches, the cue ball will follow 10 inches after contact. I don't think that the circumference enters directly into the problem.

Thank you for clarifying this. Now to go one step further. Is the any correlation between how much of the OB that is hit to the distance that the CB will travel?

ie; Would a half ball hit then cause the CB to travel 2 or 4 or? times as far?

 

[DaveK]

... I'd find it really interesting to see some examples or tests that show just how far that distance is.

Shoot with a stiped ball rather than a plain cue ball and you can easily see when/where a ball picks up natural roll, or looses it's backspin, under various conditions and strokes. I highly recommend this approach when learning drag shots or stun-run-through shots, or even long stop shots. It's also a lot of fun to do this on brand new Simonis

Dave

 

[catscradle]

Then why does it roll forward at all, and why can I vary the distance it rolls?

Not trying to be argumentative here, it just comes natural. Actually trying to learn something , but I'm not totally convinced yet.

Far be it from me to speak for Mike, but ... I think what he means is that it has acquired some roll but not full natural roll. In other words it has "underspin" and is still sliding to some extent when it hits the object ball. Whereas if you hit it with top, it has full natural roll almost immediately with virtually no skid.

Tom,
As far as your proposition goes, I would buy that the vast majority of players (myself included) don't put meaningful overspin on the ball, but what about super strokers like Mike Massey? Do you think he can do so?

 

[Patrick Johnson]

... Is the any correlation between how much of the OB that is hit to the distance that the CB will travel?

ie; Would a half ball hit then cause the CB to travel 2 or 4 or? times as far?

For a half-ball hit (30 degree cut) with natural roll on the CB, the CB and OB will travel equal distances after impact (this is also true for a 45 degree cut with stun).

I'll bet Ron Shepard's Amateur Physics for Amateur Pool Players (which Bob Jewett keeps handy for us here) has the formula for all cut angles.

pj
chgo

 

[spoons]

Shoot with a stiped ball rather than a plain cue ball and you can easily see when/where a ball picks up natural roll, or looses it's backspin, under various conditions and strokes. I highly recommend this approach when learning drag shots or stun-run-through shots, or even long stop shots.
Dave

This is a great little practice method that I use pretty often. In fact I rarely use an actual cue ball anymore when I practice, unless the ball set I'm using has the TV ball.

My point though, is that at differing speeds, even using the striped ball, it's difficult to tell when the cue ball has attained full natural roll as opposed to being most of the way there.

But say we knew that on average (I'm making up numbers here), a center ball strike on the cueball at 10mph meant that the cue ball would travel 6 feet before fully attaining natural roll, and the same contact point at 5mph would send the cue ball 3 feet before fully attaining natural roll. Because of the variables in play, it's obviously not an exact figure, but I would still find that to be useful information.

I don't get too heavy into the phyiscs of the game except for the pure enjoyment of learning, but I, like most people, do use certain reference points or rules of thumb when I play, and I could see this being one of them if I knew what the actual numbers were.

It's also a lot of fun to do this on brand new Simonis

YES!! New Simonis is the pool player's tree house.

 

[Bob Jewett]

... Is the any correlation between how much of the OB that is hit to the distance that the CB will travel?

ie; Would a half ball hit then cause the CB to travel 2 or 4 or? times as far?

Yes. There is a fairly simple geometrical construction that gives both the direction of the following cue ball for any amount of object ball cut and the relative speeds. This is described and illustrated in my next column in Billiards Digest. Most of the ideas have been discussed in previous BD columns at: http://www.sfbilliards.com/articles/BD_articles.html

There is a small problem in getting from speed to distance, since the distance is proportional to the square of velocity. Twice the speed results in four times the rolling distance.

To answer for a specific case, if a rolling cue ball hits an object ball half full, the two balls will go almost the same distance.

 

[frankncali]

One question ....

Say I have a shot
If I were to hit the shot with the same speed twice but change the tip height above center what makes the balls go different distances after contact with the OB? Or do they?

CueTable Help

 

[klockdoc]

Originally Posted by klockdoc
... Is the any correlation between how much of the OB that is hit to the distance that the CB will travel?

ie; Would a half ball hit then cause the CB to travel 2 or 4 or? times as far?

For a half-ball hit (30 degree cut) with natural roll on the CB, the CB and OB will travel equal distances after impact (this is also true for a 45 degree cut with stun).

I'll bet Ron Shepard's Amateur Physics for Amateur Pool Players (which Bob Jewett keeps handy for us here) has the formula for all cut angles.

pj
chgo

You know when I was writing this, somehow the relevance didn't click. I think I remember this conversation before.. Thank you.

And thanks for the book information

 

[Tom Simpson]

One question ....

Say I have a shot
If I were to hit the shot with the same speed twice but change the tip height above center what makes the balls go different distances after contact with the OB? Or do they?

CueTable Help

What matters is what is the CB doing when it hits the OB. If it has some backspin, you get some draw action. If it's skidding, you get stop/stun action. If it has some roll, you get some follow action. If it has full natural roll, you get more follow action. So, for a given speed, the question is: At the moment of impact with the OB, where is the CB in its process of going from backspin or skid through to full natural roll?

 

[frankncali]

What matters is what is the CB doing when it hits the OB. If it has some backspin, you get some draw action. If it's skidding, you get stop/stun action. If it has some roll, you get some follow action. If it has full natural roll, you get more follow action. So, for a given speed, the question is: At the moment of impact with the OB, where is the CB in its process of going from backspin or skid through to full natural roll?

Tom

I was more thinking about two top shots. Just varying the height above center. I understand the drag principal. Is there a maximum natural roll for all shots?
I just always assumed that the more top a cueball had on it at impact then it would travel farther forward due to it keeping more top spin after contact. I assummed that a ball rolling would lose more forward momentum
without the top spin.
Interesting stuff and thanks for coming here and discussing. I dont know if I will ever use the info but its nice to nkow what is happening.

 

[Bob Jewett]

One question ....

Say I have a shot
If I were to hit the shot with the same speed twice but change the tip height above center what makes the balls go different distances after contact with the OB? Or do they? ...

I assume you are going to hit the cue ball softly enough that the cue ball will be rolling smoothly when it gets to the object ball for both cases. If so, you are really asking whether the cue ball will be moving (and rolling) faster for one shot compared to the other. Faster movement = faster rolling = more top spin = more movement after contact.

This is actually a tricky question that most people will get wrong. It is covered in Ron Shepard's paper mentioned earlier, and it has been in the techno-threads fairly recently.

The answer is that if you start at the bottom of the cue ball and gradually hit higher, the cue ball will at first go faster for a higher hit because there is less draw with a higher hit and that draw slows the cue ball. (Hitting what will be a follow shot with draw is a well-known technique called a drag shot or draw drag.) As you hit too high, however, the cue ball gets lots of spin but not much initial speed due to the very off-center hit. It turns out that for a given stick speed, you want to hit at about 61% of the height of the cue ball to get maximum speed on the cue ball after normal rolling sets in. Any higher or lower than this hit will result in less speed (and less spin, and less travel after contact).

The 61% number depends slightly on the weight of the cue stick and the kind of tip you use but not much. 50% would be the middle of the cue ball and 75% would be as high as you can hit without miscuing (on a good day). See problem 3.11 in Shepard's paper: http://www.sfbilliards.com/Shepard_apapp.pdf

61% is also where you want to hit lag shots to make the distance insensitive to exactly how high you hit the ball. Some people have been known to advise hitting lags with draw; I think they shouldn't advise.

 

[frankncali]

I assume you are going to hit the cue ball softly enough that the cue ball will be rolling smoothly when it gets to the object ball for both cases. If so, you are really asking whether the cue ball will be moving (and rolling) faster for one shot compared to the other. Faster movement = faster rolling = more top spin = more movement after contact.

This is actually a tricky question that most people will get wrong. It is covered in Ron Shepard's paper mentioned earlier, and it has been in the techno-threads fairly recently.

The answer is that if you start at the bottom of the cue ball and gradually hit higher, the cue ball will at first go faster for a higher hit because there is less draw with a higher hit and that draw slows the cue ball. (Hitting what will be a follow shot with draw is a well-known technique called a drag shot or draw drag.) As you hit too high, however, the cue ball gets lots of spin but not much initial speed due to the very off-center hit. It turns out that for a given stick speed, you want to hit at about 61% of the height of the cue ball to get maximum speed on the cue ball after normal rolling sets in. Any higher or lower than this hit will result in less speed (and less spin, and less travel after contact).

The 61% number depends slightly on the weight of the cue stick and the kind of tip you use but not much. 50% would be the middle of the cue ball and 75% would be as high as you can hit without miscuing (on a good day). See problem 3.11 in Shepard's paper: http://www.sfbilliards.com/Shepard_apapp.pdf

61% is also where you want to hit lag shots to make the distance insensitive to exactly how high you hit the ball. Some people have been known to advise hitting lags with draw; I think they shouldn't advise.

Thanks Bob
The 61% part makes good sense to me and clears up alot of thoughts. Years ago I noticed that on shots where both balls were kind of close and I needed the ball to follow that I had to hit the ball harder with more top spin then if I lowered down on the CB. In other words I could use the same speed of hit and lower down on the cueball getting the same or better distance after contacting. Hitting the same speed with alot of top seemed to kill the CB after contact.
Now I know ( it think ,,,lol) that this is becuase even though the same shot speed was used that anything higher than the 61% kills the intial speed of the CB. I think that is right.

thanks

 

[Tom Simpson]

I was more thinking about two top shots. Just varying the height above center. I understand the drag principal. Is there a maximum natural roll for all shots?

I just always assumed that the more top a cueball had on it at impact then it would travel farther forward due to it keeping more top spin after contact. I assummed that a ball rolling would lose more forward momentum
without the top spin.


Natural roll is just natural roll (not breaking the friction with the cloth either forward or backward). The CB try to will "propel itself" in the direction of any roll that's in the ball. After collisions with balls or rails, any remaining roll or backspin in the CB will act more strongly. This is because some of its speed has been removed, but most of its spin remains.

Yes, a rolling ball does lose forward momentum due to friction with the cloth. You get more follow action by arriving at the moment of impact with a faster rolling CB. The overspin sometimes visible in the CB after the impact is the rolling speed that is still in the CB after the collision takes some of the forward speed out.

 

[mikepage]

Tom or Bob,

This is probably an over exaggeration of the shot, but, isn't over-spin happening here?
[...]

OK here's the little video I was trying to post yesterday. It's not the elusive getting overspin with the stick. Rather it just illustrated the common overspin that follows any collision of a rolling ball.

http://www.youtube.com/watch?v=XI-NMgYbMyA


Oh, and it shows what happens in Klocdoc's diagram

 

[frankncali]

I was more thinking about two top shots. Just varying the height above center. I understand the drag principal. Is there a maximum natural roll for all shots?

I just always assumed that the more top a cueball had on it at impact then it would travel farther forward due to it keeping more top spin after contact. I assummed that a ball rolling would lose more forward momentum
without the top spin.


Natural roll is just natural roll (not breaking the friction with the cloth either forward or backward). The CB try to will "propel itself" in the direction of any roll that's in the ball. After collisions with balls or rails, any remaining roll or backspin in the CB will act more strongly. This is because some of its speed has been removed, but most of its spin remains.

Yes, a rolling ball does lose forward momentum due to friction with the cloth. You get more follow action by arriving at the moment of impact with a faster rolling CB. The overspin sometimes visible in the CB after the impact is the rolling speed that is still in the CB after the collision takes some of the forward speed out.

Such as top stopping a ball hung up in a pocket. Years ago we used to set up a shot with a ball hung up and take turns trying to see who could get the cueball to come back down table the farthest. You could not use top or it would kill it. Natural roll or as close to center (without being center) would always do the trick.

 

[Patrick Johnson]

I just always assumed that the more top a cueball had on it at impact then it would travel farther forward due to it keeping more top spin after contact.

For topspin, speed = spin. The only way the cueball will have more topspin at impact is to be going faster at impact. So "the more speed a cueball has at impact the farther it will travel after impact due to it keeping more topspin after contact".

Bob's point above is interesting: to get maximum speed with follow, you should hit at about 60 percent of the height of the cueball (about 1/4 inch above center).

pj
chgo

 

[klockdoc]

Such as top stopping a ball hung up in a pocket. Years ago we used to set up a shot with a ball hung up and take turns trying to see who could get the cueball to come back down table the farthest. You could not use top or it would kill it. Natural roll or as close to center (without being center) would always do the trick.

Yes, we used to try this shot too. But, I always used low on the CB. Not a draw shot, but just struck low and the CB will come down table very easily.