Tangent Line conundrum

Dr. Dave's 30 degree rule accurately predicts for me where a rolling cue ball will go after the shot. (Use a peace sign with your fingers to see where cue ball will go.)

And Dr. Dave's 90 degree rule works with a stun shot. (Use a L with your hand to see where the cue ball will go.)

If the 30 degree rule shows the cue ball will scratch, I shoot with stun instead of follow and the cue ball does not not scratch. Simple as that!

I have almost elliminated my scratching using these rules. Also I am using these rules to get position after my shots. After many months of using these rules, I can now sometimes leave perfect position after every shot.

The bottom line is that I can get the cue ball to go one direction after the hit when hitting with follow, and a different direction after the hit with stun. Try it is all I can say. Set up the same exact shot. Shoot with follow and see where cue ball goes. Then shoot with stun and see where cue ball goes. This is very usefull stuff to know...
 
Hickok said:
The initial direction of the cueball after contact is 90 degrees to the line taken by the object ball. Right?

I've been playing a long time and have learned (the hard way) to expect an angle closer to 86 degrees. Four degrees isn't much. But on stun shots with a pocket 3 or 4 diamonds away waiting to gobble the cue ball, four degrees becomes huge.

I was playing around recently with the VP simulator trying to figure out why 90 degrees doesn't work for me, and was surprised to find that it too shows a 'tangent line' of about 86 degrees.

Now, have I finally gone mad? Or is the 90 degree tangent line only a rough approximation offered to beginners?
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if by initial you mean the first 1/64" after contact,,,,maybe so. however the cb almost never takes a 90* angle off the ob's line because of the cb's natural forward roll.

the closest to this possibility would the very most thin of thinnest cuts wherein forward roll and cb angle almost become one,,,or an extremely well executed stun shot.
 
Hickok said:
...Or is the 90 degree tangent line only a rough approximation offered to beginners?
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As many have pointed out, throw is one cause of the deviation from ninety degrees. Another is that the collision is not perfectly elastic, as pointed out in Dr. Dave's technical articles, and by Bob Jewett many times. The component of the cueball's velocity in the direction of the line between centers (at the moment of impact) would be completely removed if the collision were perfectly elastic. Instead, after it's over the cueball still retains a little bit of this component....not much, but some.

Using Dr. Dave's figure of .96 (I think) for the coefficient of restitution, which is a measure of this inelasticity, a three-quarter ball hit should produce about a 4 degree reduction from 90 degrees due to this alone. Added to this, a touch of inside english could easily produce another 4 degrees (or more) of reduction due to frictional throw of the object ball (stun shot). So the deviation can approach 8 degrees or more. (Technically, for extremely small cut angles, the deviation from 90 degrees due to inelasticity is almost 90 degrees.) As the cut angle increases, the effect of the inelasticity on angular deviation diminishes.

And as Cane mentioned, mismatched weights are yet another cause. A heavier cueball will diminish the separation angle while a lighter one will increase it.

Jim
 
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bruin70 said:
if by initial you mean the first 1/64" after contact,,,,maybe so. however the cb almost never takes a 90* angle off the ob's line because of the cb's natural forward roll.
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The 90-degree rule is for stun shots only. The 30-degre rule (measured from a different line, mind you) is for "follow" or "roll" shots (and 1/4-3/4 ball hits).
 
Throw versus the actual contact point

Many of you are very close to what I have always believed to be correct.

The cue ball does actually leave the "contact point" at 90 degrees to that point. The object ball, however, does not. The friction from the contact of the two balls "throws" the object ball off line slightly. If you look at the angle between the cue ball and the object ball it is less than 90 degrees depending on the friction induce throw. It is, however, 90 degrees to the contact point.

So, the cue ball does not leave the point of contact exactly 90 degrees from the path of the object ball, but it does leave at 90 degrees from the original contact point.

Obviously by Dave's video, this can be a very short journey. If the hit is soft and the cue ball is rolling the forward spin of the cue ball will make it deviate from that path almost instantly. Draw or backspin will tend to react slower because the cue ball will be traveling faster in relation to how much spin it has on it. It has to because the spin is against the cloth and that friction has to be overcome.

This just my opinion, and I may be completely wrong, but it works for me.

Royce Bunnell
www.obcues.com
 
RBC said:
Many of you are very close to what I have always believed to be correct.

The cue ball does actually leave the "contact point" at 90 degrees to that point. The object ball, however, does not. The friction from the contact of the two balls "throws" the object ball off line slightly. If you look at the angle between the cue ball and the object ball it is less than 90 degrees depending on the friction induce throw. It is, however, 90 degrees to the contact point.

So, the cue ball does not leave the point of contact exactly 90 degrees from the path of the object ball, but it does leave at 90 degrees from the original contact point.

Obviously by Dave's video, this can be a very short journey. If the hit is soft and the cue ball is rolling the forward spin of the cue ball will make it deviate from that path almost instantly. Draw or backspin will tend to react slower because the cue ball will be traveling faster in relation to how much spin it has on it. It has to because the spin is against the cloth and that friction has to be overcome.

This just my opinion, and I may be completely wrong, and it works for me.

Royce Bunnell
www.obcues.com
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Keep in mind that everything on these video clips is accurate and perfect, based on the stroke applied, the cue shaft used, the cloth used, the ball weights, the conditions of the say,.. And yes it is not possible to slow roll a stun shot simply because to cause the CB not to spin or turn requires some speed, so in reality this never happens anyway in the real game.

Given this, I believe these clips provided an excellent illustration showing what can happen and what we can change which is far better than seeing nothing at all. I enjoyed the clips and it works for me too.
 
pete lafond said:
...Given this, I believe these clips provided an excellent illustration showing what can happen and what we can change which is far better than seeing nothing at all. I enjoyed the clips and it works for me too.
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Yes, this is what is important to me. Seeing a scratch shot and being able to alter the direction the cue ball will take to avoid scratching. I see a lot of players who don't know they can do this. They see a scratch shot and think their only option is to shoot a bank shot. So knowing these things gives me a big advantage over these other players.

My favorite example is an angled shot shooting the OB into the side pocket and CB will scratch into the corner. Well if you shoot it with stun, the CB will not scratch. This is a *very* handy thing to know.

A friend and I were teaching a beginner this. Well wouldn't you know it, she gets a similar shot, except in this case shooting with stun is a scratch and the thing to do is use follow to avoid scratching!
 
RBC said:
....The cue ball does actually leave the "contact point" at 90 degrees to that point. The object ball, however, does not. The friction from the contact of the two balls "throws" the object ball off line slightly. If you look at the angle between the cue ball and the object ball it is less than 90 degrees depending on the friction induce throw. It is, however, 90 degrees to the contact point.

So, the cue ball does not leave the point of contact exactly 90 degrees from the path of the object ball, but it does leave at 90 degrees from the original contact point....
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Yes, throw alters the object ball's path from a perfect 90 degree path from the tangent line, but inelasticity and unequal ball weights change the cueball's path from the original tangent line too. Inelasticity causes it to move forward of the line, while mismatched weight can do either. Usually the cueball tends to get smaller and therefore lighter, and the consequence of this is to have it move back of the tangent line. But for small cut angles and balls not too severely mismatched, the inelasticity wins and the cueball moves forward of it.

There is still another effect which Bob Jewett has noted, and which may not be insignificant. The finite contact time (i.e., more than infinitesimal due to compression) tends to increase the angle of the object ball's direction to more than 90 degrees from the original tangent line, if you fix this line at the precise moment when the balls just begin making contact. But it also, I think, alters the cueball's direction to more forward of this "original" tangent line too. My simple view suggests that it changes the cueball's direction by the same amount as the object ball's, so the separation angle is probably not affected by this particular phenomenon. In fact, I think the conservation laws demand it.

Jim
 
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