why ain't my tangent lines tangent?

[ShootingArts]

DVD's

Joe Villalpando's DVD's that I have do a super job of explaining tangents and how to adjust the cue ball's path with draw and follow too. My problem is that my baseline seems to be the slightest bit off. Tangent lines are plenty accurate enough for shape but I'm looking for the final adjustment that lets me make a ball well out from the pocket or after a carom and kick or bank playing one pocket. I note that you do say to hit slightly below center to stun the cue ball. It could be as simple as that I am not hitting the cue ball low enough normally although I have verified it skids into the first ball.

Hu

I believe the 90 degree tangent rule is accurate if the cue ball is hit slightly below centerball (or stun). Hitting with follow will cause the cue ball to deflect slightly less than 90 degrees and hitting more of a stun or draw shot causes deflection of slightly more than the standard 90 degrees.

Dr. Dave explains this and many more pool fundamentals on his Colorado State site. Check it out:

http://billiards.colostate.edu/

Dr. Daves' produced dozens of excellent videos showing pool concepts including the notorious 90 and 30 degree rules. Should be required viewing/understanding for anyone looking to improve their game above towards a B or A level.

 

[TATE]

Joe Villalpando's DVD's that I have do a super job of explaining tangents and how to adjust the cue ball's path with draw and follow too. My problem is that my baseline seems to be the slightest bit off. Tangent lines are plenty accurate enough for shape but I'm looking for the final adjustment that lets me make a ball well out from the pocket or after a carom and kick or bank playing one pocket. I note that you do say to hit slightly below center to stun the cue ball. It could be as simple as that I am not hitting the cue ball low enough normally although I have verified it skids into the first ball.

Hu

I guess were getting into physcis here, so I'll stay out of it, but it probably has to do with the force of the collision and friction between the balls. After all, if the object ball is the subject of throw, why wouldn't the cueball be subject to it as well? Since I know nothing of physics, I use my common sense, which gets me into heaps of trouble.

Anyway - I'm staying out of it. Exit stage left.


Chris

 

[mikepage]

I guess were getting into physcis here, so I'll stay out of it, but it probably has to do with the force of the collision and friction between the balls. After all, if the object ball is the subject of throw, why wouldn't the cueball be subject to it as well? Since I know nothing of physics, I use my common sense, which gets me into heaps of trouble.

Anyway - I'm staying out of it. Exit stage left.


Chris

Sorry, when the physics door got opened, the left stage door bolted shut ;-)

This has already been mentioned, but I'm not sure people know the correct use of the words elastic and inelastic when it comes to collisions. So with proper googanic apologies to Jude, I'll elaborate a bit.

Suppose we're given that the cueball absolutely, completely, SKIDS into the object ball.

And suppose we're given that the cueball and object ball are EXACTLY the SAME size and weight.

Can we expect an initial 90 degree angle between them? (Or, equivalently, if it's a full hit that the cueball stops dead?)

The answer is NO.

The problem is that both energy and momentum must be conserved in the collision.

So think of the full hit. Imagine the cueball slides full into the object ball at EXACTLY 10 mph, the cueball stops dead, and the object ball takes off at EXACTLY 10 mph. This scenareo seems to conserve both momentum and energy. Before the collision the energy and momentum were all in kinetic energy and momentum of the cueball. After the collision both are all in kinetic energy and momentum of the object ball. And all the numbers work out.


But wait! Did you hear a click sound? If so, then at least some energy went into vibrating your ear drum. Where did that energy come from?

This is a problem.

We have:
(energy before) = (energy after), or

(cueball energy before) = (object ball energy after + clicking-sound energy)

We would have had to create new energy to get the object ball moving at exactly 10 mph.

OK, so you might suggest the cueball is stopped dead and the object ball goes off at, say 9.9, mph, giving up some energy to account for the clicking sound. This--a little object-ball kinetic energy loss to account for the clicking energy--sounds like a reasonable solution UNTIL you think about conservation of MOMENTUM.

This solution fails to conserve momentum!

If the object ball is going at 9.9 mph, then the cueball must be going forward at 0.1 mph to conserve momentum.

The clicking sound energy is just one part of the kinetic energy that is lost in the collision. A collision that loses no kinetic energy at all (and would have no sound) is known as an ELASTIC collision. Energy loss is known as collisional INELASTICITY. Energy goes not only to the clicking sound but also also into the balls. If we checked carefully the temperatures of the balls a bit after the collision, we'd find they're warmer. The upshot of this is that the cueball moves forward of 90 degrees a bit.

 

[ShootingArts]

seems like we are headed towards a consensus

Seems like we are headed towards a consensus that the ninety degree rule is slightly flawed.

Is the cue ball moving forward slightly after the collision and then fairly rapidly assuming a ninety degree path or is the angle itself slightly different from ninety degrees from the moment of impact?

A thanks to all contributors, everyone's post is much appreciated.

Hu

 

[Patrick Johnson]

I'm not a fan of center-ball hits for this exact reason...I personally think they should be avoided.

It's often just the right spot to hit for a little follow.

It's just another spot on the CB; no need to avoid it. Sometimes you need to hit it, like any other spot.

pj
chgo

 

[Jal]

... After all, if the object ball is the subject of throw, why wouldn't the cueball be subject to it as well? ...

You are absolutely right, physics or no physics. But the "throw" of the cueball doesn't affect the 90-degree rule, since the cueball merely loses (or gains) speed along said tangent line. In other words, were it only for this, the tangent line would remain the tangent line that we've grown to know and love.

See Mike Page's explanation for what really shifts (slightly) the cueball away from the tangentential direction. There's also another effect due to the finite contact time and compression of the balls during a collision, but this is very tiny at typical cut angles and speeds.

Jim

 

[Jal]

...The clicking sound energy is just one part of the kinetic energy that is lost in the collision. A collision that loses no kinetic energy at all (and would have no sound) is known as an ELASTIC collision. Energy loss is known as collisional INELASTICITY. Energy goes not only to the clicking sound but also also into the balls. If we checked carefully the temperatures of the balls a bit after the collision, we'd find they're warmer. The upshot of this is that the cueball moves forward of 90 degrees a bit.

Nice explanation, but a slight bone of contention with this part.

According to what I'ver read (a couple of articles), most of the lost mechanical energy is due to the permanent deformation of a thin layer of surface atoms. Apparently the balls get forged into smaller and smaller spheres (apart from abrasion), until one day they disappear...or are lost.

Jim

 

[cuetechasaurus]

I have a relatively simple method of finding center. This is easier to do with a regular cueball, the measles ball makes it a little harder to see.

When you are taking your practice strokes, on your forward strokes, put the tip as close to the cueball as you can without touching it. If you have standard lighting for your pool table you should be able to see a reflection in the cueball of your ferrule. If you are not in the exact middle of the cueball, you will see it in the reflection that it appears slanted. When you find the exact center of the ball, you will see an equal and perfect reflection.

I have a tendency to think centerball is a little to the right of where it is, and using this helps, as long as I first recognize that my perception of center is off.

 

[Patrick Johnson]

Hu...Bob Jewett already answered your question with the most likely accurate response...you're simply NOT striking the CB at REAL center. MOST poolplayers have a misconception of where center is (i.e.: many pro players maintain that they break with a 1/2 tip of draw. They don't...they just don't know where accurate center is!). This is mostly due to a visual misperception by each individual, due to the elevation of the cue, or the chin being a certain distance above the cuestick. Even if your chin were directly on the cue (ala A. Fisher), your eyes would still be several inches above the surface of the table (resulting in your eyes perceiving center ball at an incline...which is where the misperception comes in). The only way to accurately "see" center is to lay your head sideways on the cloth. Of course we can't play this way. The radius of the CB is 1 1/8 inches from the surface of the table. A piece of chalk is exactly 1 1/8 inches in diameter. If you hold a piece of chalk on the cloth on a diagonal, the middle of your ferrule should be at the point on the chalk. If you do this with the CB and a stripe, turned horizontally, you'll see what I mean. I teach my students where center really is, and how to adjust for it. Hope this helps...

Scott Lee
www.poolknowledge.com

Masters chalk is a little bigger than 1-1/8 inch across, but that just means it works better, because the effective center of the cue ball - the spot the tip touches when the stick is pointed through the center of the cue ball's mass - is actually a little above the "horizontal equator" (because the stick's always at least a little tilted).

pj
chgo

 

[Rarelymisses]

It's good to see that many people have pointed out that the cue ball cannot depart on the tangent line because the collision is inelastic. Now to stir the pot a bit, the angular variation from the tangent line is not a constant. The more straight-on the shot, the greater the departure from the tangent line.

 

[seymore15074]

It's often just the right spot to hit for a little follow.

It's just another spot on the CB; no need to avoid it. Sometimes you need to hit it, like any other spot.

pj
chgo

Well, I mean I don't really avoid it...I just don't swear by it. I agree completely that it is just another spot on the CB.

 

[Bob Jewett]

It's good to see that many people have pointed out that the cue ball cannot depart on the tangent line because the collision is inelastic. Now to stir the pot a bit, the angular variation from the tangent line is not a constant. The more straight-on the shot, the greater the departure from the tangent line.

For those interested in the details and a graphical way to think about the imperfections in the 90-degree rule, here is a whole article about it that was written by Dr. George McBane and appeared in Billiards Digest in 2001:

http://www.sfbilliards.com/articles/2001-02.pdf

 

[Andrew Manning]

I don't have any experimental data about the degree of elasticity of pool-ball collisions, but I don't think the inelasticity of the collision is significant enough to have a visually noticeable effect.

I think the more likely culprit is that you're shooting with a slightly elevated cue, and so your soft stun shots go 90 degrees, but your hard stun shots involve the CB hitting the OB while slightly airborne. The 90-degree rule only takes two dimensions into account. If there's a third dimension (vertical) to the direction of the collision's impulse, there's going to be a third dimension to the directions of the balls afterwards.

Of course, there's gravity in one direction, and the shock-absorbing qualities of the cloth in the other, so the third dimensional component disappears pretty quickly after contact (the CB stops bouncing and starts rolling/sliding), but the energy that went in that direction has already been subtracted from the energy that could have made the CB go the right two-D direction.

So basically, and airborne CB will go forward of the tangent line it would have had were it not airborne. If the problem you're seeing is more pronounced on hard shots, concentrate on keeping your cue level on hard shots. The tangent line won't be the only benefit of this levelness; it will also minimize swerve from intentional or unintentional side-spin.

-Andrew

 

[Robert Raiford]

However with medium or firm speed the cue ball either carries forward a bit before taking off on the tangent or it comes off the object ball at slightly less than 90 degrees.

Ball inelasticity is definitely noticeable at the table, especially with older balls. I find I have to adjust by 3-5 degrees to account for it in 3-cushion depending on ball condition.

However, since you notice the difference only on harder shots, I suspect that Andrew Manning is right and it's the increased speed that is causing the cue ball to be airborne at contact and carom less than you expect. This should be more pronounced the closer you are to the object ball, which is amusing when you start putting the cue ball closer to the object ball to try to increase accuracy. The "carries forward a bit before taking off on the tangent" part suggests that you err on the side of too much draw rather than too little when attempting a stun shot.

Bob's test is good because it significantly reduces the effect of your slightly-elevated cue on the outcome.

Robert