If the tip and cue ball only touch so briefly, why the difference in power?

[Andrew Manning]

I like to quibble the details:

1. The energy from the cue is vectored. One in forward energy and one is rotational.

True (although I've not heard vector used as a verb before).

2. The farther you hit from the center, the more rotational energy(spin) you get and less forward energy(cb speed).

True.

The total energy doesn't change, just proportions.

Not really true. If all of the cue's energy was transferred to the ball, this would be true. But the cue keeps moving forward after it hits the ball, meaning it keeps some of its energy. Furthermore, I'm pretty sure (don't have any research to back this up, but I am pretty sure) that the cue retains more of its energy (transfers less) in an off-center hit than a center-ball hit. In other words, the cue doesn't slow down as much as when it hits the ball off-center, and so for the same stroke, the off-center ball gets less total energy.

3. Cue speed and mass determine the total energy, unless you have a tight grip.

Cue speed and mass determine the total energy of the CUE, but as I pointed out above, that's not the end of the story. Also, many smart people argue that the tight grip doesn't really matter, because the flesh of your hand is always going to be flexible enough that your arm mass does not act as part of the cue mass. I think I'm on their side of that argument; considering the briefness of the contact, I think the by the time your flesh reaches the end of its flex and your arm's momentum can augment the momentum transfer from cue to ball, the ball has already left the tip and your arm is really only affecting the follow-through.

-Andrew

 

[Patrick Johnson]

considering the briefness of the contact, I think the by the time your flesh reaches the end of its flex and your arm's momentum can augment the momentum transfer from cue to ball, the ball has already left the tip and your arm is really only affecting the follow-through.

I think this is right, and I think they even tried lining the robot's grip with bubble wrap to simulate this during the Jacksonville Video Project. Maybe Bob Jewett will tell us about it.

pj
chgo

 

[jsp]

I like to quibble the details too.

1. The energy from the cue is vectored.

This wording is not precisely correct. Energy is not a vector...it's only a scalar quantity. However, force and momentum are both vector quanitities.

2. The farther you hit from the center, the more rotational energy(spin) you get and less forward energy(cb speed). The total energy doesn't change, just purportions.

That depends. If you hit the CB such that you impart the same amount of energy into the CB, then you're correct. Total energy is conserved, and you just change the relative amounts of rotational and translation energy change, while the total energy of the CB remains the same.

However, if you hit the CB such that you apply the same force impulse to the CB, then you'd get the same translational energy (disregarding the effects of squirt) for any case, but you'd have more rotational energy the further you hit the CB off center (assuming you don't miscue). That means, for the same force impulse (force times time) applied to the CB, you'd get more total energy the further you hit the CB away from center. I posted a somewhat trick-question quiz a couple weeks back regarding this scenario.

3. Cue speed and mass determine the total energy, unless you have a tight grip.

I admit that I've always thought that the grip hand does have a non-negligible effect on the total mass of the cue at impact. As Andrew stated, many smart people think that your hand contributes practically nothing to the the cue's mass. They're probably right, but I'd like to see lab data before I completely admit it.

 

[KMRUNOUT]

There is no magic to it. The tip is on the cue ball for such a small amount of time, all that matters is where it hits the cue ball and how hard. It doesn't matter if you get the tip to the cue ball all herky jerky or smoothly, only that you make good contact (don't miscue) where you need to in order to execute the shot.

I would submit that you are losing control when you try to hit the cue ball hard... you are not hitting the cue ball where you want to. With the slower stroke you're hitting your target, or getting closer to it anyway. It's not easy to hit the cue ball where you want when you shoot hard, that's the trick.

You probably won't believe this but follow through doesn't matter either, assuming you do all the rest of it right.

This is it right here in a nutshell. Great post. I know that hitting the ball softly but in the desired location can make all the difference in the world. For example, a nice smooth medium-soft stroke through the very bottom, maximum low position can draw the ball 8 feet easy on Simonis cloth. A very hard stroke through a 1/2 tip low may not achieve this.

One way to get both (hard stoke and accurate tip placement) is to RELAX your arm. Do not try to muscle the cue through the ball. Let your arm just hang and swing. But swing FAST. An image I've tried that works well is to simply imagine the tip moving fast at contact, and to imagine that zero effort is required on my part to do that. Relax my wrist, relax my arm, especially relax my HAND (because tightening your grip can lift up the back of the cue and drop your tip, or vice versa-depending on your hand and grip), and just swing effortlessly through the ball, but FAST.

Hope this helps.

Kerry

 

[Jal]

...Not really true. If all of the cue's energy was transferred to the ball, this would be true. But the cue keeps moving forward after it hits the ball, meaning it keeps some of its energy. Furthermore, I'm pretty sure (don't have any research to back this up, but I am pretty sure) that the cue retains more of its energy (transfers less) in an off-center hit than a center-ball hit. In other words, the cue doesn't slow down as much as when it hits the ball off-center, and so for the same stroke, the off-center ball gets less total energy.

Yes, this is very true. Jsp remarked earlier that you would have more energy if you applied the same impulse (average force x time) on an off-center hit. But as he indicated, there is a catch. For the same cue speed as a centerball hit, the impulse is reduced enough so you in fact get less energy into the cueball.

Nevertheless, hitting at about 1/4R above center results in the cueball having the most speed, and therefore the most energy, if and when it reaches natural roll (not sliding anymore). But the key word here is 'if'. On a brisk shot, the cueball never reaches full roll even if travelling the length of the table.

Cue speed and mass determine the total energy of the CUE, but as I pointed out above, that's not the end of the story. Also, many smart people argue that the tight grip doesn't really matter, because the flesh of your hand is always going to be flexible enough that your arm mass does not act as part of the cue mass. I think I'm on their side of that argument; considering the briefness of the contact, I think the by the time your flesh reaches the end of its flex and your arm's momentum can augment the momentum transfer from cue to ball, the ball has already left the tip and your arm is really only affecting the follow-through.

Also very true.

Jim

 

[Cornerman]

I think many people on this board are familiar with those high speed films of the cue hitting the cue ball. I think it was Bill Porter who said that no matter what they did during those tests they could not increase the contact time between the tip and the ball.

If that is the case, where does the extra power in a stroke come from? I think most people would agree that it doesn't come from hitting the ball harder. For myself, I know that I get the most stroke on a ball when I'm hitting quite softly. It feels like I'm just moving my hand to my shoulder, no more effort than that.

So, if you can't increase contact time, and hitting harder doesn't seem to help, where the HECK does stroke come from, with regards to physics?

We'll always have a problem when a poster asks a question that has a physics answer, but the poster doesn't really understand the physics terms.

Just to be completely confusing... given the same amount of work, the smaller the time means greater power. But, that really doesn't mean anything to anyone for practical purposes either. LOL!!!!

Spin and shit is determined by tip offset and speed of the tip just at the point when it contacts the cueball. I think most people think about "acceleration," but it's the speed that's more important. In fact, most people will hit the cueball close to zero acceleration (constant velocity). But, again, those are physics terms that won't mean anything to the non-physics person.

Fred

 

[mikepage]

[...]

Spin and shit is determined by [...]

Hey Fred, if you're going to use new physics words, would you at least define them ?

sheesh...

 

[Bob Jewett]

... I think the by the time your flesh reaches the end of its flex and your arm's momentum can augment the momentum transfer from cue to ball, the ball has already left the tip and your arm is really only affecting the follow-through. ...

That's correct. There were a bunch of video sequences in the Jacksonville Project that studied the speed of the cue stick during a shot. The cue stick is slowed down by 50% during tip-to-ball contact and then recovers slowly to about 80% of its original speed as the moving hand re-accelerates it. Of course, the ball is long gone while that happens. From the times involved, it's possible to figure out that the flesh of the hand is about 100 times softer than the cue tip. It is that difference in softness that keeps the hand (and arm and body) from having any significant effect on the shot. Of course, it is your hand and arm and body that gets the cue stick up to the right speed and moving in the right direction to make the shot, but it is the cue stick that does all the work during the shot and your hand/arm/body cannot help during tip-to-ball contact.

 

[Alex Kanapilly]

That's not some mystical "stroke" ability that nobody can describe or understand. If you have trouble getting draw, it's because you don't hit the ball low enough, and that's often because your subconscious won't let you.


Not at all what I was talking about. I have no problem drawing a ball the length of the table. Sometimes though I stroke the ball and it is completly different. Like someone said, I believe it has to do with acceleration at contact.

You can test this. Use a striped ball (just for any easy way to determine where you actually hit the ball) and measure where you leave a chalk mark on a hard draw shot that doesn't go as far as you thought it should. Then compare it to a nice "soft" table length draw shot.

I think you will find that on the shots that don't draw back as hoped, you're hitting the ball higher than you thought.

 

[crosseyedjoe]

I think many people on this board are familiar with those high speed films of the cue hitting the cue ball. I think it was Bill Porter who said that no matter what they did during those tests they could not increase the contact time between the tip and the ball.

If that is the case, where does the extra power in a stroke come from? I think most people would agree that it doesn't come from hitting the ball harder. For myself, I know that I get the most stroke on a ball when I'm hitting quite softly. It feels like I'm just moving my hand to my shoulder, no more effort than that.

So, if you can't increase contact time, and hitting harder doesn't seem to help, where the HECK does stroke come from, with regards to physics?

Force = Mass x Acceleration, since the mass of the cue stick is constant then the acceleration affects the force your delivery.

 

[Deadon]

We'll have to agree to disagree. I believe it has nothing to do with acceleration and everything to do with tip/cue ball accuracy. However, the way you stroke does affect your stroke accuracy, and accelerating evenly ("smoothly") throughout the stroke sounds like it might help achieve greater accuracy and consistency.

The problem I have with the idea is that it redirects your attention toward one means of accomplishing your goal and away from the goal itself.

pj
chgo

Patrick;

If you accelerate through the ball you impart more energy on it. More energy makes it spin faster if hit away from the center and may not increase the forward speed as much purportionally.

Mike

 

[Bob Jewett]

Force = Mass x Acceleration, since the mass of the cue stick is constant then the acceleration affects the force your delivery.

Just to help with the confusion, the above statement is nearly meaningless from the technical, physics perspective. What I think the OP wants is more speed in the cue stick. That can only be obtained by accelerating the stick, since it starts from zero speed at the back of the backswing. Remember: acceleration is a change in velocity (or speed). The acceleration is obtained by applying a forward force to the stick with your hand. At the instant of contact, any acceleration of the stick due to your hand pushing forward at that instant is of no consequence.

 

[Bob Jewett]

.. If you accelerate through the ball you impart more energy on it. More energy makes it spin faster if hit away from the center and may not increase the forward speed as much purportionally. ..

I think that Pat was trying to point out this truth: Any acceleration at the instant of tip-to-ball impact is wasted. The important acceleration occurs before impact. It is the velocity of the stick at impact that is important, not the acceleration at that instant.

You seem to be confusing velocity and acceleration. They are very different things. If only acceleration were important, backstroke would be limited to a couple of inches since the highest acceleration happens in the first half of the forward motion.

 

[crosseyedjoe]

Just to help with the confusion, the above statement is nearly meaningless from the technical, physics perspective. What I think the OP wants is more speed in the cue stick. That can only be obtained by accelerating the stick, since it starts from zero speed at the back of the backswing. Remember: acceleration is a change in velocity (or speed). The acceleration is obtained by applying a forward force to the stick with your hand. At the instant of contact, any acceleration of the stick due to your hand pushing forward at that instant is of no consequence.

SUPERSTAR, jsp, can you explain this to me.

Remember: acceleration is a change in velocity (or speed). The acceleration is obtained by applying a forward force to the stick with your hand.

I always thought you move something by transferring kinetic energy to the object and the result of the acceleration times the mass of the object is the force.




But damn, way to complicate a simple answer.

 

[Patrick Johnson]

Originally Posted by Bob Jewett
Just to help with the confusion, the above statement is nearly meaningless from the technical, physics perspective. What I think the OP wants is more speed in the cue stick. That can only be obtained by accelerating the stick, since it starts from zero speed at the back of the backswing. Remember: acceleration is a change in velocity (or speed). The acceleration is obtained by applying a forward force to the stick with your hand. At the instant of contact, any acceleration of the stick due to your hand pushing forward at that instant is of no consequence.


SUPERSTAR, jsp, can you explain this to me.

He's saying that the tip bounces off the cue ball on impact, your fleshy hand can't stop it from doing that, and the cue ball is long gone before you can possibly get your tip back up to speed. Therefore acceleration's only contribution (and it's an important one) is getting the stick up to speed before impact.

pj
chgo

 

[crosseyedjoe]

He's saying that the tip bounces off the cue ball on impact, your fleshy hand can't stop it from doing that, and the cue ball is long gone before you can possibly get your tip back up to speed. Therefore acceleration's only contribution (and it's an important one) is getting the stick up to speed before impact.

pj
chgo

Now I'm in nowhere land.

 

[Patrick Johnson]

...acceleration's only contribution (and it's an important one) is getting the stick up to speed before impact.

So what? Well, I think knowing this tells you not to "pace" the acceleration of your stroke so you have some left over to apply directly to the cue ball. It tells you that you get the most use of your acceleration by using it all up in the part of the stroke before impact.

In this special sense, the sense of how to time your stroke to deliver the most power to the cue ball, you don't stroke *through* the ball but *at* it. (This doesn't contradict the idea of "following through" for form.)

pj
chgo

 

[Jal]

So what? Well, I think knowing this tells you not to "pace" the acceleration of your stroke so you have some left over to apply directly to the cue ball. It tells you that you get the most use of your acceleration by using it all up in the part of the stroke before impact.

This is more likely untrue than true, imo. We'll never know for sure unless we get a chance to see many accelerometer graphs of players' real life strokes.

But if "accelerating through" results in a force vs time curve that is simply stretched out in time, to some reasonable approximation, then that does get you more cue speed. This kind of curve represents some "pacing" early on in the stroke.

If the rise time of the curve is about the same as when not "accelerating through", (ie, no pacing takes place), but the acceleration is maintained at a larger level throughout the rest of the stroke, then of course even more cue speed will be realized.

Jim

 

[Patrick Johnson]

This is more likely untrue than true, imo. We'll never know for sure unless we get a chance to see many accelerometer graphs of players' real life strokes.

But if "accelerating through" results in a force vs time curve that is simply stretched out in time, to some reasonable approximation, then that does get you more cue speed. This kind of curve represents some "pacing" early on in the stroke.

If the rise time of the curve is about the same as when not "accelerating through", (ie, no pacing takes place), but the acceleration is maintained at a larger level throughout the rest of the stroke, then of course even more cue speed will be realized.

Jim

This is more likely untrue than true, imo.

Why?

if "accelerating through" results in a force vs time curve that is simply stretched out in time, to some reasonable approximation, then that does get you more cue speed.

What does this mean? Any acceleration that occurs after contact is wasted, isn't it?

pj
chgo

 

[arsenius]

Thanks again to everyone who has replied to this thread. I've really learned a lot. To be honest when I asked this question I wasn't 100% certain about what I was asking, but I tried to ask it as best I could anyways. Now I understand my question better and the answer.

So, the most powerful stroke will be the one that is going the fastest when it hits the ball as far from the center as possible (without miscuing).

I'm sure this hasn't been done, but has this been quantified in any way? I think it would be fascinating to find out something like "To draw the cueball ____ meters, you must stroke the cue at at least ____ km/hour and ____ mm off of center" or something like that. If you did the test under "optimal" conditions you could always assume that your own conditions were worse. It would be really cool to find out how much faster Larry Nevel's arm can move than mine too!