Why does the cue ball do that?

G

gwvavases

AzB Silver Member
Silver Member
When players with a great break do so, their cue balls smash the rack, bounce back, and then "surge" forward a couple of inches to park nicely in the center of the table. How does that happen?

Everything I've read and been taught (and from what I've observed pro's doing), tells me to hit the cue ball low or maybe a tip below center. It seems to me that with the amount of force applied, some amount of draw should be imparted on the cue ball. Yet, when the pro's break, and the cue ball bounces back, the only thing that I can think of that can account for that little "surge" forward a couple of inches would be a tiny amount of follow.

So...how does that happen?

George in VA
 
My guess would be that the cue ball picks up some "follow" or top spin when coming into contact with the rack.

The cueball when hit so hard actually leaves the surface of the table slightly, which causes it to jump off the headball when you break.

We all know that when two balls come into contact at an angle (when cutting a ball), they impart english on one another in a gear-like effect.

I imagine this is the same thing that happens on the break. But instead of english imparted along the vertical axis (i.e. left/right engligh) you get spin imparted along the horizontal axis (i.e. top/bottom). Since the cueball strikes the rack while its still in the air, it contacts the headball above the head ball's horizontal axis. The gear-effect (one would assume) would impart "top" english on the cueball from the perspective of the breaker.
 
The total effective mass of the rack is much higher (9, 10, or 15 times as big in the ideal case) than a single ball (the cueball). Therefore, upon impact, the cueball bounces back off the pack (something light bounces off something heavy). The tiny bit of topping (follow) counteracts this bounce-back and keeps the cueball in the middle of the table or close to the headspot.

Here's another instance where this can cause grief if you don't understand the principle. When two balls are frozen together, and you hit them on the line of contact with a cue ball with backspin, you'll draw the cueball a fair bit farther than normal. .... Why ? Because the (single ball mass) cueball bounces back (off the two ball mass of the plant) and has some momentum in the direction of the draw before the backspin kicks in. The problem is the opposite with topping/follow. Because the cueball is moving backwards right after impact, some of the topspin goes to stop the cueballs backward speed, leaving less spin to take it forward. The problem becomes worse if there are three balls tight together (the effective mass is higher). Set them up sometime, it's rather interesting and a great way to impress the ladies with your power draw stroke :D


Dave
 
Hey Dave,

There two things happening here:

1. The cue-ball is bouncing back.

As gwvavases points out, this is because the total mass of the rack is greater than the cue-ball. Hence, no matter what spin you use, assuming you hit the 1-ball full and hard, the cue-ball will rebound.

2. The cue-ball then surges forward.

What happens after the cue-ball settles on the table is completely based on how much top or bottom you imparted on the cue-ball when you broke. If there is no spin whatsoever, the cue-ball should move little (or not at all). If you have a lot of draw, it will rocket right back toward one of the head-corner pockets completely unobstructed. In all liklihood, you'll have two object balls racing behind it just to remind you that things would have been great had you parked whitey. If you have a lot of follow on your break, it will create a ridiculous arc, avoid 42 collisions and inevitably scratch right in one of the foot-corner pockets. If you scratch this way, the 9-ball is a 98% favorite to drop, too.
 
Jude Rosenstock said:
Hey Dave,

There two things happening here:

1. The cue-ball is bouncing back.

As gwvavases points out, this is because the total mass of the rack is greater than the cue-ball. Hence, no matter what spin you use, assuming you hit the 1-ball full and hard, the cue-ball will rebound.

2. The cue-ball then surges forward.

What happens after the cue-ball settles on the table is completely based on how much top or bottom you imparted on the cue-ball when you broke. If there is no spin whatsoever, the cue-ball should move little (or not at all). If you have a lot of draw, it will rocket right back toward one of the head-corner pockets completely unobstructed. In all liklihood, you'll have two object balls racing behind it just to remind you that things would have been great had you parked whitey. If you have a lot of follow on your break, it will create a ridiculous arc, avoid 42 collisions and inevitably scratch right in one of the foot-corner pockets. If you scratch this way, the 9-ball is a 98% favorite to drop, too.
Click to expand...

:D another shining example of how %98 of statistics are made up on the spot.:D
 
StevenPWaldon said:
My guess would be that the cue ball picks up some "follow" or top spin when coming into contact with the rack.

The cueball when hit so hard actually leaves the surface of the table slightly, which causes it to jump off the headball when you break.

We all know that when two balls come into contact at an angle (when cutting a ball), they impart english on one another in a gear-like effect.

I imagine this is the same thing that happens on the break. But instead of english imparted along the vertical axis (i.e. left/right engligh) you get spin imparted along the horizontal axis (i.e. top/bottom). Since the cueball strikes the rack while its still in the air, it contacts the headball above the head ball's horizontal axis. The gear-effect (one would assume) would impart "top" english on the cueball from the perspective of the breaker.
Click to expand...

I think you're probably right about this, because I'm talking about breaks that are very obviously struck with LOW at very high speed. There's not an awful lot of follow imparted on the cue ball after impact, or it would bounce back, then run straight ahead through the rack. That's actually a pretty cool looking break, but it gives you NO control over where whitey ends up. So, I think you're right: the cue ball hits the head ball with backspin (coz it was hit that way) while it's airborne, and the collision imparts just enough "follow" through the gearing action to stop the cue ball's backward spin giving it just a slight enough amount of forward spin to cause the "surge".

I usually break with a center-ball hit, and I don't get nearly as much speed as the pro's. Don't try, really, coz I tend to lose control. I'd much rather have whitey "pop and squat" (i.e. hit the rack, bounce up and back a little, and park right there). If I break hard with follow, it will bounce back a little, then charge ahead plowing through the rack. If I break with low, it does just as Jude says: it comes screaming straight back at me without expending its energy on the rack. Either way, it's a total crapshoot as to where it ends up. Can't have that!

Thanks for your replies, guys.

George in VA
 
I always try to hit the cue ball as center as possible and the head ball as center as possible. I have found that that is the easiest way to control the cue ball after impact. Just my 2 cents.

BVal
 
George;

The answer to your question is they cue just above center, not below, not at the center. Then the cue ball behaves as Jude explained. Contact with the rack does not impart any high or low on the cue ball. There are many different breaks. Some cut breaks use low outside, some inside. It all depends at what angle you contact the head ball. By angle, I mean full, or to either side hit.

Also you will sometimes see the cue ball back up because the it has left the surface of the table and is coming down as it hits the head ball. Which makes it roll back away from the hit.
 
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DaveK said:
The total effective mass of the rack is much higher (9, 10, or 15 times as big in the ideal case) than a single ball (the cueball). Therefore, upon impact, the cueball bounces back off the pack (something light bounces off something heavy). ...
Click to expand...
While the effective mass of the rack is higher, it is not "much higher" than the mass of one ball. If it were, the cue ball would bounce back with close to the speed it goes into the rack with. (The bounce-back fraction of speed is (M-m)/(M+m) where m is the mass of the ball and M is the effective mass of the rack.) From the geometry of a head-on break, and assuming the collisions happen one at a time and no energy is lost in the collisions, the effective mass of the rack looks to be about 9/7 of the mass of one ball, and the cue ball should bounce back with 1/8 of its incoming speed.
 
Not sure of the physics and the like...

But some top (just above center) with a solid stroke (follow through) will send the ball though the remaining rack one more time after the rebound.

I've played the 8 a few times like this over the years...but the cue ball wants to leave the table if you misstroke it ime.
 
StevenPWaldon said:
My guess would be that the cue ball picks up some "follow" or top spin when coming into contact with the rack.

The cueball when hit so hard actually leaves the surface of the table slightly, which causes it to jump off the headball when you break.

We all know that when two balls come into contact at an angle (when cutting a ball), they impart english on one another in a gear-like effect.

I imagine this is the same thing that happens on the break. But instead of english imparted along the vertical axis (i.e. left/right engligh) you get spin imparted along the horizontal axis (i.e. top/bottom). Since the cueball strikes the rack while its still in the air, it contacts the headball above the head ball's horizontal axis. The gear-effect (one would assume) would impart "top" english on the cueball from the perspective of the breaker.
Click to expand...



perfect explanation!
 
I like to the hit the ball with a stun shot with the cueball as far to either side as comfortable (near the rail). On each break I move the cue ball a little right or left and watch the wing balls as they move toward the corners. Once a wing ball goes in, I break from the same place during the rest of the match.

If the ball surges forward, you are too far above center.
 
Bob Jewett said:
...From the geometry of a head-on break, and assuming the collisions happen one at a time and no energy is lost in the collisions, the effective mass of the rack looks to be about 9/7 of the mass of one ball, and the cue ball should bounce back with 1/8 of its incoming speed.
Click to expand...
Could you expand on this?

JIm
 
Jal said:
Could you expand on this?

JIm
Click to expand...
Assume that the balls are all slightly separated and that the collisions occur one at a time. You will get the head ball coming back towards the cue ball at a velocity that is fairly easy to calculate. This is just a series of two-body problems.

If you assume a tight rack, the problem is quite a bit more complicated and I'm pretty sure you have to do numerical integration to find the solution to the 10-body problem.
 
StevenPWaldon said:
I imagine this is the same thing that happens on the break. But instead of english imparted along the vertical axis (i.e. left/right engligh) you get spin imparted along the horizontal axis (i.e. top/bottom). Since the cueball strikes the rack while its still in the air, it contacts the headball above the head ball's horizontal axis. The gear-effect (one would assume) would impart "top" english on the cueball from the perspective of the breaker.
Click to expand...

I could be wrong, but I can't imagine that the gear effect is very significant compared to whatever follow is put on the cueball initially.
 
Bob Jewett said:
Assume that the balls are all slightly separated and that the collisions occur one at a time. You will get the head ball coming back towards the cue ball at a velocity that is fairly easy to calculate. This is just a series of two-body problems.
Click to expand...
Thanks Bob. I get it now. Your figure of only 9/7 was quite a surprise (although it shouldn't have been given the relatively slow rebound speed.)

Jim
 
Not only can you not assume that all collisions happen at different times (if the rack is tight), but you cannot also assume that the collisions are perfectly elastic (when using 15 balls; two ball collisions are different). The effective mass is nowhere near 9/7 the mass of one ball.

If you would like, I could waste like 3 hours of my life proving this by going through the physics of the collision, but I assume everyone can imagine that 15 balls acting like the mass of ~1 ball is counterintuitive.

As for the gear theory, it is true that follow put on the cue ball would have a greater affect on it continuing to roll forward, but a cue ball struck dead center, after leaving the table and striking the head ball while off the surface would also have some induced follow due to the off-center collision on the head ball.
 
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PKM said:
I could be wrong, but I can't imagine that the gear effect is very significant compared to whatever follow is put on the cueball initially.
Click to expand...
You're probably right. The cueball picks up some follow even if hit centerball. If the contact time with the cloth is significantly greater than the duration of contact with the tip, the cueball will actually pick up more follow on that initial bounce immediately after being struck by the cue, than if it were to slide uniformly along the cloth thereafter. It's what happens during that first rebound off the cloth that's important. The question is how long it takes it to compress and then decompress the cloth compared to the tip.

Jim
 
StevenPWaldon said:
My guess would be that the cue ball picks up some "follow" or top spin when coming into contact with the rack.

The cueball when hit so hard actually leaves the surface of the table slightly, which causes it to jump off the headball when you break.

We all know that when two balls come into contact at an angle (when cutting a ball), they impart english on one another in a gear-like effect.

I imagine this is the same thing that happens on the break. But instead of english imparted along the vertical axis (i.e. left/right engligh) you get spin imparted along the horizontal axis (i.e. top/bottom). Since the cueball strikes the rack while its still in the air, it contacts the headball above the head ball's horizontal axis. The gear-effect (one would assume) would impart "top" english on the cueball from the perspective of the breaker.
Click to expand...

Mental gymnastics, if that is true, then as the cue ball came down on the head ball, it would acquire backspin from the perspective of the breaker, and roll back toward the center of the table. See my earlier post.
 
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