The Myth of Top Spin???

... the speed of the surface of the cb can never be greater than the speed of the cue tip. Not sure what people are refering to as "over spin". Would that be any thing like being to throw a ball faster than your hand can move?
Sigh. More false statements.

When struck in the center by a cue stick, the cue ball moves away at about 130% of the incoming speed of the stick.

When playing with spin, the edge of the cue ball is moving away from you at something like twice the speed of the stick at the end of stick-ball contact, depending on how far out on the ball you hit.

Not that very many of these ideas have any application in play, but it's startling how many bogus statements we're seeing in this thread. I think it comes from too many kids being raised on Road Runner/Coyote physics.

In fact is it possible to shoot with a high enough stroke that the bottom of the cue ball is moving backwards (!!! yes, really!!!) as the center of the cue ball moves away from you.
 
DoubleA said:
Check your physics texts, you will see that the flange actually has an easterly velocity. When the train completes it's journey, all parts of the train will have moved to the east.
No. The bottom of the flange is always moving backwards. But the bottom of the flange is not a fixed set of atoms. All parts of the train are moving east on average, but at times some of them are moving west, and the bottom of the flange is always moving west.
 
mikepage said:
OK fair enough.

How's this

A point on the regular part of the wheel moves forward like a hopping frog.

A point on the flange moves forward like a hopping frog who takes a step backwards after each hop --like a retrograde motion

I love this !!!

How about, a point on the flange (which at some point during it's rotation goes below the track) is like an eastward hopping frog who places one foot backwards each time to start his next hop. The one foot backward is a western motion.

Oh, never mind. I think a graph is better.

Fred
 
Cornerman said:
I love this !!!

How about, a point on the flange (which at some point during it's rotation goes below the track) is like an eastward hopping frog who places one foot backwards each time to start his next hop. The one foot backward is a western motion.

Oh, never mind. I think a graph is better.

Fred

I'd prefer a video of a frog hopping the wrong way on an airport walkway
 
3andstop said:
Yeah, I'm sure that plays into it also, but if you've ever force followed a secondary break shot into the rack in straight pool, at times you can actually see the cue ball sitting there overspinning like a race car peeling out with his front brakes on. :) Then, after parting the mass somewhat, the CB proceeds forward with what is left of the rotational momentum.

http://www.zippyvideos.com/9225341667625366/overspin/
 
I would be interested in knowing what physics books you get your information from. You can never store more energy than you input. This is laughable, check your laws of motion. Since these laws and logic seem to have been replaced with a need to be "right", I respectfully submit this as my last post.
The physics is in pretty much any high school text book. The particular fact about the cue ball moving faster than than the cue stick is explained by Bob Byrne in his "Advanced Technique" book.

Two objects colliding with unequal masses is a standard situation in physics that is often used to introduce changes of frames of reference to simplify the problem. Here is an example of how that works. Suppose we want to know what would happen to a stationary tennis ball when struck by a freight train. Imagine some kids have dangled a ball from a bridge in the path of a train. How fast will the tennis ball be going after being hit by the 60MPH train (the bottoms of the flanges of which are moving backwards)?

The answer, ignoring any loss of bounce in the ball and wind resistance, is that the ball will be going 120MPH right after the collision.

How to see this easily? First imagine a stationary train parked on a siding. The kids have made a tennis-ball cannon that can shoot the ball at 60 MPH. They shoot it at the front of the train.The train's speed doesn't change perceptibly -- it is unmoved by the tennis ball -- but the ball bounces off at 60MPH in the other direction, more or less. I hope this result is comfortable for everyone. If not, please try throwing a tennis ball at a heavy object like a freight train or a concrete wall. It really does bounce off.

Now, in the case of the moving train, we simply "change frames of reference" and get on the moving train with the engineer. To us, it looks like the dangling tennis ball is coming at us at 60 MPH. After the collision, the ball bounces away from us at 60 MPH relative to us on the train, more or less. That means that the ball is moving 120 MPH relative to the ground.

The point of this discussion is to show that if a heavy object hits a stationary light object, the light object will be moving faster than the heavy object was. The case of the train and the tennis ball was extreme. For objects of closer weights, the change is not so large.

When a cue stick hits a cue ball, it slows to 50% of its initial speed, and the cue ball leaves at 150% of the initial stick speed, similar to the tennis ball being struck by the train. (Actually, because the tip is not perfectly springy, the number will be closer to 130%).
 
Jal said:
Doesn't it max out at about .58?

Jim
I thought that the stick could contact a trapped ball at 45 degrees and not slip. Doesn't that imply a coefficient of 1.0? In any case, the tip-ball coefficient of friction is normally a lot larger than the ball-cloth coefficient. More importantly, I think, is that the tip-ball forces are much, much larger than the ball-cloth forces during most tip-ball contacts.
 
Bob Jewett said:
No. The bottom of the flange is always moving backwards. But the bottom of the flange is not a fixed set of atoms. All parts of the train are moving east on average, but at times some of them are moving west, and the bottom of the flange is always moving west.[/QUOTE]

Let me ask you this...and a yes or no would be fine.

There is a difference IMHO between "moving west" and "moving in a westerly direction."

If the train tracks were coated with a substance as slick as ice, then the wheels under a stopped engine would spin when the throttle is advanced.

In that event a tick mark on the bottom of the flange would actually move to a position on earth west of where it was when at rest.

But in your riddle, the train is moving at 60 mph and not slipping on the track.

Therefore, while the tick mark on the bottom of the flange would move in a westerly direction relative to a tick mark on the top of the flange which is moving east...would it not be true that while the bottom tick mark is rotating in a westerly direction, it is in fact, moving east relative to its original position on the earth?

Thanks,
Jim
 
av84fun said:
Bob Jewett said:
No. The bottom of the flange is always moving backwards. But the bottom of the flange is not a fixed set of atoms. All parts of the train are moving east on average, but at times some of them are moving west, and the bottom of the flange is always moving west.[/QUOTE]

Let me ask you this...and a yes or no would be fine.

There is a difference IMHO between "moving west" and "moving in a westerly direction."

If the train tracks were coated with a substance as slick as ice, then the wheels under a stopped engine would spin when the throttle is advanced.

In that event a tick mark on the bottom of the flange would actually move to a position on earth west of where it was when at rest.

But in your riddle, the train is moving at 60 mph and not slipping on the track.

Therefore, while the tick mark on the bottom of the flange would move in a westerly direction relative to a tick mark on the top of the flange which is moving east...would it not be true that while the bottom tick mark is rotating in a westerly direction, it is in fact, moving east relative to its original position on the earth?

Thanks,
Jim

yes --

as in yes it would not be true
 
mikepage said:
yes --

as in yes it would not be true

Thanks mike. Sorry to have asked the question in the negative!

BTW I can't get your overspin video to play. Anyone else having difficulty?

Jim
 
Question...

If you strike downward on the front of the cue ball, won't it deflect backward for a moment before changing direction and accelerating foreward?

Would this be an example of "over spin"?

This would lead me to believe that "over spin" is indeed possible, though my stroke is not good enough to produce this with a level cue.
 
jongreve said:
Question...

If you strike downward on the front of the cue ball, won't it deflect backward for a moment before changing direction and accelerating foreward?

Would this be an example of "over spin"?

This would lead me to believe that "over spin" is indeed possible, though my stroke is not good enough to produce this with a level cue.

i figure that'd just be backspin in the other direction...

-s

p.s. can someone show me a video of, if not overspin as such, a CB travelling faster than it comes off the stick in any situation?
 
Last edited:
av84fun said:
Therefore, while the tick mark on the bottom of the flange would move in a westerly direction relative to a tick mark on the top of the flange which is moving east...would it not be true that while the bottom tick mark is rotating in a westerly direction, it is in fact, moving east relative to its original position on the earth?

Thanks,
Jim
Relative to the earth, the bottom point of the flange would be moving west.

I found a graph that illustrates. Scroll down, and you'll see the exact problem we're talking about.

http://www.physclips.unsw.edu.au/jw/rolling.htm

And Mike's answer to your question is the same as mine. Yes, it is not true.

Fred
 
Jason Robichaud said:
CB would never travel past OB if it didn't. Must be easy to get the BCA instructor cards!

Like what most people already pointed out, the ball doesn't spin faster than the roll. The forward momentum is transferred to the object ball, but the angular momentum is mostly conserved, that is why the CB "follows".
 
Bob Jewett said:
I thought that the stick could contact a trapped ball at 45 degrees and not slip. Doesn't that imply a coefficient of 1.0? In any case, the tip-ball coefficient of friction is normally a lot larger than the ball-cloth coefficient.
A maximum offset (tip contact point) of about 1/2R implies a coefficient of static friction of up to about 0.58. Not sure about the trapped ball though?

Bob Jewett said:
More importantly, I think, is that the tip-ball forces are much, much larger than the ball-cloth forces during most tip-ball contacts.
So true.

Jim
 
LAMas said:
CB move forward soon after impact by the cue - it appears.
http://www.youtube.com/watch?v=Yhnlrdjq8uc
But in this thread we are discussing a very specific shot -- one in which the bottom of the cue ball is actually moving backwards (!!!) as the ball leaves the tip. It is absolutely clear in the two videos you gave that the bottom of the cue ball is moving backwards after the cue ball hits the object ball(s) and has lost all of its forward speed. That happens on all full-ball follow shots. That's absolutely standard and well understood. What we are discussing here is a pretty fringe topic of whether you can get the "excess" or "retrograde" spin right off the tip.

In fact, you can get the action shown in those videos with a partly sliding cue ball -- one that has forward spin but is not rolling smoothly on the cloth. You could call this 90% follow if a smoothly rolling cue ball is 100% follow and "excess" top spin is 110% follow.
 
jongreve said:
Question...

If you strike downward on the front of the cue ball, won't it deflect backward for a moment before changing direction and accelerating foreward? ...
That would be a kind of follow-masse shot, I suppose. How much elevation were you thinking of?

But I think we are mostly discussing level sticks here. With masse shots (full or partial), you can get huge amounts of spin in relation to the speed of the ball.
 
Back
Top