OK pool scientists, help me understand this: a pitcher moves his hand at 90mph so the baseball moves at near 90, but how fast does the break hand/cue move compared to the speed of the cue ball to the rack? There must be a ton of loss. 50-70% loss in speed transfer? Example: is the hand moving at 50mph and the cue ball 20 mph? Thoughts...
Break Speed vs Hand Speed
- Thread starter BarTableMan
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There probably isn't much difference in speed. Try throwing a baseball with a pool stroke. I highly doubt you will get anywhere near 50 mph
RBC
Deceased
The cue out weighs the cue ball by around 3 times, so one might think that the cue ball would travel faster.
I'm not really sure that's true though. I'm no physics guy, but I think the rules say that you can't get more out than you put in.
I sure wish Patrick were here. He would have a really good answer to this question.
Royce
I'm not really sure that's true though. I'm no physics guy, but I think the rules say that you can't get more out than you put in.
I sure wish Patrick were here. He would have a really good answer to this question.
Royce
Correct. CB moves a little less than 1.5 times the stick speed if you are using an 18oz cue. It would be exactly 1.5 times as fast, but there is some loss of energy transferred because the tip is softer than the ball, friction of cloth, etc.
I'm not Patrick, but I explained all this last week in another thread, complete with spiffy graphs and everything. No one responded to it.
Go figure.
Royce,
I think you're on the right track. You are *not* getting more out than you put in. You put in an 18oz cue moving at X speed. You get out a 6 oz ball moving at Y speed. Conservation of momentum means that with NO other losses, the ball will move 3 times faster than the stick. Of course there are MANY losses: tip compression, sound, heat, the cue which keeps moving after impact, etc. Realistically, you probably are getting about 1.5 times the cue speed out of the cueball. That is really a guess though.
KMRUNOUT
Sorry I missed it. What was the title of the thread? I would love to read it.
Correct. CB moves a little less than 1.5 times the stick speed if you are using an 18oz cue. It would be exactly 1.5 times as fast, but there is some loss of energy transferred because the tip is softer than the ball, friction of cloth, etc.
I'm not Patrick, but I explained all this last week in another thread, complete with spiffy graphs and everything. No one responded to it.
Go figure.
You have the basic idea, but you are neglecting the momentum that the much heavier cue still carries after the collision. In a perfectly elastic collision, only a portion of the momentum of cue gets transferred to the CB. The cue continues to move forward after contact, but at a slower speed. The fact that it is still moving means that it still has momentum (mass x velocity) that has not been transferred to the CB.
It turns out that 1.5 times the cue speed is the theoretical maximum the the CB can have after the collision, and energy losses resulting in lower cue speed are subtracted from that figure. How much loss can't be predicted by using a simple formula because there are too many variables. You would need to use some method to actually measure the before and after speeds.
Although momentum is a useful concept all by itself, this problem can't be addressed using just that formula. You have to use formulas for energy as well. Somewhere on Dr. Dave's website is a short paper he wrote which shows how to derive the necessary equations. I used his information to create the Excel graphs I mentioned above. I'll look for it after I've had a few cups of coffee and provide a link. Really useful stuff to know, for cue designers and players alike.
I would also guess that 18 oz wouldn't reflect the total weight applied to the object ball because your hand and possible more of your body is also connected to the cue. Not to mention that if your cue is not perfectly aligne to the vertical and horizontal center of the cue ball (which is impossible due to the rails) you will have energy loss due to the cue striking the ball at a different vector than the cue ball will travel and due to energy used in creating english.
http://forums.azbilliards.com/showthread.php?p=4626042#post4626042
I don't know how to link to individual posts, but here is the thread. My posts start at post #6.
Most of the engineers that looked into all this seem to think that the mass of the hand, arm, and body contribute negligibly or not at all because the collision is so brief. I can't say I'm in complete agreement with this, nor do I disagree. I just haven't seen any writeups of the experiments they did regarding this aspect of the problem.
I do agree with the other things you brought up, though. This all assumes a center ball hit. It's just a starting point to look at the problem IMO.
The cue continues to move forward because your arm is pushing it forward not because of the force you imparted to the cue when it first started moving (say if you attached it to a pendulum and pushed that). You arm muscle is what continues to push it forward. For the cue to stop after it hits the cueball, the cueball will have to be heavy enough that your arm muscles could not push though it, or you would have to stop your arm forward motion at the instant of contact and let go of the cue. If you could do that, then you could measure exactly how much energy goes from the cue to the cueball and how much is left in the cue.
What would happen if you were to try a stop shot with a sliding CB that was the same size as the OB but weighed 18oz instead of 6oz?
Are we talking about the cue motion or the ball motion? I was just talking about the cue motion. A big part of why it keeps moving forward is not just because it has more energy in it than it moved to the cue ball but because of your arm. I was pointing out that you can't use the fact that it moves forward to base any type of force transfer to the cueball/object ball unless you have the cue mounted on a pendulum and just let it move forward without additional force behind it.
How the cueball and OB move after contact are different things from how the cue would move.
In your example, if we're talking about close to perfect energy transfer, the object ball will move with 2x the force of the cueball.
I thought my example would isolate the point I was making, but I guess it just caused more confusion.
You don't need to suspend the cue as a free-moving pendulum to see what happens after contact. Next time you are at the table, set a ball up at mid-table. Use an open bridge to minimized the friction from the bridge hand and just throw the cue at the ball. If you do this right you will see that the cue continues it's forward motion after contact with the ball, even though you are no longer holding it. Now, it won't go sailing over to the next table, but it will retain a significant portion of its original momentum.
I'm not discounting at all what you are saying about the effects of the hand and arm on the post-contact forward movement of the cue, just that these effects are quite negligible to the net reaction of the cue and the CB after a brief (.001 second) impulse.
uhhhhhhhhhh wat
But the cue would keep moving forward after contact with the CB, even if you weren't touching it. If you completely let go of it the instant before impact, it would still follow the CB down the table after impact.
High school physics (conservation of momentum and conservation of kinetic energy) tells us that when a heavy moving object strikes a lighter stationary object, the light object moves forward faster than the heavier object was originally moving, but the heavier object also continues moving forward.
Or in other words, take Sloppy Pockets' word for it, because he clearly knows his basic physics.
-Andrew
Tony, here is the link to Dr. Dave's proof that I promised to post:
http://billiards.colostate.edu/technical_proofs/new/TP_A-30.pdf
It covers a lot more than what we are discussing here, but the first part covers conservation of linear momentum pretty well. He shows how he derives the equation (section 7) that can be used to determine CB speed as it relates to cue speed, assuming a center ball hit, neglecting frictional forces, and also assuming a perfect elastic collision (e=1).
In section (8) he substitutes into this equation to arrive at the initial speed of a 6oz CB right after a collision with an 18oz cue, which turns out to be 1.5 times the stick speed. In reality, this will be slightly lower for the reasons stated above, but I just used the 1.5 figure to make the charts because I wanted to generalize the effect, not determine the actual CB speed. IMO the real-life CB speed is irrelevant to the way the game is played because we can't determine it anyway.
Thanks, but it's really Dr. Dave (and others) that worked all this out for us over the years. I'm just a student of the game.
Dr. Dave is the Teacher.