Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

Perfectly centered break of a perfectly aligned pool ball rack

mathoverflow.net

Post reply on HN
Print view View original

41–50 of 69 posts

Re: Perfectly centered break of a perfectly aligned pool ball rack

#41

Post by taliesinb »

jimbelk wrote: post #33
taliesinb wrote: post #23

Really cool problem! And great animations. I was disappointed to find that the associated notebook ( http://math.bard.edu/belk/code/BilliardsHertz.nb ) doesn't work correctly in either Mathematica 9 or 10 -- I think the author used 8. It seems like NDSolve falls down: NDSolve::smpf: Failure to project onto the discontinuity surface when computing Filippov continuation at time 0.`. >> and the solution shows the cue bo…

Sorry about that. I really ought to upgrade to 9. Apparently Mathematica added some new features in version 9 that are intended to better handle differential equations with discontinuities (see http://reference.wolfram.com/mathematica/tutorial/NDSolveWhe... ). Apparently this is getting tripped up at the boundary between the two behaviors. One simple thing that might work is to change the force law to either force[di…

Thanks for the help. Piecewise has the same problem, whereas the If is taking forever. How long should I wait before I kill it?

Re: Perfectly centered break of a perfectly aligned pool ball rack

#43

Post by zodiac »

Someone wrote: post #32
hmsimha wrote: post #20

Earlier quoted context omitted.

I'm no physics expert, but I can account for the second one -- if there's no friction, the balls just keep rolling

Not necessarily; it might be possible to have some balls collide in such a way that at least one comes to a stop. At least one ball will keep moving though, and it isn't clear to me whether it is possible to prevent that moving ball from, eventually, hitting that stopped ball again. I think that would require at least three balls (ignoring pockets)

> it might be possible to have some balls collide in such a way that at least one comes to a stop.

can you give an example?

Re: Perfectly centered break of a perfectly aligned pool ball rack

#44

Post by gordaco »

IgorPartola wrote: post #24

Somewhat meta, but at what point does Stack Overflow transition from being a useful, time-saving tool and become a waste of everyone's time? The top answer author clearly put a ton of time on the answer. We are all now reading this. Is anyone able to apply any of the knowledge they gained from this Q&A to anything productive?

Stack overflow is clearly both an useful, time-saving tool and a "waste of time" (in the sense that it doesn't look directly productive). If you don't like these kind of questions, don't worry, there are plenty of useful questions and answers with a direct work-related application. The answers for your questions are not going to disappear just because there are another questions answered. You know that Stack Overflow has section on things like gaming and role-playing, right?

By the way, I'd have loved an example like this a year ago, when I had a physics subject where elastic and inelastic collisions were studied. I'm pretty sure I'm not the only one.

Re: Perfectly centered break of a perfectly aligned pool ball rack

#45

Post by a_soncodi »

manglav wrote: post #39

While this is great for a theoretical model, most professionals also try to put forward spin on the cue ball, to offset the collision and break apart the rack further. You may have noticed small white burn marks from the heat generated by the cue ball spinning on the felt, from the location where most people break.

This is mostly incorrect. The chosen spin depends on several factors including the breaking position (as in breaking from "the box") and ball count. In this case it appears to be Eight Ball.

The burn marks are caused by the downward force due to an inclined cue. Similar marks occur when performing a legal jump. This is why many tournaments provide a cloth scrap for breaking.

Furthermore, the cue ball may hop to the rack and impact the head ball above its equator, causing the cue ball to jump in the air, reducing the likelihood of being kicked into a pocket. Top spin or forward roll is not required for this.

Source: I play pool semi-professionally.

Re: Perfectly centered break of a perfectly aligned pool ball rack

#47

Post by manglav »

a_soncodi wrote: post #45
manglav wrote: post #39

While this is great for a theoretical model, most professionals also try to put forward spin on the cue ball, to offset the collision and break apart the rack further. You may have noticed small white burn marks from the heat generated by the cue ball spinning on the felt, from the location where most people break.

This is mostly incorrect. The chosen spin depends on several factors including the breaking position (as in breaking from "the box") and ball count. In this case it appears to be Eight Ball. The burn marks are caused by the downward force due to an inclined cue. Similar marks occur when performing a legal jump. This is why many tournaments provide a cloth scrap for breaking. Furthermore, the cue ball may hop to the r…

Agree on the spin, I was assuming a perfectly vertical break.

Abou the burn marks, see here http://www.pooldawg.com/article/pooldawg-library/table-maint...

http://www.cuesight.com/types-of-pool-table-felt.html

Yeah, I hate it when the ball pops up, but my opponent looks cool catching it, heh. I also play pool. Best game for lounging I feel.

Re: Perfectly centered break of a perfectly aligned pool ball rack

#48

Post by prakashk »

ballstothewalls wrote: post #31
sillysaurus2 wrote: post #25

Earlier quoted context omitted.

There's a Mathematica 10 now, already? Interesting. I'm still a happy 8 user, myself. Is there much reason to upgrade? It'd probably be good for them to pay more attention to backwards compatibility...

I think he is confused. Wolfram's site only has Mathematica 9 listed

> I think he is confused.

You don't mean that the GP (taliesinb) is confused, do you?. He works for Wolfram Research, developing Mathematica.

Re: Perfectly centered break of a perfectly aligned pool ball rack

#49

Post by IgorPartola »

IgorPartola wrote: post #24

Somewhat meta, but at what point does Stack Overflow transition from being a useful, time-saving tool and become a waste of everyone's time? The top answer author clearly put a ton of time on the answer. We are all now reading this. Is anyone able to apply any of the knowledge they gained from this Q&A to anything productive?

Going to reply to my own post since I cannot edit it anymore to clarify.

First, I am not saying that this is strictly a waste of time. In fact, I do love learning from things like this.

Second, I am not saying we must always be working instead of stopping to try something different. I personally find switching focus very important and I think it's a great tool for a great many people. I find that when people learn something slightly outside of their field they suddenly are hit with a lot of new ideas they did not have before, so learning about random solutions to random problems can be a great way to spend your time.

What I was trying to ask (not assert) was how answers like this come to be. I am somewhat astonished that I can ask pretty much any question on any given subject and a subject matter expert somewhere in the world will then proceed to spend hours putting together a detailed essay or demonstration (or both) of the concepts involved. What I wanted to know is who are these experts, why are they giving their expertise away for Internet Points (tm) and is this detracting from their actual work.

My other question is whether it's possible that at some point the number of soft, open-ended questions on SO will reach a critical mass where most participants will flock to those rather than the specific and hard questions and detailed expert answers that SO became famous for. Clearly we are not there right now, but is it possible and is it going to happen?

Re: Perfectly centered break of a perfectly aligned pool ball rack

#50

Post by IgorPartola »

jimbelk wrote: post #38
IgorPartola wrote: post #24

Somewhat meta, but at what point does Stack Overflow transition from being a useful, time-saving tool and become a waste of everyone's time? The top answer author clearly put a ton of time on the answer. We are all now reading this. Is anyone able to apply any of the knowledge they gained from this Q&A to anything productive?

This is a good question, and I sometimes do put too much time into answering questions on Math Stack exchange. Right now, though, I'm fairly happy with the time I put into this simulation. First of all, there's the fact that I did this on a Friday evening, after I came home from work. (I spent about six hours on it, and maybe another hour or two on Saturday morning). There are probably more productive uses of my time…

Thanks for both the answer on SO/MS and your comment here. You definitely shed some light on what I was wondering about.
Post reply on HN