okay, thanks. seems you are quite well-versed in these matters

Probably too much (-;

Got to go, have a nice day.

hehe. see you

hello

is anyone actually using this thing?

hi. sure

bit bored, just thought i'd check it out!

well, this room was deleted until very recently though...

so i just read.

mkay

apparently i'm meant to moderate this too, heh.

looks nice and friendly so far. :)

by the way, I wonder if both you and David are in Europe and it's natural to stay till late hours :-)

okay, moderate away then :-)

i am indeed. not sure where david is from

hah, no need. the less i interfere the better probably

it's only 0020 here. where are you based?

prague, czech republic. 0125 (apparently my clocks go wrong again)

ah right.

i guess you vary between one and two hours ahead of GMT

yeah, I guess the site doesn't need much moderating which is good. except for some closed questions it's quite problem free

right. one in the winter, two in the summer

yeah. and tagging posts as community wiki!

(some people just want to "milk" reputation)

I think community wiki is a strange concept

i can see why you think that. it has a few uses though

it was probably meant as a wiki (i.e. everyone can edit everything and keep it up-to-date) but it became a means to disallow people gaining rep :-)

mainly for subjective/opinionated questions

that are still acceptable

that's sort of a byproduct, but yeah.

(in theory)

yeah, I get that. but it doesn't make sense to let other people edit your post just because it subjective. quite the contrary

heh

at least I think that you can edit other people's posts in CW mode, right? it's not just about denying reputation gains

well the theory is that it's more of a collaberative effort i guess

rather than subjective

hence various people should be able to edit the post

and everyone (noone?) gains credit

indeed, you can edit others' posts

yeah. that makes sense for things like big lists and such. but over at MO (which is the only other SE site I am visiting more often) they use it for every soft-question which is a bit strange. or maybe not

i find that a bit strange, yeah

a bit of a misuse, at least as seen by the original SO creators

i don't frequent MO much heh. the level of questions there is a bit scary i think!

oh hey, I completely forgot I was still logged in here

hi david.

yeah, it's definitely scary. I didn't post there yet and am not sure I will in the near future (though some topics are quite within my reach, perhaps). but I still enjoy reading clear explanations by some of the best mathematicians in the world. and soft-questions are very enjoyable there too

yeah, the mathematical physics ones ought to be. (some of them.)

but a lot of abstract maths that might as well be chinese to me.

yeah, I would probably also forgot if it weren't for the new app-tab feature in firefox beta :-)

and yeah, i've gotten some very nice replies on mathematical logic and proof theory on there!

so did i, but apparently not.

if you try to make the question appear intelligent :

mathoverflow.net/users/602/noldorin

yeah, I quite like studying group theory (even thought I might go on to research at one point), so I know something about that. and I enjoy reading about modern math like category theory, algebraic geometry and algebraic topology so that I can follow notation and discussion. but I wouldn't be able to provide an answer of my own at all

yeah...most of which are (thankfully?) irrelevant unless your a string theorist

@noldorin that is neat!

or related field...

gives one a boost. maybe I'll ask something too :-)

@Marek: cheers. yeah, go for it!

so as long as we're here, any more ideas about site promotion?

maybe trying to convince some well-established physics blogs to add us as a link?

@DavidZaslavsky yeah, good point

I think there would be no harm in sending an email to them

i looked at the AI site which just started today...it makes the level of questions on physics look absurdly high!

@Marek absolutely, it can't hurt

if we attract a good range of graduate and undergraduate topics, then that wil go a long way

so long the only notable visitor to the site i've seen is Peter Shor

still, not bad :)

Agreed @Marek ... I'm not really up on which physics blogs are the big ones but I suppose it couldn't hurt to ask

if you have any in mind

Yeah, I wonder how Peter Shor found this site and whether he could help us with promotion

I imagine we would have to convince him that it's worth promoting, first

which probably will not be easy

I'm sure the man has high standards ;-)

I have like 10 in my feeds. I am not sure which ones are big though. Reference Frame certainly but Motl has so many links there that no one would notice one more :-)

that's probably true of every established physicist with a well-known blog though

I've actually been wondering whether we could target Facebook Questions

there are a bunch of those about physics

hehe

interesting idea

may be lowering the bar a bit much

never really used the feature

me neither, but FB insists on showing them to me anyway

facebook questions? what is that about?

@Marek, @DavidZaslavsky he did answer a question or two i think, but was mainly expressing his concerns on meta!

@Marek basically FB implemented a version of Stack Overflow integrated into their site

so people can ask and answer questions and vote on the answers

yeah, I remember him also mainly from meta

@DavidZaslavsky it wasn't really based on SO

but yeah

oh. and does it really work?

i'll check it out. somehow

@Noldorin sure, I guess they may not have based it on SO but it's functionally the same thing as far as I can tell

it's hard to believe that SO played no role in inspiring that

Actually the examples I see are mostly not that bad

e.g. "How does a band gap arise in crytalline structures?"

fair enough

that's a fair undergrad question

i'm not sure i could answer it very well :S

and "What qualifies as an observer in a wave function collapse (quantum mechanics) and what distinguishes an observer from any other matter?"

I think we had one like that

hah, that sounds philosophical

but yeahg

pretty good questions actually. for FB surprisingly good

yeah... and they all get multiple comprehensive answers too

facebook.com/pages/Physics/…

well, mostly

anyway I guess I could start keeping an eye on new FB questions about physics, and if any of them are answered on this site, I can post a link

I am not sure how the thing works. Either I got something else displayed or I am blind

and just hope that people start to prefer physics.SE to Facebook

yeah, that's a good idea

@Marek huh, maybe there is some sort of access control in place... I'd guess you may at least need to be logged in

but whatever, I don't think you're missing that much ;-)

ooh good one:

"What's your favorite dimensionsless quantity, and why?"

lol

I am logged in. but if it's a facebook app I'd expect that it would required some authorization. and it didn't

yeah, I am probably not missing anything but I am curious and want to see it :-)

I don't think it's an app, it's an integral part of the site, like photo albums

does $\TeX$ work here by the way?

hm, apparently not

evidently not :(

oh. then I just don't know what to do with it. I think computers don't love me these days

if only you could ask a Facebook question about how to access Facebook questions ;-)

maybe we should start fb.SE :-)

lol

actually that would fall under webapps.SE

you actually could ask there

lol

I think I would be too embarrassed

I dunno, it seems like a perfectly legitimate question

@DavidZaslavsky i just see my classmate's physics-related wall statuses :P

well, maybe for someone who's first day on the internet :-)

in any case, i'm still quite discouraged by the lack of decent level, well-formed, quantitatve (or even conceptual) questions

I think if we want to improve in that respect, we'll have to start doing it ourselves

i.e. asking the questions ourselves

quite possibly yes

i think we all agree more or less on the problem though

eh?

but ourselves shouldn't me us three, right? I don't want to flood the system (even if I had so many decent questions)

yeah, at least I can definitely agree that the more of these well-formulated questions we get, the better

yeah, I agree there's a problem

haha yes, if it's just us 3 that's not ideal

lots of vague, imprecise questions

the thing is for me, when i have a question that needs answering, usually 5 minutes talking with professor (or a friend that's smarter than me) does the job!

so i'm not likely to post a lot, without forcing it

you have a knack for easy questions then :-)

true... although for the sake of building up the site, it might be better to post the question here first

or very knowledgeable friends

s/easy/simple/

@Marek: i don't think so, just i know the help professor (or tutor) to ask

@Marek or i rein in my curiosity sometimes, for the sake of sanity ;)

and time

I don't think I possess that quality

i'm not sure it is a quality

but it's a practicality :)

I tend to spend intense bursts of energy on something and after a while I am not really interested anymore

heh

yeah, i know that feeling

me too

string theory is a best example. it shows up every once in a while so I decide I finally learn it, start reading books and papers... for about two days usually :-)

@Marek: i wouldn't worry, unless that's where you're going with your PhD!

it's a very specialist area

curiously, physics.SE is keeping my interest still. I guess that is a good sign

and goes inand out of vogue frequently

...anyway I think it's time for me to go home

and bedtime for me

good night!

same here

so good night everyone

ciao

you too (both)

hi all

hey

I believe that we need some inline tool to draw graphics... the wagon problem is 99% that

That would be indeed nice but I doubt people from SO would implement it

I haven't been following the discussion as of lately though. What's the current opinion of the majority? :-)

By the way, certainly chat is a better place for a discussion than the comment area under the question.

It looks messy

we have 2 solutions with movement on the side, one without and one with back and forth

(the chat is fine, the question is messy) :-)

I am reading the discussion under your answer. Will take some more time though :-)

No worries

Okay, so let's sort it out.

ok

You agree that the horizontal portion of the total momentum is conserved, right?

yes

And you also agree that the water has a momentum toward left?

some water, yes

not globally

Yeah. But when you integrate over all of over you obtain total momentum for all of it. It will be non-zero

*over all of water

mmm

thinking

yes, in the frame of reference of the cart

Okay

now, to conserve horizontal momentum, something has to compensate for this left moving water. Mark (and I) says that it's the wagon moving right

And you? Are you saying that it's all just structural forces?

hang on

sure

so the system has the nozzle pointing down

you need to use this somewhere

sure. I use it for conservation of horizontal momentum

ok

go on then

you used the wagon

so the water has momentum to the left

the wagon has momentum to the right

so the water "pushes" the wagon on the right side

how do you use the nozzle?

because - if the nozzle is horizontal, then I 100% agree with your analysis

I don't think nozzle is that important

the water steals momentum from the cart

because it has momentum when it leaves

ok ok

what's important is that the water flows to the left. This, in itself should impart a momentum to the wagon to the right

no

purely from the conservation principle

Actual dynamics is not important

no there's something missing

so what is it?

as the water flows out, at least relative to the wagon, it has no horizontal momentum

sure

so where is it gone?

because if you say that the wagon is pushed to the right as the water moves left, then the wagon is pushed to the left as the water is made to flow down

that's true, I guess

so you are saying these contributions are actually equal?

I have to think about this some more

they have to be, because the flow does not have horizontal momentum

you mean the flow leaving the wagon

yes

so, water at the top has zero hm, water flowing out has zero hm

okay, let's say I agree for now (I really have to think it through though)

what about the center of mass?

it's apparently not conserved if the wagon doesn't move, right?

you think that's okay?

why should it be conserved without the earth?

if all the movement was horizontal, then the CoM would be conserved horizontally

but it's not the case

you mean if all the movement was vertical?

but the total momentum is zero

nope. think about walking

?

ok sorry bad example

I mean, what Mark has written earlier is completely true

if you consider the wagon

and the water in it

that the derivation of center of mass with respect to time gives you momentum

this is zero and conserved. so that means the center of mass should not move

then mass is not conserved, nor is the center of mass

total mass

it surely is

why not center of mass? I just told you why it should be (because of momentum). with which part of that statement do you disagree?

thinking

the derivation of the com versus time is not zero

because the (vectorial) momentum is not conserved

sure. i am talking just about the horizontal part

you can separate that

i have to think this through

okay. meanwhile I'll think through your argument about where does the water momentum go to

fair enough

hmm, I can't come with anything better that to agree with you that there would indeed be a horizontal force slamming into the nozzle which stops the horizontal momentum of the water. however, I think this force is smaller than the force induced on the wagon by water moving to the left

still, I don't have any means to compare these two forces directly. that would need a lot more thinking