The h Bar

2:00 PM

@Mew, hey, I'm back. I have about 10-20 minutes and then I'll have to go (again). Yeah, I saw the site. Cool! I'll keep looking at it. =)

ACuriousMind

@MikeMiller GW? Generically, deformation invariants could be interesting, depending on what they are

Mew

@heather, thanks obvioiusly a lot of work to go but a starting point. I'll create a meta and privacy policy in about 24 hours. once the meta is in place it will help the site develop

heather

@Mew, I'm looking around, and I'm so impressed; is there anything I can do to help after signing up?

I do know some python and a bit of HTML/CSS and a bit of JavaScript, if that helps any. And I'm willing to spend time figuring things out =)

Mew

@heather, for the moment I guess just seeing what improvements. (e.g. someone pointed out comments weren't available, which I've now added) I will also look at what permissions I can give to trusted users who can assist.

I have to go to bed now but I'll hvae more progress in 24 hours

laterz

heather

@Mew, sweet, thank you. I'd be glad to moderate in any way if necessary, of course. Have a good night. =)

Mike Miller

2:09 PM

@ACuriousMind gromov written

ACuriousMind

@MikeMiller Ah. I don't know much about that, but I think it is interesting to know they are deformation invariant, that corresponds to a symmetry of the path integral that computes GW invariants, I guess

0celo7

@ACuriousMind dog is sick, cat is depressed :(

ACuriousMind

@0celo7 Cat is depressed? oO

How does one even diagnose that?

user116211

Take him to vet.

0celo7

@ACuriousMind he keep crying over the poor dog

user116211

2:18 PM

WoW. 0celo's cat loves dog.

0celo7

@MAFIA36790 dog was at the vet all day yesterday

user116211

ohh.

user116211

Would the dog be okay?

0celo7

It seems so

user116211

:)

Mike Miller

2:24 PM

@ACuriousMind So Joyce's paper is interesting, since it suggests asymmetry in a path integral :)

Andras Deak

2:42 PM

@0celo7 awww

what's wrong with the poor thing?

Ramanujan

@MAFIA36790 hi, how will you get area of triangle when equation of its sides are given?

0celo7

@AndrasDeak ear infection, intestinal issues, stomach issues

The vet probably thinks he's being abused

Andras Deak

aww

Abused how? Worst thing that would suggest is being fed crap, right?

0celo7

3:07 PM

@AndrasDeak he's fed fine

@AndrasDeak I bet it's a complication from him randomly getting worms last month

Probably eating fox poop...stupid dog

user116211

Fox poop?

user116211

That's bad.

Andras Deak

@0celo7 I know:P

he could always be sensitive to something, like too much protein

but yeah, those worms are probably related:)

hope he gets well soon

Bernard Meurer

@MetaEd Wait a second, you're a software guy?

MetaEd

3:32 PM

I'm afraid so.

I still like little stopwatches and arrows though. Can I stay?

Sanya

:33249798 but those kind of questions are just the point of the new site? :|

Bernard Meurer

@MetaEd I'm a software guy :)

Andras Deak

so...both of you have to leave?

Sanya

@AndrasDeak we're inclusive here

Bernard Meurer

@AndrasDeak And you from the looks of it

Andras Deak

3:43 PM

@BernardMeurer define "software":D

but probably, yeah

Bernard Meurer

@AndrasDeak MatLab, Scipy? You're doomed

Mike Miller

Luckily, everything I do is worthless, so I fit right in

Andras Deak

Well I'll have you know it's written as MATLAB. Hmph.

Bernard Meurer

@AndrasDeak Yeah, you're a software guy lol

GNU's Not Unix!

MetaEd

@AndrasDeak The universe? Yeah, that would explain a lot.

Andras Deak

3:46 PM

@MetaEd if the universe is MATLAB, we're indeed doomed. Way too many cells in it to be efficient.

user116211

Welcome back in The h Bar @ColinMcFaul.

Colin McFaul

@MAFIA36790 Thanks, it's been awhile.

Bernard Meurer

@AndrasDeak The line of thought around here is that everything you can do with MATLAB you can do with LaTeX

MetaEd

@AndrasDeak I should probably rewrite it in APL. A Physics Language, right?

David Z

@BernardMeurer ??

There's almost nothing you can do in MATLAB that you can do with LaTeX

Just plotting, as far as I know

Bernard Meurer

3:48 PM

@DavidZ Thanks fur ruining my bait

Or well, you fell for it :p

rob

@DavidZ Turing-completeness suggests replacing "you can do" with "a sane person would want to do"

Bernard Meurer

@rob Good observation

I mean, you do have LuaTeX

David Z

@rob Yeah, I mean practically

Obviously you could implement MATLAB in LaTeX but that's very far from practical

Andras Deak

just program it in Game of Life

rob

A simple Turing-complete language

Slereah

3:54 PM

There's a ton of simple turing complete languages

They are called Turing tarpits

David Z

By the way, we have a chat session coming up: what do we have to talk about?

Slereah

And they are 100% useless

user116211

Chat session!

David Z

Ah, so it is!

Let's get started then. Welcome everyone to our biweekly(?) chat session!

Please keep unrelated discussion out of the chat room until the end of the session, or the open discussion period.

Give me a moment to construct an agenda... :-P

1. Intro, welcome newcomers, site questions (10m)
2. Recent developments in physics (10m)
3. How to handle "physics problems" questions (20m)
4. Goals of the site (20m)

It's not too formal today so we don't have to stick to the posted times that closely. We'll see how it goes.

So to start off, who here is new to chat sessions, new to chat, or new to the site?

Bernard Meurer

::raises hand::

To chat session only though

David Z

4:04 PM

@BernardMeurer 'sup :-)

Bernard Meurer

@DavidZ Hi, nice to meet you

David Z

Likewise, though haven't you been around for a while? Or is it just my imagination?

Not that it really matters! Anyway, anyone else?

@Ramanujan? @DHMO? @AndrasDeak? I seriously can never keep track of who's new.

Bernard Meurer

@MetaEd is new

Andras Deak

@DavidZ hey. I'm new to chat sessions, new to this room, I'm not active on the main site. But I'm not sure how long I'll be around and don't do much, so you can ignore me;)

I'm also having coffee right now

David Z

@AndrasDeak Nobody does much ;-)

That's cool, though, you can lurk

If nobody else wants to say hi or ask any general-interest questions or anything, we can move on to recent physics developments

Anybody have things to share?

I think it was last time that we talked about the Nobel Prizes, but let's see what has been going on since then

user116211

4:11 PM

Researcher explore balance between coherence and control with simple but complete platform for quantum processing

user116211

Physicists induce superconductivity in non-superconducting materials

John Rennie

Steinhardt thinks inflation is crap

2

David Z

hehe

user116211

Not recent development but worthy to read among other Lumo's posts: A bump at LEP near 30GeV: weak but possibly justifiable

Andras Deak

have you discussed the "maybe inflation doesn't accelerate after all" thingy?

David Z

4:13 PM

@AndrasDeak I don't think so... maybe I vaguely remember reading about that.

Andras Deak

edited

David Z

@MAFIA36790 Recent enough

user116211

Well then; Lumo is great.

David Z

@AndrasDeak Ah right. Not really an inflation thing, it's about the supernova distance measurements that showed the acceleration of cosmic expansion (which itself was a Nobel Prize winning discovery IIRC)

Andras Deak

yeah, forgot the disclaimer: read about it in non-scientific local media and I don't actually understand the original source because I don't know any star stuff:P

David Z

4:16 PM

Fair enough :-P

Basically, we can calculate the distances to very faraway supernovae by measuring their brightness, and those distance measurements had been used to determine that the expansion of the universe was accelerating. This new study seems to show that that conclusion isn't quite as conclusive as was previously thought. If I understand correctly.

from here

Andras Deak

that's what I gathered

and it also seemed to me that the original findings were a bit fundamental to our current view of inflation

so it seemed like a possibly-huge-deal to layman me

David Z

@AndrasDeak that part I'm not sure about. Maybe someone else could say more definitively.

That might not be a bad question to ask on the main site, actually.

John Rennie

@AndrasDeak the new work doesn't show the expansion of the universe isn't accelerating, just that the statistical significance of the measurement isn't as great as originally thought.

Andras Deak

ah

John Rennie

But there is other evidence for dark energy, so it seems very unlikely the measurements of an accelerated expansion are wrong.

Andras Deak

4:22 PM

the bit in the abstract about "the data are still quite consistent with a constant rate of expansion" made me wonder

of course the devil is in the details, "quite consistent"

and the figure David linked, and which I have seen, seems to say that there is acceleration

anyway, I didn't intend to hijack the discussion:D Carry on, please

David Z

Yes, let's move along to the next topic: the new proposal of a separate "physics problems" site, and how it might affect our scope here (whether that site actually takes off or not)

Relevant:

4

Q: New site proposal specifically for problem solving in Physics on Area 51

"Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics." At present Physics Stack Exchange is more inclined towards answering specific conceptual questions in Physics and not really meant for problem solving at the High School/Undergraduat...

The proposal on Area 51 was shut down because SE doesn't want to support multiple sites on the same topic. But in light of the fact that people considered it necessary, we should probably think about whether we should modify our scope and/or our closing behavior in response.

Sanya

I think it might make it easier to satisfactorily handle well written questions that are now closed as homework&exercise; I actually think it should not impact the voting pattern here too much except in borderline cases - after all, we do not want questions we would consider good at the moment to disappear from here, would we?

David Z

E.g. should we be more lenient on problem-solving questions, and absorb that proposal into this site? Or should we hand off all such questions to another, external site, and completely forbid problem-solving questions here?

@Sanya Whatever we decide would only affect future questions, it wouldn't involve moving existing questions from this site to another one.

Sanya

relevant: physicsproblems.nfshost.com a new test site has already been put up for test

John Rennie

If the proposal had gone ahead on the SE I would vote for a complete ban here.

Sanya

4:28 PM

@DavidZ yeah, totally understood

user116211

@DavidZ I would love to see Phys.SE totally shunning away such sort of queries.

Sanya

I meant I would not want that to happen in the future - bad wording on my part, sorry

David Z

@JohnRennie Indeed, though by shutting down the proposal, it seems like SE is subtly pushing us to be more inclusive about those questions.

John Rennie

Good problem solving questions could have been easily migrated to the new site. However if the site goes ahead independently of the SE that isn less easy.

ACuriousMind

@DavidZ In light of our homework policy, absorbing it is out of the question, in my eyes.

3

user116211

4:29 PM

yes.

Sanya

@DavidZ look at math SE - I for my part am gone from Physics SE if it begins like that

David Z

@ACuriousMind Well, we're throwing out our homework policy. So this is a perfect opportunity to change what sorts of questions we accept in that regard, if we would like to do so.

John Rennie

I'd be very surprised if there were much appetite for changing the homework policy to, in effect, absorb the new site.

Sanya

we have enough not very very good quality questions being tolerated as is

ACuriousMind

@DavidZ While we have continual discussion about where exactly the line for homework is in the edge cases, I think the vast majority agrees that straightforward questions that just include an exercises and ask how to solve it are off-topic

2

David Z

4:31 PM

@JohnRennie That's what we should carefully determine.

user116211

Math SE is DIFFERENT T__T

John Rennie

@ACuriousMind to be fair, even the new site would have required evidence of effort from the OP.

David Z

@ACuriousMind Of course. The question is where exactly to draw the line. We probably all agree that we want to exclude such straightforward no-effort exercises, but that still leaves a wide range of possibilities.

Take this from the demo site as an example: would that be on topic for us? (Presumably not) Would it be on topic at the new site?

ACuriousMind

@JohnRennie Yeah - but still, I think that "I'm stuck, what do I do know?" questions show effort and are nevertheless rather uncontroversially off-topic.

David Z

Actually, any of the 3 questions currently posted at the demo site could make the point just as well.

ACuriousMind

4:33 PM

Alas, the last round of "data" on that that we got wasn't really in line with that, so what do I know

David Z

@ACuriousMind yeah, though to be fair that data wasn't really in line with anything.

Sanya

@DavidZ No definitely not on topic here, not on topic on the new site probably due to not very much effort shown - but I'm not setting guidelines for a new site that I'm not (yet) part of

David Z

I think it would be useful to have some examples of questions that would be considered good and on topic at the hypothetical new site, from those who are involved in setting it up.

Better yet, to have examples of questions which would be on topic there and off topic here.

Sanya

is there a way to see all on hold/closed questions?

user116211

@Sanya Some of them are deleted by the community too.

ACuriousMind

4:35 PM

@DavidZ Which led me (an other people like Daniel) to believe that "homework" might be a proxy for not a single close reason, but a variety of them. However, I think that "Asks us to solve an exercise for them" is clearly one of these reasons, although people seem to relent on that stance as soon as they consider the exercise "interesting" themselves.

David Z

@ACuriousMind yeah, as far as I can tell, that seems to be accurate

@Sanya you can get a partial list by using the advanced search operator. It's something like closed:yes or is:closed IIRC

Andras Deak

@Sanya physics.stackexchange.com/… but as MAFIA noted, this doesn't include deleted posts

John Rennie

@Sanya physics.stackexchange.com/…

Andras Deak

and roomba eats a lot of closed crap if they're not answered

Sanya

@AndrasDeak @JohnRennie thanks

ACuriousMind

4:38 PM

BTW, it was recently pointed out to me that there's a 10k tool showing a variety of stats about closures

Sanya

physics.stackexchange.com/questions/290075/… << that is not good, but it might be ok for the new site maybe?

ACuriousMind

"Homework" currently accounts stably for between 25 and 35% of all closures.

user116211

@DavidZ Can we make a meta post for that like we did earlier for borderline posts?

John Rennie

This question?

10

Q: Degenaracy in mass of $8$ and $27$ reps of $SU(3)$ in Coleman's Aspects of Symmetry

In Coleman's Aspect of symmetry he proposes an amusing problem in the first chapter. It asks us to consider a set of eight pseudo-scalar fields transforming in the adjoint representation of $SU(3)$. We are asked to write down interactions at quartic order (ignore cubic terms) and show that: The...

homework-and-exercises quantum-field-theory lagrangian-formalism symmetry group-representations

David Z

@MAFIA36790 sure, go ahead

user116211

4:40 PM

Well then, will make it a post tonight.

David Z

@Sanya ah... well, I'd consider that no-effort

Sanya

@DavidZ that's what one would have to consider

David Z

@JohnRennie ah, OK. Yes, I would definitely consider that off topic here

Andras Deak

isn't no-effort the defining quality of bad homework questions?

David Z

@AndrasDeak it's a big one, but perhaps not the only one

Andras Deak

4:41 PM

I see

yeah, I can see that

ACuriousMind

@JohnRennie Now see, that question shows effort in solving different parts of the exercise, but no effort for the actual question. There's no reason for this to be phrased as a question about an exercise in Coleman's Aspects of symmetry, it could be made simply into a question "Why are the 8 and 27 degenerate in mass?". Add an attempt/thoughts about this and it becomes an on-topic question here.

user116211

@AndrasDeak The other one is the dearth of conceptual query.

Andras Deak

brb, googling:D

ah, I see

David Z

@ACuriousMind agreed

@MAFIA36790 at least under our current homework policy. Of course we should be open to changing that if desired.

John Rennie

8

Q: Find the minimum value of velocity

Find the minimum value of the initial velocity $u$ of the particle such that the particle crosses the wheel of radius $R$. Details and assumptions $R=2m$ $g=9.8m/s^2$ Neglect air resistance. All surfaces are frictionless. The value of $\theta$ (angle the projectile makes either with vertical or...

homework-and-exercises newtonian-mechanics classical-mechanics kinematics projectile

7

Q: Adiabatic expansion in van der Waals gas

Given a Van der Waals gas with state equation: $$\left( P+\frac{N^2 a}{V^2}\right)\left( V-Nb \right)=NkT,$$ show that the equation of an adiabatic process is: $$\left( V-Nb\right)T^{C_V}=\text{constant}.$$ I began by setting $đQ=0$ in $$\mathrm dU=đQ+đW,$$ one then gets $$0=\mathrm dU+P~\mathr...

homework-and-exercises thermodynamics adiabatic

David Z

4:46 PM

@JohnRennie on topic for the new site proposal, you mean?

The thing that kills it for here is "Can anybody suggest a way to do this question?" IMO

0celo7

Did I walk in on a chat session?

ACuriousMind

@JohnRennie Yeah, I don't see a way to make that one on-topic here, but you're probably suggesting that should be on-topic at the problem solving site, right?

David Z

@0celo7 yep

John Rennie

They are examples of questions that are homework but show effort. I think they're the sort of thing that would be suitable for the new site but have been closed here.

David Z

While we're talking about this topic, we can also consider the site's goals.

17

Q: What are the goals of this site?

This discussion question is inspired by this post on the current homework policy question. The main question is What are the goals of this site? Some things to think about when answering: What is it we want this site to represent? What are the ideals to which we should hold all of the conten...

discussion featured scope content-level

The top answer there talks about homework help, so I think determining to what extent we want to offer problem-solving help is a key factor in determining this site's goals. i.e. the two topics are related

Sanya

4:49 PM

0

A: Calculate how much air will cool from an amount of ice?

Your calculation of the heats up to the melting point are correct and useful. If we devide that heat by the heat capacity of the air and the mass of the air, we obtain 22.308K. This is the amount by which the air's temperature has dropped by the time the ice is completely molten and at 0°C. Now w...

What about this question?

John Rennie

Having gone through a few pages of the search I posted above it's striking how few questions do show significant effort by the OP. The vast majority just post the question and ask how do I do this?

David Z

@Sanya OK, so that's of the "did I do something wrong?" variety

Sanya

(I do find it rather hard to find examples of questions that are homework but show effort - maybe there really are less of them than we thought?)

ACuriousMind

@DavidZ I think the top answer also talks about that because that question grew out of the homework debate

Sanya

@JohnRennie read my mind :D maybe we would actually be fine if we just trashed homework for low effort ...

ACuriousMind

4:50 PM

And I'm still not convinced talking about our "goals" is anything but the homework debate in disguise, since the only reason we're trying to discern our goals is trying to remake the homework policy :P

David Z

@ACuriousMind It's not even much of a disguise. That is exactly the motivation behind posting that question.

ACuriousMind

Right. So it's trying to bring forward the debate by...making it more nebulous what we're talking about?

David Z

@ACuriousMind No, it's trying to establish a foundation for the debate. To establish the reasons why we would want to choose particular types of questions to be on- or off-topic.

For instance, should it be a goal of this site to help students complete their homework assignments?

John Rennie

@DavidZ I fear this will lead us back into the maze of twisty little passages, all alike.

David Z

Should it be a goal of this site to specifically avoid helping students complete their homework?

ACuriousMind

4:54 PM

For instance, the second-most upvoted answer is basically saying stop closing any questions since our goal should be (is? effectively is? It's not even clear to me whether the answer is descriptive or prescriptive) helping confused people on the internet

I don't see how that bring us any closer to making policy

David Z

@ACuriousMind I don't like that answer

user116211

@DavidZ Of course not.

0celo7

@ACuriousMind Holy crap. I can click the arrows on my new phone

user116211

@DavidZ Unless there is sufficient effort and a good conceptual query, the homework queries are dommed.

user116211

@0celo7 Yes, that's why it's iPhone7.

John Rennie

4:57 PM

The point of the new site was to be educational i.e. improve knowledge of physics by explaining how to do problems.

David Z

@ACuriousMind Given that I don't like it, as far as I can tell, what it seems to be saying is that we should consider the perspective of "general internet people" who come across this site and assume it must be for the general public to ask questions about physics. Such people are going to ask certain kinds of questions, and we should cater to those questions. So says the answer, anyway.

user116211

@DavidZ Why would it be in consideration of change? Don't we want conceptual query?

ACuriousMind

I mean, all we get as "goals of this site" is stuff like "teaching people about physics" or "being a site for physicists to ask questions" or even "the site doesn't have goals, only individualpeople have". The question is too vague to actually yield anything that will bring us closer to new policy or a clearer view of the site, in my opinion

David Z

@JohnRennie well, I'd be clear that the goal of the new site is not simply "to be educational". PSE is also educational, in a manner of speaking.

Sanya

@DavidZ the only alternative where OP really shows effort is the "I am stuck at this point in my solution attempt" - and after going through 15 pages of closed questions, I tend to think both make up such a small amount of the closed questions that we could actually try to allow them and see the effects ...

2

John Rennie

4:58 PM

@DavidZ Didactic then.

David Z

@MAFIA36790 That's the point of this whole process, to figure out (and agree on) what we do want.

@ACuriousMind It was kind of a disappointing response, but at least we have something to work with.

@Sanya This is one of those things we could decide on. Presumably such questions would be on topic on the new site, if it takes off. Should they be on topic here?

user116211

0

Q: Deriving Bernoulli's equation from Navier-Stokes equation

I can prove the Bernoulli equation in an elementary way (that is, using the conservation of energy law). But I was asked to derive the Bernoulli equation from the Navier-Stokes equations, and I have no idea how to proceed. I tried searching on the Internet, but I found no good explanations. I...

fluid-dynamics navier-stokes

user116211

No efforts at all; homework?

Sanya

@DavidZ what I am saying as a first impression is that they probably make up such a tiny number of currently already closed questions that making a new site based on them is not worth it and we would probably not lose much by allowing them

David Z

@MAFIA36790 Closeable as no-effort, I'd say... which for now means use the homework-like close reason, yes

Sanya

5:01 PM

if my first impression is correct

David Z

@Sanya Yeah, so then we take that to the community at large and see if others agree.

Anyway, our hour is up! Thanks everyone for coming to the chat session, and see you all in two weeks!

Sanya

@DavidZ we could also systematically keep a record ...

and decide with a bit of data to base that on

David Z

@Sanya That works too. Are you volunteering? :-P

Sanya

@DavidZ if you give me a tool which sends me all questions closed as homework

once a day or sth

I'd sort them

as of now, it's hard to find only those

David Z

Ah, well I don't have that handy but you could start looking at SEDE. It'd give you recent ones, at least.

Sanya

5:04 PM

well, that's the point actually - you are right

I can once a day look at all new closed questions

and see which are closed due to homework

user116211

0

Q: Molecular simulation of ideal gas- descriptive statistical analysis

I wrote a simulation of an ideal gas in Java. Now that it is complete, I want to verify the results. The image (1) show the variation of kurtosis, skewness and temperature of the system with time. All these quantities were captured a second apart.Image (2,3) show the percentile plot and cumulativ...

simulation computational-physics ideal-gas molecular-dynamics

Sanya

we don't want a historical account but an account but a statement of current policy after all

I'll try to keep a list for a week

user116211

Homework?

John Rennie

@Sanya the search I linked pretty much does what you want

user116211

At least the last query sounds so.

Sanya

5:06 PM

@JohnRennie yep, still have it open

and I planned to use that for that

ACuriousMind

16

Q: Provide a way to search for closed questions by a specific close reason

I can search for of all the questions that have been closed for any reason with closed:1, but you very rarely want to see just closed questions, you usually care about questions closed for a specific reason. This would also be a great feature for teaching new people about why questions get close...

feature-request search close-reasons

Still an open feature request :/

Also, I can't find the page listed in waffles' answer

maybe it was removed/remodeled since then

Sanya

I got to go for now, but John's link will do the job nicely - I'll try to post a sum-up on meta in a week

David Z

@Sanya excellent

@ACuriousMind yeah, perhaps, but this seems related: physics.stackexchange.com/tools/question-close-stats

ACuriousMind

@DavidZ The close reasons look clickable in that screenshot in waffles' answer, they're not clickable there

I think it's especially useless to list the custom close reasons without giving a way to see which questions they closed

David Z

Good point

ACuriousMind

5:12 PM

For instance, at the 90d page, there's:

> I'm voting to close this question as off-topic because vvvvvvvvvvvvvv

I'd like to see who wrote that to what question

but I can't

David Z

There's also the review queue history, but it's not quite what we're looking for

user116211

I remember that.

Secret

5:29 PM

Assuming chat session is over: Electro aculpunture

0celo7

6:16 PM

@ACuriousMind Do you think I should take alg top or FA next year?

Never mind. Alg top and measure theory

Obliv

@0celo7 what's measure theory like

0celo7

6:34 PM

@Obliv lots of rulers and stuff

sig figs, etc.

Obliv

HaH

Andras Deak

and "yo how long is the shoreline of Norway?" :P

probably not though

Obliv

7:03 PM

I am totally stumped in re-deriving the spherical coordinate unit vectors in terms of $\vec{e}_x,\vec{e}_y,\vec{e}_z$ again.. help

imgur.com/a/mDxqr is my best attempt at a drawing

$\vec{r}$ was quite easy to do but I'm trying to do $\vec{\theta}$ right now and I can't figure out how

okay found a better picture but it's slightly different (switches theta & phi) plaza.obu.edu/corneliusk/mp/suv.pdf

0celo7

what the hell

just use the tensor transformation laws

Obliv

@0celo7 I don't care about tensors dood I need to derive these naturally using geometrical analysis or else my OCD will literally kill me

0celo7

geometric analysis is way overkill here o.O

How the hell do you use geometric analysis here

Obliv

it was simple for r/p. not sure how to project $\vec{\theta},\vec{\phi}$ and find components though

0celo7

What the hell are you talking about...

Obliv

7:12 PM

wait for $\vec{\phi}$ i think I can do it on my own. Just $\theta$ that's giving me trouble

@0celo7 I need to show $\vec{\theta} = ...\vec{x} + ...\vec{y} + ...$

0celo7

see Cahill's book

Andras Deak

can you at least rotate a vector along z with phi?

Obliv

@andras yea that's how changing $\phi$ works

Andras Deak

actually I have no idea which ones $\phi$ and which one's $\theta$, everything looks the same in your picture

are you struggling with the polar direction or the azimuthal?

Obliv

7:28 PM

polar direction

I found one component ($\vec{y}$)

but now i can't find the $\vec{x}$ one

ill upload and show u my progress

Andras Deak

don't feel obliged:P

so if you can rotate a vector along $z$, why not write $\vec\theta$ for $\phi=0$ and rotate that about $z$?

Obliv

@andras imgur.com/a/kqiMX

it's much cleaner as you can see.

for $\phi = 0$ wouldn't $\theta = \text{undefined}$?

oh forgot to label axes. They're the same as before

OH I'M SO DUMB

I do that same thing except for the other side. imgur.com/a/EmZgM

Andras Deak

I suggest switching to $\vartheta$ and $\varphi$, much less ambiguity

Obliv

i did this initially but couldn't get y component so thought it was wrong

user116211

Who starred that?

user116211

7:36 PM

Trolls these days.

Obliv

probably ocelo7 but idc it's true

Andras Deak

I'm sure arbitrary accusations are pointless

and if all trolls just starred "I'm so dumb" messages, the world would be a happier place

user116211

@Obliv 0celo7 can't star in mobile.

Obliv

@andras so now to do this for $\phi$

i got the gist so it shouldn't be a problem

Andras Deak

@Obliv I still don't see the problem. Look at $\vec\theta$ for a $\phi=0,\theta$ point, then rotate the result about $z$

Obliv

7:40 PM

oh I think I see what you're getting at @andras but wouldn't you end up with the same idea (having to project onto a plane)

Andras Deak

probably yes, except rotation of vectors is easy and mechanical

0celo7

Easy!

It's trivial

Andras Deak

trying to be nice here:P

DanielSank

Anyone around have Mathematica?

Obliv

@andras you'd get something like this imgur.com/a/h9td4

DanielSank

7:44 PM

I'm lazy and I don't want to work this out:

$$\int_0^t \int_0^t \cos(\omega(t'-t'')) \, dt' dt''$$

Oh wait, I'm being stupid. I have mathematica on this machine :D

Andras Deak

wolframalpha.com :P

Obliv

probably easier to think of how to do it with only 2 components of the vector but too late now :P

DanielSank

Nothing to see here. Move along.

Andras Deak

Mma is even better

also, integrating linear function compositions is also easy:P

DanielSank

@AndrasDeak It fails on 2D integrals in ways I don't understand.

Andras Deak

7:46 PM

hmm

interesting, it won't even parse a 1d integral for me right now

maybe too many unknowns...

I know I could make it work in the past

@Obliv ok

user218912

hey

NeuroFuzzy

@DanielSank -shrug- wolframalpha.com/input/… :D

Andras Deak

hmm, same thing didn't work for me with $x\mapsto tp$, $y\mapsto tpp$.....

DanielSank

8:04 PM

@NeuroFuzzy :D

Obliv

8:23 PM

anyone know why the top arc length (given by $\rho_i \sin(\phi_k)\Delta \theta$) isn't just $\rho \Delta \theta$? i.e why isn't $r_i \Delta \theta = \rho_i \Delta \theta$?

I'm guessing it's because $\Delta \theta$ isn't the same both there and on the projected xy-plane

forgot to link it. imgur.com/a/CwnnL

Andras Deak

@Obliv because that arc is generated by rotating the vector around the $z$ axis rather than rotating around a point

Ah, dude, your angles are confusing the hell out of me again

you said $\theta$=polar, but in your figure it seems to be azimuthal

Obliv

In the one I just linked isn't $\theta$ polar? I thought azimuthal meant normal to the $\vec{r}\vec{\theta}$ plane

if it's the one that's the angle b/t the projected radius onto the xy-plane and the reference direction, then yes I meant $\theta$ is azimuthal

Andras Deak

polar means measured from the pole, the $z$ axis. Azimuthal describes revolution about $z$.

Obliv

Ooh.. yes sorry about that.

Andras Deak

...

my remarks above hold regardless:P

for the azimuthal arc element you rotate about z, so you need the project the spherical radius with the polar angle

Obliv

8:32 PM

when you say 'that arc' do you mean the one given by $\rho_i \sin(\phi_k)\Delta \theta$? or the other one

Andras Deak

for the polar arc element you rotate about an axis that lies in the (x,y) plane, so the full radius applies

@Obliv that sin() one

Obliv

hmm if it's rotated about $z$-axis, shouldn't that arc length be equal to the azimuthal one?

Andras Deak

you lost me

Obliv

Maybe an extreme example will clear my confusion..

@andras this is basically my question imgur.com/a/J3Ivb

no..

Andras Deak

I don't really understand that image, but surely $S_1\ne S_2$ since $\Delta\phi\ne\Delta\theta$...

Obliv

8:41 PM

yes s_1 = s_2 but that's not the problem I'm having. I'm wondering if you could write $r\Delta \theta$ as $\rho \Delta \theta$ which is not possible since $\rho$ is necessarily bigger than $r$ (hypotenuse of the triangle formed by r,$\rho$)

wait wat

omg i'm sorry @andras that picture is messed up..

Andras Deak

yes:D

Obliv

it should be the same arc length projected downward but my problem as i stated above is that I was trying to write the radius as different in both cases when the angle is the same (see my original image from the textbook)

Andras Deak

yeah, sorry, I find it hard to understand what you need

Obliv

it's okay I do this often. I figured out my issue

Andras Deak

glad to hear that:)

Obliv

9:33 PM

@secret whats up

Andras Deak

10:34 PM

@0celo7 some source for the apple port situation we discussed earlier

heather

@Mew, sorry to say this, but two junk questions came up a second ago. I flagged/downvoted them both, but I noticed that there is no way to edit a question and there is no menu for what you are flagging a question for.

I guess some policy needs to be created about what is on and off-topic there.

And I also noticed that LaTeX/MathJax isn't available.

Andras Deak

I can only see the original 3. Maybe flagging sent them away?

heather

@AndrasDeak, yeah, now I can only see the three too.

You know, it might make sense to start transferring over the homework questions on physics.SE that are good per our standards (shows work, etc).

Andras Deak

I'm pretty sure you should first pin down the very basics of the new site in particular with respect to moderation and dynamics:P

user218912

10:53 PM

hiii

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