The h Bar - 2014-10-01

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12:03 AM

Are you there, @DanielSank?

I'm leaving now. Please let me know by leaving a comment on the question if you are looking for further clarification.

12:20 AM

@DanielSank I assure you (and anyone else), I am not trolling ... I am slowly building up my confidence in participating here and my other SE memberships. Apologies to those who thought I was trolling.

@Omen: I was joking.

Sorry about that, sarcasm (even in good humor) doesn't transfer well in writing. My fault.

@millsmess: I'm here. I guess I think of online chat as an asynchronous communication protocol :)

@millsmess: Anyway, I still don't get it. If I extend the area somewhere where the force is zero, then the pressure goes down.

On the other hand, if I extend somewhere were the force is equal to the average force on the already existing area, then the pressure stays the same.

So it does matter where I happen to be when I fiddle with the size of the area.

I'm here again.

Ah, let me clarify: suppose I have an area $A$ such that the pressure is some positive number.

Ok

Well, actually, that's assuming we have a good definition of pressure.

Now say I extend $A$ such that the edges are now creeping into a place where the force is zero. Then my net area is going up while my force is not, so the pressure is going down.

Pressure = total force / total area.

At least, that's a good functional definition.

12:27 AM

OK, that's a good working definition

Right.

Alright, the pressure goes down

I agree there

The point is that F_n(A) does not increase

Whereas if the edges are extended in to a place where the force is NOT zero, then F_n(a) does increase.

Yes, agreed, of course.

So depending on how the edges are extended, F_n(a) has different values

yep.

12:31 AM

So, if I increase the area in some ways, the force increases at a different rate than if I increase the area in other ways.

Yeah

And if we let p = dF/dA, then p changes depending on how we choose to increase the area, since dF/dA is simply the rate at which the force increases as I increase the area

Of course.

That's not what it says in your original comment... I think.

So, p is ambiguous, since it depends on how we choose to increase the area

Hmm. I can edit if necessary

Hm, yeah, I think I read too much into your comment.

It's worded fine. I just misinterpreted it.

My apologies.

12:33 AM

Okay, thanks.

Let me know if you need any further clarification.

@millsmess: Nah, I'm just an idiot :)

OK, thanks for your input!

What up, @Omen?

2 hours later…

3:05 AM

@DanielSank: I'm gonna have to go ahead and disagree with you here. I have yet to find a reason to combine Scripts with Compiled software.

And by Scripts, I mean any scripting language

@KyleKanos: To each his own.

I'm not really sure this is a topic where disagreement or agreement even makes sense.

Also, if you use any build tools whatsoever you are using scripts...

But again, what works for you is what works for you.

@DanielSank That is fine

But writing a compiled code

And then a Python/Perl/Shell script to run said code

That's what I think is inherently wrong

If you can make a set of modules that work together, you can write a control all in the same language

No interface issues to deal with!

@KyleKanos: Ok, whatever. If you think that various programming practices used by thousands of people are "inherently wrong" then I'm not going to try to change your mind.

Do your programs use some form of input file to decide what to do?

Yes, Fortran namelist file

Built into the language

That's not what I mean. I suppose you process data at some point?

3:14 AM

VisIt: wci.llnl.gov/simulation/computer-codes/visit

I don't know what that means. Give me a clue.

I'm not an astro person.

Sorry, I hit enter before adding the link

Ok, let's pretend I'm not going to grok what the heck that package does for you right now.

It's a visualization software

That's not what I meant.

3:16 AM

That's how I process my simulations....

Don't you ever have some large data set which you want to "do computery stuff" with?

Ah.

You do simulations only?

Yep

Ok... so how do you decide over which parameters to run the simulation?

So I'm an astrophysicist, not an astronomer ;)

Yeah yeah I get that.

3:17 AM

@DanielSank Actually, my work has only 1 real unknown variable :)

omg

Other people have done what I'm doing in 1D, so I'm using their initial conditions and mapping it to 2D

Do you have to run the sim over a range of parameters or not?

Yes I do. I'll blow up stars of 1.4, 10, & 25 solar masses

And vary my parameter eta between 1e-4 and 1e-6

So 9ish runs

Ok, so now imagine you have ten variables instead of one and you like to change the values those variables take based on reasons. It's awfully convenient to do that in a scripting language rather than in some hard-coded header file.

But again, to each his own.

3:20 AM

Fortran Namelist files aren't hardcoded

It's simple File-IO

And the data takes a few days to make on ~1000 processors

So running multiple sets at once is actually blocked by cluster IT guys

I have to do it manually

If you can do logic inside of namelist files then that's great, maybe no reason to use scripting.

I've never used Fortran, so I just don't know.

But anyway, the notion that scripting is inherently wrong is kind of... statements like that usually come from people who have never had a reason to use the thing they're trash talking.

I didn't say scripting was wrong, I said that writing a script to run the compiled software (or as you used, "glue") is wrong in my books.

Anyway, it's late & I've gotta get to bed

Fine, whatever. It's only "wrong" if you use it in a way that's not useful. Saying that something in software is just "wrong" usually results from just not having had to do that thing.

@Waffle'sCrazyPeanut: What's the story with your handle?

Waffle's Crazy Peanut

@DanielSank Pardon me, what does that mean?

@Waffle'sCrazyPeanut: What does your user name mean?

Waffle's Crazy Peanut

3:33 AM

@DanielSank Oh well, I don't really know :D

Haha. I like it anyway.

Waffle's Crazy Peanut

That was a mixture of names. "Peanut" is from a hat from last winterbash IIRC, "Crazy" is from my previous name (Crazy Buddy), and I put it together with "Waffles" (for no good reason) - doesn't have a serious history

Waffles are delicious. Good choice.

Waffle's Crazy Peanut

@DanielSank Yeah, when I made the change everyone liked it. That's why I thought to keep it!

@KyleKanos That's nice! :)

How do you do that thing where you reply to specific comments?

Oh nevermind. Apparently I'm illiterate.

Waffle's Crazy Peanut

3:42 AM

@DanielSank Just hover over a transcript, and you'll find an arrow (like a mirror of return key) to the right side (along with flag and star). Click on that arrow...

Yeah, I was staring right at it...

must be brain cancer.

Waffle's Crazy Peanut

Hah!!! :D

@DanielSank: There are a lot of interesting questions on meta.SE, where you can learn more about the SE engine (browse them when you're free) :)

@Waffle'sCrazyPeanut: Oh, that's cool. I never thought to read meta.SE.

Waffle's Crazy Peanut

@DanielSank: Start with the FAQ

1378

Q: FAQ for Stack Exchange sites

Community FAQ For sites in the Stack Exchange 2.0 network To see a list of commonly used words and phrases, see the glossary. For official guidance from Stack Exchange, visit the Help Center. Asking questions How do I write a good title? How can I get answers fast? Where can I ask a ques...

@Waffle'sCrazyPeanut: Thanks, this is quite useful.

Waffle's Crazy Peanut

3:55 AM

You're welcome ;-)

1 hour later…

5:01 AM

Is there anyone here? I have a pretty simple question that I can't seem to figure out.

Hi. I'm not sure I will be able to help you though. I'm also new on this site.

But still, you could ask your question.

OK. Mine is just a question about kinematics.

My eyes lit up. Go ahead.

Say I have a graph of displacement vs. time for an object falling in 1D due to gravity.

The graph is curved, of course, with the velocity becoming greater in magnitude as t increases.

I can perform a regression to find a value for a/2 (based on the formula d=vt+1/2at^2), and that value is right around 4.9. Perfect.

So then if I linearize my data by graphing d/t vs t, the relation should have a slope of about 9.8. But it doesn't—its slope is 4.9. Why is this?

What exactly does linearize mean here?

5:08 AM

I mean, I know that what I've done is divided each term by t to get d/t=v+1/2at, so I should still only have half of the acceleration. How can I relate this to the well-known kinematics formula v=vo+at?

@UserAnonymous My graph is no longer curved; the graph of d/t (or v if you prefer) vs. t is straight with slope 4.9.

Assuming v in that formula d=vt+1/2at^2 stands for v0, if you take the derivative w.r.t. t on both sides, you will get v = v0 + a*t, since the factor of 2 picked up from the derivative will cancel out with the coefficient 1/2. So, it is fine.

However, the trouble is d/t is not the velocity, it is d/dt of d.

i.e. dd/dt (I'll prefer to use s, and say ds/dt)

That's because d/t will measure displacement over an extended time, divided by that extended time.

@UserAnonymous Right, that's what I was thinking. I suppose then that the problem is how I merely divided by t.

So, it is not the same as instantaneous velocity, which v is supposed to denote.

Yeah. So how can I graph this correctly?

Graph what?

5:16 AM

Eurk. I am supposed to somehow linearize my graph of d vs. t and glean the correct values for a and v0 from that.

If your a was constant, and you are using a regression to fit d vs t graph, the first and second coefficients will tell you a and v0.

If I understood correctly what you are doing.

You have understood correctly, and I have performed such a regression, but my instructions clearly state that I need to linearize the data and perform a regression so as to get not only values of a and v0 but also uncertainties for them using LINEST.

What is LINEST? OK I'll google.

Oh! Hold on. I just figured out the source of my concern.

@UserAnonymous A Microsoft Excel function that finds the slope of a line, the intercept of a line, their uncertainties, R^2, and something else.

OK. That's even better. So, you don't have to do the least square fitting manually. It will straightaway give the equation.

5:21 AM

Anyway, one of the questions is asking if the value of v from my graph will match a calculated value of v using the formulas. But it won't, because my graph gives an average and the formula will give an instantaneous value. My issue was that I thought I was supposed to somehow get a graph of instantaneous values, which is not the case. I just need to find a and v0, and I've done that.

Great.

So, now I will do what I came to chat to do.

Thank you for your help, though. It provided the context necessary to figure out what my issue was.

I'm glad I was of any use. :)

@ACuriousMind - Whenever you read this, I just wanted to know if it is possible for you to slip in a pedagogic book reference for that particle definition question.

I mean this question physics.stackexchange.com/questions/138037/…

I mean that wiki link is helpful, but wiki is wiki. A book ref would be more preferable.

I'm sure you will have a suggestion, I can clearly see that the Wigner classification comment came just after I posted the question. So, I conclude you are well versed with it.

Thanks in advance.

1 hour later…

6:46 AM

0

Q: Should we allow answers to show up before migrating/holding/closing questions?

In my answer to this meta question I argued that the decision of whether or not a question should live on Physics SE is best approached by judging whether answers to that question would be likely to come from and be useful to other physicists. At the time, we were discussion questions with engine...

7:13 AM

On a separate note

42

Q: Why does my wooden door disperse light into a rainbow color spectrum?

On a clear morning, light comes through this window (viewed from the inside) It then hits the door on the opposite side (so viewing from the outside the door straight on the other side) There is nothing particularly special going on. However, if I go through the door and turn around to fa...

visible-light everyday-life dispersion

How does Floris keep going on with his magical answers?

5 hours later…

11:49 AM

@UserAnonymous I'm afraid I cannot recommend any book where this would be discussed, sorry. Perhaps someone else can.

2 hours later…

1:52 PM

I'm sure someone already talked about this but @JohnRennie how did you not get the Refiner badge? You have over 2700 answers. Surely you've edited at least 50 of those questions within 12 hours of answering them

Note that it's a new badge

I just checked and it says he has 55 eligible answers for the badge

Retroactively applied, of course

4 other people have it

2 hours later…

3:40 PM

@Danu, @ACuriousMind: Maybe it's just me, but I don't think "See also <x> site" counts as "When to use this tag" (in re: here and here).

3:56 PM

@KyleKanos I thought about it for a bit (didn't just blindly approve), and decided it could definitely not hurt.

If you feel strongly about it, I don't mind it being reverted either...

I don't think that Tag Wikis, as infrequent as they are read, are meant to be advertisements for other pages. That's just my thinking

Meh. I think it's useful information.

They're meant to describe (very briefly) the subject matter & applications

Reading 'advertisement' into it is really looking for harm where there is none in my eyes

I'm in the process of re-writing those few that guy managed to squeeze in there

3:58 PM

Or do you have good reason to believe that?

The guy has offered 6 suggested edits on excerpt, all of them "see also earth science stack exchange"

3 managed to get through

which may very well just mean he really thought it would be useful to refer people searching for a good resource to earth science SE

there don't have to be any bad intentions

He's also a moderator at Earth Sciences

Okay, that changes matters :)

I agree with you then

@KyleKanos and I apologize for forcing you to correct it!

Meh, no worries.

I'm learning stuff as I'm doing it.

4:15 PM

@Jim hey, could you tell me how you can view the number of eligible answer you have?

@KyleKanos Hm, you're right. I didn't realize that it was the excerpt and not the wiki itself when I approved that, I think.

And wow, how did John manage to avoid that one

@ACuriousMind wait.... I still didn't know that!

@Danu: Your studies have started now, haven't they?

4:31 PM

@Danu go to the data explorer and there are some links to queries that let you check your progress on those new badges

here's one: data.stackexchange.com/physics/query/229450/…

I need to pay attention to the data explorer site....

Apparently I'm 32/50 of the way there

@KyleKanos Beware of that link

What link?

Oh

Duh

Why?

Try dmckee

user ID 520

returns 95 posts

and he doesn't have the badge

Hmm

4:45 PM

Here's my attempt

data.stackexchange.com/physics/query/229615/…

but it still doesn't get the awarded badges right

so something's still wrong

That's odd

is it taking into account the >0 score of the answer?

LOL it says all anonymous users must fill a captcha.

And, I'm anonymous :)

@KyleKanos Sun not sun :(

@ACuriousMind that's OK. But where did you learn it from?

I'm indifferent to capitalization of planets & stars

4:48 PM

it kind of sticks out when the wiki makes a big deal about Earth and earth though

and for some reason I can't 'Improve' a tag wiki suggested edit

Yeah, you have to re-do it

But you'll get +2 if you get it approved ;)

oooh 6 rep on the line

I can't suggest until the current one clears the queue though

bit of a pain

@UserAnonymous You always need to sign in again on the data explorer, for some reason.

But even then, I'll still be anonymous :)

I'm user anonymous

@KyleKanos Yes. (Or I think ` AND Posts.Score > 0` does that. But SQL is not my forte.) I can't see what's wrong with the query, I'll probably drop it soon.

4:54 PM

Hmm

I don't know SQL really at all

5:06 PM

@EmilioPisanty: This one seems to be working: data.stackexchange.com/physics/query/229431/…

2

5:17 PM

Here's one that really works for the site:

And I've Forked it & modified it to show all with score users > 0

data.stackexchange.com/physics/query/edit/229633

2

nice, 22

it's clear that Qmechanics is the king of edit+answer

@Danu That's because he's the kind of editing

true

king*, I assume

With over 12000 edits, Qmechanic leads the field of editors by a large margin indeed.

3

@KyleKanos Nice

I'm trying to parse it but I can't see whether it deals with rejected edits or not

It doesn't seem to be including this one physics.stackexchange.com/review/suggested-edits/1793

but I can't see why

5:27 PM

That is weird

@ACuriousMind holy f*cking f*ck

@KyleKanos Oh, I see. Rejected edits never make it into PostHistory

I guess that makes sense. And the queries are easier.

6 hours later…

11:25 PM

0

Q: apparent bug in reputation screen

I have noticed various inconsistencies in the display of reputation on the user page (the display you get when you click on the "reputation" link where you see all "events" that build the day's rep). I was lucky (?) to have an answer that hit the rep cap both yesterday and today - but it is being...

user54412

11:41 PM

Ah, writers - such optimists. Would it be cruel to inform them that all those private industry plans to send a person to Mars will, without a doubt, kill that person?

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