The h Bar - 2019-10-04

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3:51 AM

A floating device created to clean up plastic from the ocean is finally doing its job, organizers say / cnn

↑ speaking of fluid mechanics :)

There has been some interesting quantum research recently according to science daily, one of these the headlines talked about measuring without collapsing. I need to check the paper in more detail to ensure it is not just another variation of weak measurement scheme and is really something novel

3 hours later…

7:05 AM

morning

7:25 AM

Beginner's guide to temperature scales:

user image

3

rankine missing

Rankine is dead on cold side and really cold outside for 100R

In other news, British retailers have been stockpiling German sausages because of fears there will be shortages if there is a no deal Brexit.

They are preparing for the wurst.

1 hour later…

8:36 AM

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@RyanUnger is that the fancy new mathematical GR

9:05 AM

@JohnRennie Don't forget Felsius

9:20 AM

@Slereah Either that, or the ravings of a lunatic who's really good at Latex. Possibly both. :D

3 hours later…

12:07 PM

@Slereah that looks good. what paper is this

12:22 PM

@RyanUnger arxiv.org/pdf/gr-qc/0208079.pdf

@Slereah I know that guy

is he at princeton

no

but that's more geometric analysis than GR

something like this is real GR arxiv.org/pdf/1706.07764.pdf

1:03 PM

Why do you care if a solution decays at polynomial rates in some weird Sobolev space in the special case the initial conditions are "sufficiently small"

user228700

1:27 PM

@SirCumference Hi! :-D How're you doing!? Long time!

Hey! Pretty good, how's life been with you? :)

user228700

Same here. :-) Did you graduate yet???

Nah, I've still got another year

user228700

Ahhh, okay. Two years for me!

user228700

How's college? Which major did you choose?

1:29 PM

Oh btw I noticed you pinged Blue and Balarka earlier. The latter doesn't seem to come around here much anymore, though Blue still pops in from time to time

user228700

@SirCumference Ah, that's a shame. Guess Balarka's also found a life outside of the internet. :-)

@KaumudiH Been pretty good, still Physics. Looking forward to leaving though lol

how about you? you were electrical engineering right?

@KaumudiH oh, no, I meant he went over to the math chat instead lol

user228700

@SirCumference Hahaha, I know that feel.

user228700

@SirCumference Ah, okay.

user228700

@SirCumference No, Computer Science!

1:32 PM

well there's my awesome memory again

user228700

:-) It's been a few years, lol, you really can't remember all this.

yeah fair

user228700

How's your sleep schedule? x'-D I seem to recall that it was pretty f-ed up.

user228700

Did you finally visit a doctor about it!?

@KaumudiH just had a chat about it yesterday, not much better lol

@KaumudiH oh yeah i should do that...

just now starting to remember our old conversation about that

user228700

1:35 PM

user image

user228700

Lol.

user228700

@SirCumference Wow. X'-D

ok several conservations haha

user228700

Hahaha, yeah. You were hallucinating, too, at one point!

user228700

Still doing that?

1:37 PM

yeah but more rarely lol

user228700

Oh, God. X'-D

just when im falling asleep i might hear like some incomprehensible voice, nothing schizophrenia-like

user228700

You remember how Secret used to (perhaps still does) have extremely vivid dreams and he would later post them here?

@KaumudiH Yep I remember, he occasionally still posts them. though a lot less frequently

man i'm getting some weird sense of nostalgia

user228700

@SirCumference Wow. Nice! Well, I started having similarly vivid dreams sometime back. They're crazy. I remember almost everything, and I dream every. single. night. Gets pretty tiring.

1:41 PM

ah man i envy you

i don't think i can remember any of my dreams these days, not since childhood

user228700

@SirCumference Hahaha, me, too. :-)

user228700

@SirCumference Dude, no. It sucks! It's very tiring.

user228700

@SirCumference This sucks, too, though. :-(

@KaumudiH which part? getting pulled out of them in the morning?

or just in general

user228700

In general. It doesn't feel like sleep at all, just a different version of waking.

user228700

1:44 PM

Y'know what I mean? My sleep is a lot less restful when littered with dreams.

well i did experience some weird pseudo-sleep when i tried melatonin this week

though i'm not sure if that's the same lol

user228700

@SirCumference You tried melatonin!? What was that like?

Er, I'm not convinced I was either awake not sleeping

user228700

That sounds uncomfortable...

Like it was just me lying in bed trying to sleep, but hours were passing like minutes

Weird feeling lol

user228700

1:47 PM

@SirCumference I've experienced this before, but not for the whole night.

user228700

So you're on melatonin now?

Eh I try it when I have e.g. a test

user228700

Just like that? You don't need a prescription?

Nah it's over the counter

At least in the US

user228700

Nice.

user228700

1:50 PM

You still have, like, extended conversations here?

user228700

Or do you mostly lurk?

It's died down a bit since people like obe, Bernardo, Balarka, etc. left

Well those are the main people that I can think of rn

user228700

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I think Bernardo left when 0celo was banned, but now that 0celo is back hopefully he'll return

user228700

"loser"?

1:53 PM

That obe?

user228700

@SirCumference Ahhh, I see. I heard about that from John. He and I have kept in touch over Google Hangouts.

user228700

@SirCumference Yeah!

@KaumudiH Oh wow

@KaumudiH oof

user228700

@SirCumference He's the only one from around here with whom I've been in touch. We chat once in a few weeks. Sometimes for a few minutes, and sometimes for much longer.

user228700

Why did obe leave?

1:55 PM

No one knows, though based on that I assume he's been going through rough times

Obliv also left a good while ago

Heather pops in from time to time

user228700

@SirCumference That's good to hear...

user228700

Was wondering about her. She must be in high school now, right?

Welp I gotta head to a mandatory TA session, but it's been cool chatting with you again :)

@KaumudiH Oh yeah she's a junior

user228700

@SirCumference Ah, okay. :-) Same here, man. (Not the TA session. :-P) Guess I'll catch you later! It's nice to have friends in far places, haha.

2:07 PM

@RyanUnger I don't know why I even said it, really. I mean, I do obviously know about gender pronouns. It's just that it's not an issue I care about. I think I said I didn't know what they are in order to half-heartedly start a small discussion on the matter, or something. Bit sad, really, I suppose.

2:20 PM

And the whole situation doesn't surprise me, to be honest. You only have to peruse SE Politics to see how ultra-Liberal the Stack Exchange directors are.

2:30 PM

2

Q: Work done by friction in a comlplicated path

A block of mass $M$ is taken from point $A$ to point $B$ in a complex path by a force $F$ which is always tangential to the path. We also have coefficient of friction as $K$. What will be the work done by force $F$ when it reaches point $B$ from point $A$? Given that the vertical displacemen...

energy-conservation

Maybe I am just making a silly mistake, but I am pretty sure $N=mg\cos\theta$ does not apply in this instance. Yet almost every answer being posted is assuming this.

@AaronStevens I think it still holds here, as long as you account for the fact that theta is always changing so N is always changing. The force F is always tangential to the path, so it shouldn't affect the normal force; were that not the case, it wouldn't hold

@JMac But the acceleration needs to have a component perpendicular to the path for it to change directions

@JMac In other words, along the direction perpendicular to the path $N-mg\cos\theta=ma_\text{perp}$

@JMac For a more usual example, when an object is sliding down a circular bowl, we don't say $N=mg$ at the bottom

2:46 PM

@AaronStevens That's a good point. I think mentioning the curved path more directly in your comments should help point that out. I'll try to word some comments differently to see if people can understand what you're getting at

I like Unique's logic... In India publishers often ignore that aspect, so apparently when answering we should too...

@JMac I didn't know the physics education in India was so standardized haha. I highly doubt that is actually true though.

But this is one of my intro physics pet peeves. Students get so used to $N=mg\cos\theta$ they start to forget where it comes from and when it is actually applicable.

@AaronStevens It frustrates the problem solver in me. I love questions where you have to make an assumption to remove a variable so that you can actually solve it. Assuming N = mg cos theta is not a good assumption to make here based on the problem statement. Like the wording of the question actually makes that impossible

@JMac Yes, exactly. I don't think this problem is actually solvable without additional assumptions. I think if you assume $F$ is constant, and if you are given the final speed of the object, then I think it would be doable.

@AaronStevens Or if you were given the path you should probably be able to get something in terms of F

and if F is constant i guess

@JMac Yes, that too. More is needed somewhere

3:12 PM

@JMac I am really surprised at how much push back this is getting

@AaronStevens IKR. I think I might know why the book is assuming that (and OP didn't copy that detail in their problem) I found the textbook question, there are a few magic words here that actually might make the assumption half-valid, but no one mentioned it yet

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@JMac So maybe the book is trying to assume that $a_\text{perp}\approx 0$?

@AaronStevens Exactly. "Slowly" might be the magic word here, but no one has included it in their questions and answers

if v was approximately 0, then acceleration when turning would be approximately 0

@JMac Yeah that would do it. I would make an answer, but really I have VTC for it being a homework problem with really little effort shown.

@AaronStevens Yeah, I'm going to give Unique a hint (since he seems to have access to this text), but I want to see if he can understand the important difference between the written question by OP and his "correct" answer

3:20 PM

@JMac If the question was worded differently to really be about these concepts, then maybe. But this really is framed more as just a homework problem looking for a solution. It just so happens to involve some pretty subtle points that are pretty important.

@JMac Good luck with that. That user has a history of not being the best to have a conversation with

@AaronStevens I just want to see if he can figure it out even when I draw him right to that fact. Because he's the one who mentioned the textbook it was in, and I found the major difference right away when I googled it

@JMac Same. "slowly" becomes an important word often haha

@AaronStevens Yeah. I googled it because I highly suspected there would be a keyword like that. It's like I was saying earlier, I love questions where you have to make an assumption to remove a variable so that you can actually solve it. Generally there's at least one keyword that helps you get there, and I figured OP might have missed it when copying.

@JMac Well then, let's see what ends up happening I guess.

@AaronStevens physics.stackexchange.com/questions/506329/… swing and a miss

3:28 PM

@JMac I already told Unique that constant speed has nothing to do with this

1 hour later…

4:33 PM

8

Q: How can we argue that the electron has $g \approx 2$, but not the proton?

I'm suddenly getting confused on what should be a very simple point. Recall that the $g$-factor of a particle is defined as $$\mu = \frac{ge}{2m} L$$ where $L$ is the spin angular momentum. For any classical system where the charge and mass distributions are identical, $g = 1$. However, one of t...

quantum-field-theory particle-physics quantum-spin

@knzhou This should be HNQ

(but can't because of MathJax)

5:08 PM

@EmilioPisanty Aha, that's why!

That's alright though. I can't see anybody on SE, but not already on Physics.SE, being interested in it.

@knzhou I reckon it would help to raise the stakes and the visibility

@knzhou I made that same mistake earlier =P

24 hours ago, by PM 2Ring

@EmilioPisanty That will never hit the HNQ, due to the MathJax in the title.

Greed has gotten the better of me, I wrangled together a TeX-free title. :P

6:04 PM

Hi, everybody.

@KaumudiH Hi, Cow Smoothie! It's been a while.

2 hours later…

7:50 PM

@DanielSank hi, Dan

user image

For you ;-)

"pndjo"?

8:04 PM

@SirCumference I probably shouldn't spell it out

"dumb" is a reasonable translation

3 hours later…

10:41 PM

I have a question concerning Lie derivative acting on a metric g (I used Schutz "geometrical methods of mathematical physics "). I didn't understand how to show $\xi^{k} g_{i j, k} = -2 \xi_{l} \Gamma_{i j}^{l}$?

Schutz used the definition of the Lie derivative acting on the metric as

$\left(\mathfrak{E}_{\xi} \mathfrak{g}\right)_{i j}=\xi^{k} g_{i j, k}+\xi_{, i}^{k} g_{k j}+\xi_{, j}^{k} g_{i k}=0$

he lowered the index k with

$\xi_{k}=g_{k l} \xi^{l}$

Then if we admit the equality in question we can finally write

1 hour later…

11:48 PM

@EmilioPisanty hahahahaha

Yo me siento pndjo todos los dias...

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