The h Bar

12:35 AM

@bl00 wtf is enriched UC?

user218912

@0celo7 what?

user218912

what do you mean?

I have no clue

4% enriched UC showed up on my homework

and I have no clue what it means.

user218912

what homework?

user218912

nuclear engineering?

12:37 AM

yes

user218912

no idea, googling doesn't help either.

ah

Uranium Carbide.

user218912

that's what I thought

user218912

but I've never seen uranium carbide before

So it's UC with 4% U-235.

user218912

12:39 AM

so I thought it would be dumb to say that since I thought it didn't exist

user218912

yes

@bl00 it's used in experimental reactors IIRC

user218912

experimental reactors?

user218912

aren't all reactors experimental?

Huh?

user218912

12:40 AM

idk...

user218912

i'm so dumb

user218912

was evaluating this integral with time in the exponentials but it's supposed to be at t = 0.

Hey @0celo7, what's the diagnosis?

I need stronger glasses and I shouldn't waste the doctor's time like that.

@0celo7 You suddenly need glasses in one eye?

That's suspicious.

12:53 AM

I've had glasses in one eye already

But they stopped working

which was the whole point

although the prescription he wrote has corrections for both eyes

-1 R, -0.5 L

But my left eye is fine

er

fuck, I always mix up left and right

-1 L, -0.5 R

My right eye is fine

So I don't know why he wants a correction on both eyes.

1:15 AM

@bl00 holy shit I'm stupid

I spent the last hour deciphering this thermal utilization calculation

I was looking at the wrong cross sections.

user218912

lol

user218912

nice

why does the math chat get weird dudes so often

@BalarkaSen You mean that Inquirer dude?

i think he used to be called Twink

oh

1:23 AM

he's probably severely autistic, he's been stalking Ted for a while

sad really

i remember that guy, but never realized this is he

I think it's him

Similar attitude

We get weirdos too.

One of them is banned for a year

i liked that dude

good trolls are always likable

Me too

I named my former blog after him

heh

there's an algebra troll i like in the math chat, i think he's syzygy now.

he's rather condescending sometimes, but his trolls were pretty solid.

1:28 AM

@bl00 Hmm. Possible pitfall. Do I separate the uranium from the carbide in the total number of moles of stuff in the reactor?

user218912

@0celo7 depends what the question is asking.

user218912

I remember when I used to be good at chemistry.

user218912

like 3.5 years ago

well I need relative ratios of all the stuff in there

so because I have separate cross sections for U and C I have to separate them

user218912

brb getting a drink

1:33 AM

@BalarkaSen Hey maybe you can explain what the smash product is telling me

I haven't had time to read Hatcher this week but I've been pondering it

I understand that $S^m\wedge S^n= S^{m+n}$.

mhm

So I'm trying to picture $S^1\wedge S^1=S^2$.

and?

it's not working :P

I don't understand Hatcher's comment about collapsing $X\vee Y$

I understand what it means, sure

But I can't picture it.

Think of a torus as a 2-cell attached to the wedge of circles, in a way so that that wedge is precisely wedge of a pair of meridian and longitude in the torus

Can you do that?

1:38 AM

Yikes, this cell stuff is freaky.

I'll think about it and get back to you. (Doing other things now)

I also have to think about something for ted, can't forget that

@BalarkaSen Yes, I think that works.

Definitely.

Unfold the torus to a square

That's it.

Now pinch off the wedge of circles (that's what quotienting means!). Your 2-cell gets attached to a point (the equivalence class of the wedge of circles inside the qt space). That's a sphere.

Ah ok.

So that would be like collapsing the border of the square?

That's it, yes.

Cool, thanks.

No problem

A word of advice: I mostly don't care for smash products, but it's useful in homotopy theory. I don't know why one would care for join of spaces. So read these constructions only if you're interested.

1:45 AM

@BalarkaSen Well, I think right now I'm only looking for the basics of covering spaces and fundamental group.

You'll find them in chapter 1

I want to read Wolf's book on constant curvature spaces and I want to be more familiar with that stuff because he uses it extensively.

@BalarkaSen Yeah

I wish I knew more geometry nowadays

I wish I knew algebraic topology.

I just can't seem to sit down and read it

I really enjoyed the first half of Bott & Tu but that was basically geometry.

Obliv

@acuriousmind I figured out my issue

kind of.

2:00 AM

What was your issue?

Obliv

I realized that the reflected wave in my experiment was a standing wave. So, I still don't know what happens if you stand closer or further away from the point source (I am told to believe that the amplitude of the wave varies)

@0celo7 issues with understanding how we hear sound waves w.r.t. position you are at

Hello again

I had to leave because I had to do some stuff and didn't think I'd be back

So, I'm confused.

A set $A$ can be a subset of itself.

And sets can have other sets as elements.

But I read somewhere that $A$ isn't always $\in A$.

Why not?

@heather What are the elements of $A=\{a,b\}$?

Alfred Centauri

2:25 AM

@heather If A is the set of all silverware in my house, then A, not being silverware, is not an element of A.

@0celo7, it's just any set, I guess.

@AlfredCentauri, but then why can sets be sets of sets? (Sorry, convoluted, I know.)

@heather what

@0celo7, sorry, I misunderstood what you were asking. $a$ and $b$, right?

@AlfredCentauri, and why can $A$ sometimes be $\in A$ then?

It can't

What can't?

2:35 AM

$A\in A$

Huh...then why did I think it could be, I wonder? Ah, well. Misconception cleared up. Thanks!

user218912

well do you know why?

@bl00, I think so. It's like AlphaCentauri said: if you have a set, like A = {1, 2, 3}, A is not in A; to be, the set would have to be like {1, 2, 3, (1, 2, 3)} but then, if you set that equal to A, then you have to have that added in, and so on.

If that makes any sense.

user218912

what's the real formal reason for it though

@bl00 Hell if I know.

What is it?

user218912

2:42 AM

idk I thought you knew

I don't know PhD set theory.

Stan Shunpike

hello

@bl00 so, explain

user218912

I don't know!

user218912

I already said idk the proof

2:51 AM

@bl00 Because you can't prove it.

user218912

why not?

It's an axiom.

user218912

oh, k

I should write a book.

user218912

but you don't have a ph.d

2:54 AM

no one would read it

but it would be nice to have

user218912

i'd read it.

user218912

depends what it's on though

No you wouldn't.

We would begin with a proof of the Steenrod theorem.

i.e. derive the necessary and sufficient conditions for Lorentzian metrics

user218912

you're right I wouldn't

what a noob.

user218912

2:56 AM

flagging.

you won't

you don't have the guts

user218912

only one way to find out

user218912

but it's not bad enough to get you banned for 30 mins.

user218912

so it's pointless

lol

as I thought

user218912

2:58 AM

:(

user218912

you need a new avatar?

no

user218912

what were you pinging me this morning for then?

I changed my mind.

@bl00 quiz me on QM

I have an exam tomorrow.

user218912

idk what your course covers

user218912

3:04 AM

do my qft problem set 1 for review

user218912

it has qm stuff

A: What is the formula for the composition of two axis-angle rotation vectors?

The exam is the first quarter of Sakurai.

Keshav Srinivasan

3:25 AM

If anyone wants to know how to find the composition of two axis-angle rotation vectors, check out my question and answer here:

0

The formula is given in this excerpt from a journal paper. It was discovered by the French mathematician Olinde Rodrigues in 1840, which was before the invention of vectors or even quaternions (which were invented before vectors). The composition of $\alpha\hat{l}$ and $\beta\hat{m}$ (where the...

Physics Meta

4:14 AM

0

Q: Repeating question

New user here. I asked this question a few days back. (first post) Relative motion-Acceleration And i did not get a satisfactory answer for the above question. so I framed my question better this time and asked again as I dont see any activity with my previous question http://physics.stackexch...

ffahim

How swimming make us float????

user218912

it doesn't work for everyone, clearly. that's why people drown.

ffahim

4:33 AM

Well think about an athlete or a normal person who knows well off swimming... so why doesn't he drown ? Any explanation? Is it buoyancy or Newton's law.... I just assumed ... if anyone knows the actual phenomenon then pls tell me ???? :-)

user218912

@ffahim I think it's buoyancy and conservation of momentum.

user116211

What did the doc say @0celo7?

user116211

I suppose you got a spec.

user116211

Maybe ;/

user228700

4:50 AM

@0celo7: So u don't have eye cancer or whatever. Cool.

user228700

5:02 AM

@JohnRennie: Morning sir :-) I read that post (regarding the collision of a ball with a wall) that u had linked yesterday, and the OP uses the phrase "momentum came from the ball". Is this a correct way to phrase it?

@Kaumudi remind me which question that was - have you got the link?

user228700

Yep:

user228700

physics.stackexchange.com/questions/175095/…

Q: What is meant by "space is expanding"?

Ok. I think the OP means that initially the ball was the only object with a non-zero momentum. This is true in the rest frame of the wall.

user228700

Ah, OK, thank you :-)

user116211

5:14 AM

0

Is it just the distance between galaxies that increases? Or is the distance increasing between the molecules in my body? Does expansion affect universal constants?

space-expansion

user116211

Lack of research efforts?

general-relativity quantum-mechanics black-holes string-theory gravity cosmology quantum-field-theory

6:04 AM

@SirCumference none of the usual black hole metrics have a limit on the black hole mass. There is a practical limit to the size of current black holes because they've had only 13.8 billion years to grow. But there is no theoretical limit.

user116211

7:10 AM

Lawrence B. Crowell

7.1k 13 20

user116211

Lawrence B. Crowell

5.8k 1 3 16

quantum-mechanics general-relativity black-holes cosmology spacetime string-theory space-expansion

user116211

I wonder whether they are the same person.

Yes, Lawrence left then rejoined. He doesn't seem fussed about merging the accounts.

user116211

7:22 AM

ohh.

user228700

@JohnRennie: Busy, sir?

GPhys

five hours of filtering LIGO data later

@Kaumudi Bad timing! I have a work activity that starts at 08:30 i.e. right now. Back in 15 minutes.

GPhys

I've.........slightly lowered the noise

:D

actually I think that's good enough, now I just have to go and find all the harmonics of the things that are obviously noise and add filters to kill those

@JohnRennie Hello.

user228700

7:31 AM

@JohnRennie Oh, that's OK, I'll wait :-)

user116211

@DanielSank o/

user228700

@GPhys I feel like u deserve a high-five for ur hard work so here: 🙋

@MAFIA36790 \o

user228700

@DanielSank: Hi :-)

@Kaumudi Hi.

user228700

7:33 AM

Hey, @DanielSank: Are you busy?

@Kaumudi Only a little.

Doing my Russian lessons.

What's up?

user228700

Oh...

user228700

Well, I came across this problem that I did solve but I'm a little confused about the wording, which didn't allow me to solve the question w/o taking a quick perk at the solutions first.

user228700

I was wondering if it was indeed improper wording or just my general dumbess...

Okay, where's the problem?

user228700

7:44 AM

This:

user228700

user116211

@DanielSank I translated online a Russian pdf file today both by Google Translate and Microsoft software.

user116211

The output was a mess.

I'm back!

user228700

While trying to solve the problem the first time around, I kept thinking about all the different possibilities...

user228700

7:46 AM

@JohnRennie Yaay! :-)

@Kaumudi Work in the centre of mass frame and the problem becomes straightforward.

user116211

@Kaumudi It is in HC Verma.

user116211

Or related.

user116211

He worked in COM Frame.

@JohnRennie It's a conservation of energy problem...

Am I missing something?

7:47 AM

That is, the frame where each block is moving towards the centre at 1m/sec

user228700

@DanielSank No, u're not...

user228700

I had to use both, conservation of energy and momentum.

Wait, when it says "...front block kept at rest..." does it mean that the front block is fixed in place or that it's not moving at the beginning but is free to move?

user228700

@MAFIA36790 Oh, really?

@Kaumudi switching to the COM frame implicitly uses conservation of momentum, but in the COM frame you only need conservation of energy.

user116211

7:49 AM

@Kaumudi yes, more or less same sort of problem has been given in the illustrated examples.

user228700

@DanielSank Not moving but free to move.

user228700

@JohnRennie When we switch to the COM's frame, what we're essentially doing is having the COM be at rest, yeah?

@Kaumudi Ohhhhhhh. Yeah use the COM.

@Kaumudi Yes.

@Kaumudi Yes.

user228700

Hm...

user228700

7:52 AM

This switching isn't making that much sense to me...

user228700

I guess I need to think about it some more :/

Shing

Hi everyone

user116211

Hello.

user228700

Hi :-)

user228700

Well, my original question was different...

user228700

7:55 AM

When I was attempting to solve this question for the first time, I was trying to list all the different possibilities...

@Kaumudi Ok, please ask original question. Meanwhile, when we say "switching" we just mean to express the motions of the blocks from a different reference. For example, from your current reference, your computer is not moving, but from the point of view of someone driving past you, your laptop is moving.

@Kaumudi ok...

user228700

Since the table is frictionless, the case in which the rear block collides with the front block and then everything comes to a stop is ruled out, correct?

user228700

..?

@Kaumudi yes because momentum wouldn't be conserved.

user228700

Right.

user228700

8:00 AM

Okay, the next possibility is that the two blocks collide, and then, they move together.

user228700

The last possibility is that the two blocks collide, and then the block with the spring gains more velocity and speeds away, while the other block moves slower...

user228700

Yeah?

user228700

..?

Perhaps, but where are you going with this?

user228700

My book says "Find the max. compression of the spring".

user228700

8:10 AM

I am to take this as "Find the max. compression of the spring", possible in all the cases taken together, yes?

user228700

See, the thing is, I wasn't immediately able to identify which case it was 'cause they didn't specify.

user228700

And then I looked at the solutions and saw that they'd taken it to be the case in which both blocks move together (w/ the same velocity, of course) after collision.

user228700

And that they'd done this 'cause of the "max. compression" thing.

user116211

@Shing, Try to make the units upright in your edits. Use \mathrm for that.

user228700

Do u understand what I'm getting at..?

8:21 AM

@Kaumudi Yes I understand why you're confused.

You've identified several possibilities for how moving objects can behave.

You've assumed from the start that you should consider all of these cases.

Shing

@MAFIA36790 got it, thanks for the reminder

@Kaumudi Note that the small block could even reverse direction.

Imagine the small block is 1 gram, and the big block is 200 kg.

The 1g block surely "bounces" off the 200kg one.

But in any case, how could the blocks move together after the collision? We have a spring there.

user228700

@DanielSank Exactly.

user228700

@DanielSank Oh, yeah, it could...

@Kaumudi It's good to consider the cases qualitatively. Good job. However, note that if you simply solve the dynamics of the problem, the correct case reveals itself automatically.

user228700

8:28 AM

@DanielSank Huh. I didn't even think about the fact that they won't move together 'cause of the spring in b/w...to me it seemed like they could? :/

user228700

@DanielSank What do u mean by solve the dynamics?

@Kaumudi Suppose I toss a coin. It could land heads up, and it could land heads down, but if you know the initial velocity and orientation, you can just compute exactly how it lands.

user228700

Hm...

user228700

So, in this case, I should've placed myself in the frame of reference of the COM and then...what?

@Kaumudi I'd go with conservation of energy.

user228700

8:33 AM

Hang on, they can't move together 'cause of the spring...how so?

Uh, well what do you mean by "together"?

I'm imagining the blocks are touching, which means the spring is compressed and therefore pushing the blocks apart.

user228700

Right...

user228700

OK, then it definitely doesn't make any sense to say that they move together.

@0celo7 "it is pshysics question and it got stuff"

user228700

Damn, I have some serious problems when it comes to conservation of momentum. I wonder if it's just 'cause I'm super dumb or if my brain is complicating otherwise simple concepts...

Q: Two Cases Of Harmonic Motion Caused By Gravity

8:37 AM

@Kaumudi I have some idea of why you're confused, but perhaps it's best if you keep explaining and I'll just pipe up when you need help.

user228700

Thank you :-) I'm gonna go check out some problems on YouTube to make sure my brain is doing the reasoning correctly...

user116211

8:58 AM

I just saw the comment @yuggib. I've figured the problem out myself; nevertheless, thanks for showing interest :)

user116211

9:22 AM

0

I'm a highschool student and we learned not so long ago Simple Harmonic Motion, and I'm trying to analyse "similar" cases which I thought of. Here we have a body (with mass $m$) being affected by the gravity of a body whose mass is $M$, yet it doesn't collide with it (it just goes through its ...

gravity newtonian-gravity differential-equations harmonics

user116211

Homework?

9:58 AM

Yo y'all, is there some specific reason why there's a meta tag duplicates and also exact-duplicates?

shouldn't the latter be a synonym of the former?

David Z

@EmilioPisanty not that I can think of

@DavidZ I suggested the synonym but without some mod magic it'll just linger in limbo for all eternity

user228700

10:56 AM

Does anyone here listen to "Dear Hank & John", the podcast?

11:17 AM

@Kaumudi Never really bothered, I don't quite grok podcasts as a format

but I do follow vlogbrothers pretty closely

at least since (roughly) the time that crash course astronomy came out

Hello

user228700

11:46 AM

@EmilioPisanty Ah, OK. I must mention though, that the podcast is hilarious and I never fail to learn something new from it :-)

user228700

@EmilioPisanty Nice! :-D Have u been to any nerdfighter meetings/any of their shows?

user228700

@EmilioPisanty Ah, I too was introduced to them via Crash Course (Chemistry, in my case). Been a hardcore nerdfighter since April of 2014 :-)

nature.com/news/…

$$\text{Whole} > \sum \text{parts}$$

@Secret, that reminds me of a poster I once saw...

"be $\geq$ you ever imagined"

Complex system is all about the above equation (and also the $\leq$ version). We have to be always ready for surprises when we mix A with B

user228700

11:59 AM

@heather: Hi :-)

@Kaumudi, hello! =)

Working on anything especially exciting?

@DanielSank oh my god

How is that real?

@0celo7, hello! Another day of school on the horizon.

Q: Is there any guarantee that there will be only one point on the isotherm curve of a system that has a certain Y coord?

0

When reading this part of the "heat and thermodynamics" book of Zemansky, is there any guarantee that there will be only ONE intersection of the $Y=Y_1$ and the isotherm line? There must be. Because otherwise the concept of temperature would be self-disproving.

thermodynamics temperature measurement

Is it even possible to exist a substance whose isotherm is not monotonic?

Thermometer

A Thermometer is a device that measures temperature or a temperature gradient. A thermometer has two important elements: (1) a temperature sensor (e.g. the bulb of a mercury-in-glass thermometer) in which some physical change occurs with temperature, and (2) some means of converting this physical change into a numerical value (e.g. the visible scale that is marked on a mercury-in-glass thermometer). Thermometers are widely used in industry to control and regulate processes, in the study of weather, in medicine, and in scientific research. There are various principles by which different thermometers...

fine...water...

user228700

12:21 PM

@heather Hang on, how old do u think I am..? :-P

25?

Probably doing a PhD in chemistry

user228700

@0celo7 Wait, how old are you?

18

19

user228700

Have u seen this:

user228700

12:36 PM

Yeah, I'm 17.

So?

user228700

But, but. U just said...oh, never mind. U tend to forget things very soon?

What?

Dunno what you're talking about

@Kaumudi oh, yeah, I thought that avatar looked familiar

Sir Cumference

I fell asleep at 7:30

crashed

Yet I'm still so tired...

user228700

12:49 PM

@0celo7 U just asked if I'm 25. Never mind.

user228700

@EmilioPisanty :-) "Pizza. The answer is always pizza".

user116211

@Secret WTH is that?

@Kaumudi ok so you're 17

Doing a PhD at 17, wow

1:42 PM

@MAFIA36790 whole is more than the sum of its parts

Jim

@Secret This is why $1>0.5+0.5$

Slereah

Hello

user228700

@0celo7 Very funny -_-

user228700

@Ramanujan: Dude, do they teach u linear programming too? Not for school, for the exam.

user228700

Who the heck starred this: (:-P)

user228700

1:56 PM

1 hour ago, by Kaumudi

But, but. U just said...oh, never mind. U tend to forget things very soon?

user228700

@Ramanujan: Holy eff, do they teach u all this:

user228700

@Kaumudi nope

user228700

Actually, no, never mind. Most of that is in the syllabus.

user228700

@Ramanujan Hey, but damn, I forgot, u wouldn't know, right? LP is in 12th.

2:04 PM

Yep,everything over mind

user228700

"Everything over mind"?

user228700

What do u mean?

user228700

I thought u said only 11th is over.

(pura sar ke upper se gaya) what you send picture

Yep,only 11th is over

user228700

:-P Lol, I see. Sar ke upar se gaya, kyunki abhi tak aap logo ko sikhvaya nahi, ya tumhe pata nahi?

user218912

2:09 PM

guys this is an english chat.

user228700

(Excuse my Hindi, folks.)

user228700

@bl00 No fair! Besides, I feel that it's sometimes better to speak with Ramanujan in Hindi. Hey, people share stuff here in other languages that we don't get all the time. But OK :-( If u say so, no more Hindi.

user228700

@Ramanujan: Is it possible for u to find out if LP is taught to ur seniors for the exam? (NOT for school)

@Kaumudi I don't talk anything to senior till now,not possible

user228700

@Ramanujan Not possible? Wow, OK.

user228700

2:14 PM

Alright, thanks anyway. I'm gonna risk it and study LP.

Btw,why are studying something which is not for jee? @Kaumudi

user228700

It IS in the syllabus.

user228700

BTW, "pura sar ke upar se gaya"="It went over my head"

user228700

Not mind :-)

Can you answer this?chat.stackexchange.com/transcript/message/33053200#33053200 don't answer now,DHMO is trying for me @Kaumudi

Same, langue is used to understand,not to correct it,i will keep in mind

user228700

2:23 PM

@Ramanujan That's not true. The purpose of language is to make sure that the person whom u are speaking with is able to understand you as clearly as possible. I didn't understand when u used the word mind in place of the word head, so I brought this to ur attention for ur own benefit. I can be a little pedantic at times, but it's something I'm trying to grow out of. Bura mana, to I'm sorry, but yes, to help u was my only intention :-)

@Ramanujan India!

So confusing

user228700

@0celo7 Dude, I explained this to you! (The deal w/ all the exams ie.)

user228700

@Ramanujan At first glance, I dunno how to do it. I will think about it later and let u know, since I need to finish LP now. (I'm sorry! I hope DHMO helps u)

2:47 PM

@Ramanujan The answer is (4). You don't need to do any clever maths to see that. Just take $n=1$ in which case the expression becomes atan(1/3) = 0.3218 and (4) is the only equation that gives this value.

I think all Math is clever.

I've always considered myself to be possessed of low cunning rather than cleverness :-)

@JohnRennie that will surely help only when I know what's value of atan(1/3)

Teachers used to solve most of problems by same method, but as I want to study in deep,i don't want to do in that way

@Ramanujan aren't you allowed to use a calculator? If not then I agree that calculating arctangents by hand is probably a non-starter :-)

@JohnRennie calculators are not allowed

@JohnRennie one quick math problem?