The h Bar

0celo7

15:18

@JohnRennie balls

John Rennie

:-)

I'm afraid a fuller understanding of manifolds will have to wait

While I was genuinely in awe of your and Barry's discussion on the subjecty (really, I'm not trolling) it isn't that high a priority of mine.

koolman

Are you free now @JohnRennie

AccidentalFourierTransform

@koolman youre being kinda rude with JR

koolman

15:33

@AccidentalFourierTransform why

AccidentalFourierTransform

well if you have to ask why, then there is not much I can say

koolman

Is it look like that

Sorry @JohnRennie if you think I am rude

Sorry

AccidentalFourierTransform

youve been trying to get him to help you with your homework nonstop

you shouldnt do that

its impolite

koolman

Sorry

I thought he like to solve question ,thats why I ask

AccidentalFourierTransform

well, nobody likes homework ;-)

koolman

15:37

But it is actually not a homework

Secret

I can sometimes get annoyed when people ask me too many non homework questions too. Well I am just a lazy bone

AccidentalFourierTransform

@Secret thats @heather, right?

thats how I picture her

Secret

nope, I don't know what heather looks like

Therefore that is not her

AccidentalFourierTransform

well, if you dont know what she looks like, it may be her

youve been dreaming about her unknowingly

Secret

Nope. I know who the dream characters are. This girl is actually from a dream 6 years ago

(Pardon my drawing still suck at real people drawings, this is only 75% of what she actually look like)

Of all dreams, dreams involving her are quite unusual, and consistent. She is a recurring dream character for more than 12 dreams

AccidentalFourierTransform

15:43

@Secret that one is creepy

looks like the girl from the ring or something like that

user228700

All of them look like mugshots to me :-|

AccidentalFourierTransform

lol

user228700

@JohnR: Tell me, what is it like, being an adult? U can eat all the fried food in the whole world for lunch, isn't that right? :-) Where is that picture...

John Rennie

@Kaumudi.H I don'tknow.I'll tell you when I've grown up :-)

user228700

AccidentalFourierTransform

15:47

@Kaumudi.H BTW, how is your name pronounced?

user228700

The yellow sticks are french fries, aren't they? Oh, chips sorry.

John Rennie

Actually everything there was baked not fried. Though I have to concede there was a lot of oil involved.

@Kaumudi.H yes, they were chips/fries with a spicy coating.

AccidentalFourierTransform

looks tasty

whats the round thing?

user228700

@AccidentalFourierTransform I discussed this with a few people from NZ last week and after hearing me saying it, one of them said "So it's like Cow Smoothie, without the S sound" x'D

5

user228700

@JohnRennie Ah, I see. I don't know what any of the other items on your plate are.

John Rennie

15:48

The round flat things are chicken burgers with a crispy coating. And the bread rolls are savoury hot cross buns.

AccidentalFourierTransform

@Kaumudi.H oh, that's how I would pronounce it if it was a Spanish word.

cool name by the way

user228700

@JohnRennie Hot cross buns, hot cross buns...isn't that a rhyme? I've only ever heard about it in that rhyme.

user228700

@AccidentalFourierTransform Thanks! It's Sanskrit for "Moonlight" :-)

John Rennie

Hot cross bun

A hot cross bun is a spiced sweet bun made with currants or raisins, marked with a cross on the top, and traditionally eaten on Good Friday in Australia, British Isles, Canada, India, New Zealand, Pakistan, South Africa and some parts of America. The buns mark the end of Lent and different parts of the hot cross bun have a certain meaning, including the cross representing the crucifixion of Jesus, and the spices inside signifying the spices used to embalm him at his burial. They are now available all year round in some places. Hot cross buns may go on sale in Australia and New Zealand as early...

user228700

Hot cross buns!
Hot cross buns!
one a penny, two a penny,
Hot cross buns!

If you have no daughters,
give them to your sons.
One a penny two a penny,
Hot cross buns!

John Rennie

15:50

Though these were made with cheese and weren't very sweet, though they did have currants in.

AccidentalFourierTransform

@Kaumudi.H ah, so much cooler!

user228700

It seems to me like an exceedingly poor strategy, really--one a penny, two a penny? Why would anybody buy just the one?!

AccidentalFourierTransform

@JohnRennie what did you use for the coating?

user228700

@JohnRennie ...I see.

John Rennie

@AccidentalFourierTransform I bought the chicken burgers ready made I'm afraid.

The coating was weird. It was a bit like rice crispies.

anonymous

15:52

@Secret Without the long hair it looks like that of a boy :) Creepy nevertheless!

John Rennie

However it was indeed very crisp! Actually it was very nice.

user228700

I have rice for lunch, day after day after day after day after day after day after...

Secret

@Kaumudi.H Lol, those green lines are actually my notepad, I just sketch it quickly on the notepad and the lines got scanned with it

user228700

Dyou know, @JohnR: that while I was in school, I took Chapatis for lunch for 8 consecutive years?!

anonymous

@koolman What is your question?

John Rennie

15:54

@Kaumudi.H With different fillings though?

user228700

@JohnRennie There were two or three.

AccidentalFourierTransform

a couple of days ago I had indian food for lunch

indian food is superb

user228700

By the time I was in high school, my mom started experimenting and started to make different types (tamarind, lemon) of rice with potato chips occasionally.

koolman

@anonymous physics.qandaexchange.com/?qa=1139/…

user228700

@AccidentalFourierTransform No arguments here. However, NB: I haven't tasted any other type of food ever so I'm biaaaased!

anonymous

15:57

@koolman Interesting. Looking :)

user228700

@JohnR: Have u tried making rice?

AccidentalFourierTransform

mediterranean food is very good in general

John Rennie

@Kaumudi.H Yes of course, I cook rice all the time. You've seen my vegetable risottos.

AccidentalFourierTransform

greek, italian, etc

user228700

@JohnRennie Ah, the risottos! You should try to make lemon rice. So. Tasty!

user228700

15:58

And it's such a simply recipe!

AccidentalFourierTransform

I think that my favourite food is peruvian though

that shit is amazing

John Rennie

I cook rice in lots of ways, though I must admit I've never cooked lemon rice. I'm particularly fond of kitchuri rice.

user228700

@JohnRennie What is that omg, ::frantically Googles::

John Rennie

Rice with lentils.

user228700

Ohh, you mean khichdi!

user228700

16:01

That is just daal and rice! We gave it a name for the sake of giving it a name! That doesn't even count as "type of dish made from rice".

koolman

@anonymous yeah

John Rennie

kitchari

user228700

@JohnRennie Same difference.

ACuriousMind

The 2e DMG once compared the experience of leveling to waking up in the morning and realizing that you finally understand what a cosine is. The metaphor probably still holds. — The Spooniest 1 hour ago

wisdom from rpg.SE

Secret

(insert comment)

John Rennie

16:04

A little know fact ... during the days of the Raj this was adapted into the popular British disk kedgeree

Kedgeree is a family favourite and we have it whenever I visit my Mum.

user228700

@JohnRennie Nice! :-)

user228700

Everybody can use a laugh once in awhile. Look at what I found:

0celo7

@JohnRennie I don't know how you could be fascinated by basic topology

user228700

If you want me to stop, please tell me, don't ban me for spamming :-P

user228700

16:09

user228700

... ::sadly walks away::

John Rennie

@0celo7 I must go back and reread the conversation. You kept mentioning things, just as asides, and I'd think aha so that's what that means. As a hedge physicist I stand in awe of the rigorous approach, though I don't necessarily want to treat that route myself.

0celo7

@JohnRennie I really don't remember that

ACuriousMind

Uhhh...so this paper cites a paper that doesn't exist.

AccidentalFourierTransform

@ACuriousMind lol I used to do that in most undergrad assignments

I used to make up some of the references

nobody ever checks them anyway

ACuriousMind

16:25

Can anyone find "Hyperkähler geometry and three manifold invariants" by Rozansky and Witten? Harvey and Moore cite it as [32], but the given arXiv link leads to a paper by Douglas instead

Googling that exact title gives exactly one result - the citation in Harvey/Moore

This is strange

John Rennie

arxiv.org/abs/hep-th/9612216

0celo7

@ACuriousMind I have a book that cites a book that doesn't exist

ACuriousMind

@JohnRennie Huh, thanks!

0celo7

Lol @ACuriousMind you're terrible at google

John Rennie

@ACuriousMind any time you need more help with algebraic geometry just ping me :-)

ACuriousMind

16:28

@0celo7 I generally expect the papers to have the exact title under which they are cited :P

"Hyper-Kahler" is not the same as Hyperkähler!

@JohnRennie I think this is still rather differential geometry, actually ;)

0celo7

@ACuriousMind well try finding some PDE notes written in the 70s by some Brazilian guy

John Rennie

differential - algebraic - shrug :-)

0celo7

@JohnRennie you can tell the difference by the (non)presence of "Noetherian ring" in the paper

John Rennie

I knew that! :-)

0celo7

I wonder if there's a paper that has "Noetherian ring," "Schauder estimate", and "direct integral" in it

It would be all over the place

GPhys

16:35

What is the scalar product with itself? I can't get what Landau got?

AccidentalFourierTransform

$a=b\quad\Rightarrow\quad a\cdot a=b\cdot b$

0celo7

You don't know how to dot a vector with itself?

ACuriousMind

Arrgh, why would you choose $\mathbb{Z}$ as the notation for a map if you also want it do note the integers? There are so many other symbols!

0celo7

@ACuriousMind calm down

GPhys

oh

he divided by two Q_Q

0celo7

16:37

Maybe the map is the integers

GPhys

is an idiot

AccidentalFourierTransform

there are $|\mathbb Z|$ symbols

ACuriousMind

@0celo7 No, it's an embedding of supermanifolds

0celo7

I don't want to know what a supermanifold is

AccidentalFourierTransform

I wasnt gonna tell you anyway

0celo7

16:41

You probably don't know

AccidentalFourierTransform

i know everything

John Rennie

Me too

0celo7

what is the correct definition for sobolev space on a manifold with boundary

John Rennie

I just choose not to reveal some parts of my knowledge so as not to make the rest of you look bad.

0celo7

Good guy John Rennie

John Rennie

16:44

:-)

AccidentalFourierTransform

I just choose not to reveal some parts of my body so as not to make the rest of you look bad.

0celo7

Now if only your girlfriend didn't have wool, everything would be OK

John Rennie

I'm prohibited by international treaties from revealing any parts of my body

AccidentalFourierTransform

I want to nominate myself as a candidate for the moderation election of mathematica.SE

ive been told that they send you a hat

Loong

@AccidentalFourierTransform if you win

In the past, you could get swag just for running in an election.

0celo7

16:49

@JohnRennie I'm going to irradiate cerium

Wish me luck!

John Rennie

Your experimental work sounds really fun.

What reaction do you get when irradiating cerium?

Loong

@0celo7 irradiate Ce with what?

0celo7

@Loong alphas

Loong

nice

AccidentalFourierTransform

@Loong snap. I dont think I would even get one vote

0celo7

16:50

@JohnRennie I hope to damage it and alter oxidation effects

(This is actually a long term goal, my colleagues do not have sufficiently good samples for this experiment yet)

John Rennie

@0celo7 aha. I was going to say Googling found me lots of articles on changing the oxidation state but nothing on nuclear reactions.

Are you using gamma rays? I see articles on using cobaklt-60 sources ...

0celo7

@JohnRennie those papers are about cerium oxide

I'm irradiating the metal, then oxidizing it

John Rennie

Ah, yes, you did say alpha particles above.

0celo7

We have an alpha accelerator on campus I think

John Rennie

Cerium oxidises spontaneously anyway doesn't it?

Loong

16:52

@0celo7 what alpha source do you use?

0celo7

@Loong not sure, it isn't mine

John Rennie

Why would alpha irradiation make any difference?

0celo7

I haven't used it yet

@JohnRennie ion damage changes many properties

Oxidation is a surface effect, so surface irradiation could affect it

John Rennie

@0celo7 true, though I'm still puzzled as to what you expect to see in this case.

0celo7

I don't really know what I'm looking for specifically

But it us has been suggested to us by experts in the atom bomb field

anonymous

16:55

@koolman Take point A as axis of rotation. String cannot provide any horizontal force. It can only provide vertical force. So $a_x$ must be 0. Similar in spring $a_x$ will be 0. But in case (1) hinge force provides both $a_x$ and $a_y$. Where $a_x$ and $a_y$ are components of $R\alpha$. $R$ is the distance of the COM from the end $A$.

Loong

@0celo7 I used alpha spectroscopy for my thesis. I measured mostly everything from U to Cm.

0celo7

@Loong Pu?

John Rennie

I did some work on reactions of silver films with chalcogenide glasses induced by irradiation with electrons.

0celo7

We will also irradiate W and then oxidize it. Although I'm sure some ITER people have done that

Ce is just a pain in the ass. Some samples I have are powder because the oxidation is so bad.

They used to be metal...

Loong

@0celo7 Yes, I also measured Pu-238 and Pu-239,240 (alpha spectroscopy cannot separate Pu-239,240). I used Pu-242 as tracer.

John Rennie

16:59

Actually I also indirectly studied reactions induced by apha particles.

0celo7

@Loong how did you get it?

John Rennie

I did some Rutherford backscattering work (using alphas) to do depth profiling, and found the alphas also induced a chemical reaction. Though I didn't follow this up.

0celo7

I'd like to have a key or so

Loong

@0celo7 How did I get my samples? Or the Pu-242?

0celo7

The 239

Loong

17:03

I measured the 239 etc in environmental samples, mostly from northern Ukraine.

bolbteppa

@0celo7 congratulations

0celo7

@Loong so you didn't have a legit ingot?

bolbteppa

Hope you learned your lesson, whatever it was

0celo7

@Loong A collaborator of ours said that it's possible to obtain an ingot from the government if you know how.

Loong

@0celo7 I had a calibration source with about 1 kBq Pu-239.

0celo7

17:06

It's all very mysterious.

@Loong I don't know the activity of Pu off the top of my head, how much is that in kg?

Loong

@0celo7 2.2948E+09 Bq/g

so about 0.4 µg

the source looked like this: ezag.com/home/products/isotope_products/…

Sir Cumference

Anyone good at riddles?

Need help with one

John Rennie

And Google couldn't help?

Sir Cumference

@JohnRennie Surprisingly no

"What comes up the hill in the morning with three legs, and comes down at night with two?"

John Rennie

Ah, that's an old one though annoyingly I've forgotten the answer.

Sir Cumference

17:20

My friend promised me $20 if I guess it, but I pay $3 if I can't

John Rennie

Are you sure it isn't the other way around?

Sir Cumference

Nope, he just doesn't have much faith in me

John Rennie

No I mean go up with two legs and come down with three

Sir Cumference

@JohnRennie Oh, nope, that's what he wrote

ACuriousMind

@JohnRennie the classic variant is the Sphinx' riddle, roughly: "What begins life on all fours, walks on two legs for most of its time, and ends it on three?"

John Rennie

17:26

Hmm I think I have the answer to the riddle, but it's a bit on the indecent side ...

Sir Cumference

@JohnRennie Sure

John Rennie

Sure? What do you mean?

Sir Cumference

I mean, what's your answer?

John Rennie

Well have you noticed certain physiological effects when you wake in the morning?

Sir Cumference

Like tiredness?

ACuriousMind

17:28

@JohnRennie Oh dear

Sir Cumference

oh

John Rennie

@ACuriousMind bingo :-)

Emilio Pisanty

I particularly love this recent take on the dimensional analysis of electromagnetic waves. Makes it pretty hard for me to take things like this seriously, though. Good job @ACM on handling that one.

Sir Cumference

Uh, I don't think that's the answer...

Though he got a kick out of it

All right, I think I'll just ask him

...it's "nothing."

welp, there goes $3

John Rennie

The answer was nothing? Demand that your friend prove that!

Or just kick him in the third leg and tell him to stop being a twit :-)

Sir Cumference

17:32

^

Koolman

@anonymous so what could be the axis of rotation in II) case when string is attached

anonymous

@Koolman You can take any axis of rotation. Angular acceleration of body is same for all axis of rotations.

It is easier to solve if you take COM as axis of rotation.

0celo7

Cornell's REU has no stipend and no housing

Who thinks up stuff like that?

Koolman

@anonymous so for second case I am getting 4.8g/c

mgc/2 = (5/48)mc$^2 \alpha$

anonymous

@Koolman Did you consider the extra force due to the string due to angular acceleration ?

String force is not only mg

Koolman

17:46

@anonymous then

dmckee

@0celo7 Eh? I thought that the NSF program called "research experience for undergrads" requires a stipend.

If they are using that term for something else the NSF is likely to be ticked.

anonymous

It is $mg + mc\alpha/2$

@koolman

Where $c=r\cos(\theta)$, where r is distance of COM from end A.

Koolman

c is the side of plate @anonymous

anonymous

Sorry c/2

=$r\cos(\theta)$

@koolman

Koolman

So the equation should be $mgc/2 + mc^2\alpha /4 = (5/48)mc^2 \alpha$ @anonymous

anonymous

17:58

The string can provide a force only in vertical direction. The extra vertical force $1/2mc\alpha$ is given by the wire itself (indirectly by the hinge) in downward direction.

Koolman

@anonymous I could not understand how it can provide extra force in downward direction @anonymous

anonymous

@Koolman Who else can provide the force for the extra vertical acceleration ?

Koolman

@anonymous how can a string push a plate . As you are saying , it is providing force in downward direction .

anonymous

@Koolman It is not pushing. It is simply reducing its tension to allow for the vertical acceleration.

0celo7

@dmckee It's not listed on the NSF website

Koolman

18:10

@anonymous why it is reducing why not adding

Why it is only in downward direction

anonymous

@Koolman If it was added then the extra acceleration would be in upward direction and not downward.

Koolman

Ohh i see

anonymous

It is just $mg-T=ma$ form.

Koolman

@anonymous last doubt why we have not used the same concept in III)

anonymous

@Koolman Spring tension remains constant for some time.

Koolman

18:15

Oh yeah

Thanks a lot @anonymous

anonymous

@Koolman I personally have a doubt. Which force is responsible for the horizontal acceleration component of $r\alpha$ ?

Koolman

In which part @anonymous

anonymous

@Koolman Second part

Koolman

18:37

@anonymous sorry , I am not getting it . I will try it . If I found ,i will ping you

anonymous

okay :)

anonymous

19:14

@Koolman Tension is not passing through COM of plate. Hence, the component of tension itself provides for the horizontal acceleration.

sammy gerbil

@SirCumference Regarding your riddle. The answer is not indecent. It is a man (or woman). The third leg is a walking stick.

Sir Cumference

@sammygerbil Only in the morning?

sammy gerbil

I'm not sure if "in the morning" refers to each day or metaphorically to the first part of a man's life - like the other form of the riddle in which you start on all fours (crawling as a baby) but spend most of your life on 2 legs, and end up using a walking stick. If it does mean "each day" then maybe you need the extra help of a walking stick going uphill but not downhill. But I think it is metaphorical. Going uphill means you are young, going downhill means you are old.

Koolman

@anonymous what is the theory behind this

I don't found it on internet

sammy gerbil

The theory behind the riddle? Or your conversation with anonymous (which I haven't read yet)?

anonymous

19:21

@sammygerbil Regarding your answer here (physics.qandaexchange.com/?qa=1139/…) I feel it is slightly wrong. You did not take into account that in part (i),(ii),(iii) point A has an angular accn w.r.t COM. So tension and hinge forces act accordingly. A string can provide force only in vertical direction and will cause no horizontal acceleration.

The hinge can provide both vertical and horizontal accelerations. And the spring provides accn neither in horizontal nor in vertical. BTW it is better to solve all the questions keeping COM as axis.

ACuriousMind

@sammygerbil Chat can be a bit confusing - you can click on the little arrow to the left of the @ to see which message someone replied to (and hovering over the message will also highlight the message if it's still in view)

If you yourself want to reply to a specific message, hover over a message and click on the arrow to the bottom right

anonymous

The second question can be solved like this ^ @sammygerbil Read my and koolman's discussion above

sammy gerbil

@ACuriousMind Thanks for the advice.

anonymous

@Koolman Think of a compound pendulum ( a rod) which is moved to a certain angle. When you release it who causes the horizontal motion ?

sammy gerbil

@anonymous Yes I've seen that expression in the textbook solution. But I don't understand what it is saying.

anonymous

19:25

@sammygerbil The tension is not mg as you used in your answer.

The tension in the string will be modified to allow the extra acceleration

In vertical direction

It will be $mg-\frac{1}{2}mc\alpha$

sammy gerbil

@anonymous Which case are you addressing?

anonymous

@sammygerbil Case (ii).

Koolman

@anonymous i think it is the angular acceleration's component

anonymous

@Koolman Think in the Centre Of Mass frame. Draw a free body diagram. Angular accn about COM cannot cause it to move horizontally.

There must be some force which causes the horizontal motion and that is tension

0celo7

@ACuriousMind Really?

ACuriousMind

19:32

@0celo7 Hmmm?

I know you know that, so what's the matter?

sammy gerbil

@anonymous Thanks, I will look over your discussion.

0celo7

@ACuriousMind I know that?

There's no evidence

in fact, you've seen me ask many times for clarification

anonymous

@sammygerbil Be careful though. Some of the early messages contain wrong information. Towards the end we solved it though. Welcome.

0celo7

@dmckee halp

what address do I put for my school

the address of the math building?

the address of the main admin building?

dmckee

19:55

@0celo7 A grad student would use the address of the department because that is where they get their mail (school related).

Do you have a school mailbox?

IF not, can you ask the department secretary to be on the lookout for stuff for you?

It's that or ask your advisor.

Loong

20:08

@0celo7 Do you order your Pu ingot? ;-)

That reminds me how we once ordered a Pu-241 standard solution. The package arrived by ordinary mail, which made our secretary very agitated. She called our lab and demanded that we come and pick up our plutonium at once.

5

ACuriousMind

Everyone, please take note of the new meta post on chat and its moderation.

11

0celo7

@dmckee no

@Loong wow, we have to order rad stuff through the rad safety department

Physics Meta

1

Q: Chat and the moderation thereof

So, there has been considerable dissatisfaction with recent moderator actions in the h bar, physics.SE's main chat room, at least in part because people feel they have not been properly informed about the rules, policies, and consequences of violating them. We'd like to take the opportunity to co...

0celo7

20:31

@ACuriousMind is this like the moderators' MK?

ACuriousMind

What's an MK?

0celo7

@ACuriousMind Inapproriate reference that would get me banned

Although it's an allegory

@ACuriousMind Is 1. a big issue?

dmckee

@ACuriousMind I suppose he means google.com/search?q=mmm+kay&ie=utf-8&oe=utf-8

David Z

@0celo7 not in this case

0celo7

I meant the book by Hitler, but I would like to retract that statement.

(Having now read the post.)

Emilio Pisanty

20:49

Wow, did this just happen?

@Emilio Pisanty : re are you claiming that electromagnetic waves have amplitudes in meters? Yes. — John Duffield 1 hour ago

:35260914 as in, as opposed to being in V/m or dyn/statC

0celo7

@EmilioPisanty yeah, I know it's V/m. I misread

I love Duffield

AccidentalFourierTransform

physics.stackexchange.com/review/suggested-edits/161978

really? again?

anonymous

@EmilioPisanty My goodness XD He can't write an answer without quoting Einstein and causing mumbo-jumbo (Atleast I haven't seen one so far).

AccidentalFourierTransform

- The amplitude of an electromagnetic wave is measured in meters - Einstein, probably.

anonymous

Haha ^ :D

AccidentalFourierTransform

20:59

@ACuriousMind hey, thank you for taking the time to write that meta post

i think it'll help to focus further arguments in the future

Emilio Pisanty

@AccidentalFourierTransform a mod needs to have a word with muze

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