The h Bar

John Rennie

07:00

@koolman I don't think that question has an answer. The IAOR is just one of the ways of analysing the motion of a body.

koolman

Is it a point where net velocity is zero

John Rennie

No. Suppose I set a top spinning then drop it. The top is spinning about its axis, but the whole top is falling downwards so the velocity on the axis of rotation is not zero.

koolman

But all points on axis are IAOR

Am I correct

John Rennie

@koolman Yes

koolman

I have studied that for example if a sphere is rolling without slipping on ground . Then the point of contact with ground is IAOR

And according to me that point is not on the axis of rotation

John Rennie

07:18

According to Wikipedia the instantaneous axis of rotation is the line along which the velocity of the object is zero.

That would fit with the point of contact with the ground being the IAOR.

koolman

It is not necessary on axis

Is it ??

John Rennie

It depends on what you mean by the word axis. We generally consider an axis to be some obvious symmetry. So for a sphere the axis would pass through the centre of the sphere. The point of the rolling sphere problem is that this is not always a useful way to define an axis.

koolman

Ok thank you sir

@JohnRennie

Mew

07:41

Hi @TheStackExchange

John Rennie

@Mew you need at least 20 rep to be able to post in the chat

Mew

h k

user228700

What?

user228700

O.o

Mew

@JohnRennie do you think it is a good idea to have a rep limit for private messages on the new site?

John Rennie

07:47

@Mew yes. It stops sock puppet accounts from spamming everyone. Not that such a thing would ever happen of course :-)

Mew

YEp

I think I will implement this

user228700

I just found out that the bunny "Miffy" from that cartoon "Miffy and friends" is a boy.

Mew

that way if people do try and take down the site, they at least have to put in alot of effort building it up first

then there account can be blocked if they spam and to start again they will have to contribute more

user228700

My life is a lie :-( (I kept hearing she when I was a child, wtf?)

Mew

who is miffy

John Rennie

07:48

Mew

oh yeah looks like a boy

user228700

Miffy is the most adorable bunny ever to have existed.

John Rennie

He's a transvestite then :-)

Mew

lol

user228700

No, Miffy looks like a girl, frock and all!

John Rennie

07:49

When I was a child it was common to eat rabbit because in rural Somerset where I lived there were lots of them and they were a pest. The farmers used to pay us to shoot them.

But ...

I hate rabbit meat. It's soft and slimy.

So I have a lifelong downer on rabbits.

user228700

What dyou mean u have a downer? What does that mean?

John Rennie

Miffy

Miffy (Dutch: Nijntje, pronounced [ˈnɛi̯ncə]) is a small female rabbit in a series of picture books drawn and written by Dutch artist Dick Bruna. The original Dutch name, Nijntje, is a shortening of the diminutive konijntje, "little rabbit". The first Miffy book was produced in 1955, and almost 30 others have followed. In total they have sold over 85 million copies, and led to two separate television series as well as items such as clothes and toys featuring the character. A feature-length film, Miffy the Movie, was released on 30 January 2013. Two television series based on the character have...

According to this Miffy is a girl ...

user228700

Yaas! Female! Stupid YouTube Kids giving me an existential crisis.

user228700

> "In total they have sold over 85 million copies..."

user228700

Whoa.

John Rennie

07:53

@Kaumudi I wouldn't go so far as to say I hate rabbits, but they are far down my list of favourite animals.

user228700

No, I meant what does having a downer on something mean?

John Rennie

Having a downer means having an unfavourable opinion

user228700

Ah, OK, thanks :-)

user228700

@JohnRennie Just 'cause their meat doesn't taste so great?

user228700

That's a very strange way to rank your favorite animals o.O

John Rennie

07:56

@Kaumudi that and the fact I was forced to eat it. I now have a lifelong grudge against rabbits for their wilful decision to not taste good :-)

user228700

I still find that very strange. I rank my favorite animals based on how adorable they are :-P

user228700

Adorable, awesome, friendly etc.

John Rennie

The rabbit has a charming face,
Its private life is a disgrace.
I really dare not name to you
The awful thing that rabbits do;
Things that your paper never prints
You only mention them in hints.
They have such lost, degraded souls
No wonder they inhabit holes;
When such depravity is found
It can only live underground.
- Ogden Nash

user228700

What the hell happened b/w Mr. Nash and the entire population of rabbits?! :-o

user228700

I wonder why people hate rabbits so much ._. (Do not tell me it's 'cause their meat doesn't taste good! -_-)

user228700

08:03

I read "Watership Down" and have a great opinion of rabbits meself :-D

John Rennie

Do people hate rabbits? I suspect we misolagomorphs are a minority.

Ogden Nash's poem was written in a more prudish era, and rabbits are not noted for their chastity :-)

user228700

@JohnRennie Ai ai ai, I see...

user228700

Lunch time, I'm off!

John Rennie

08:45

I don't know if anyone has heard of the film Arrival that has just been released. It is based on a short story by Ted Chiang.

I recently reread this, and it's an excellent story that I highly recommend. I mention this because I know a number of us hereabouts are also members of the SciFi Stack Exchange.

The physics connection is that the story is based on the block universe idea. It takes liberties with the physics, but that's OK - after all it's a story not a textbook :-)

Mew

09:17

it says the film is 1996

DanielSank

What it tarnation is a-goin' on around here?

Why, hello, Monsieur LeRennie.

I'm watching @JohnRennie's laptop fail to boot.

user228700

09:42

@DanielSank: Yellow! :-)

DanielSank

green

user228700

I have a quick question, if anybody knows...I'm doing problems based on Bohr's atomic model and they all keep saying "Assume the mass of the nucleus to be infinite"... Why?

user228700

@DanielSank :-P

DanielSank

because mass of nucleus >> mass of electron

user228700

Ohh, that's why? Okay, thankoo!

DanielSank

09:48

welcome

user116211

Didn't you sleep @DanielSank?

user116211

Sounds like we're training a neural net to replace us :-) — John Rennie 2 hours ago

user116211

Did you forget @johnR "ultimately what Physics Stack Exchange will become"?

user116211

Skynet is coming.

user228700

10:04

OK, I've a quick question again.

DanielSank

@MAFIA36790 up late

user228700

The formula that gives us the total number of lines of diff. wavelengths emitted when an electron falls from the $n$th orbit is given by $n(n-1)/2$. This works only for when it falls all the way down to $n=1$, right?

user228700

Gah, that is correct!

John Rennie

I think that's the number of lines for all transitions from the $n$th orbital and lower ...

user228700

Well, yeah, sort of. I mean, if an electron falls down from the $n$th orbit, then there's the possibility that it might falls down to the $(n-1)$th orbit and then $(n-2)$th orbit from there and so on. It may also fall to the $(n-2)$th orbit and then fall to $(n-3)$rd orbit and so on. We've to account for all such possibilities.

user228700

10:15

If we employ the choose function (I read that this is how it's referred to outside of India) we get $n(n-1)/2$

user228700

Aiii. *might fall down

John Rennie

If the electron starts in the nth orbital there are n-1 orbitals it can fall into. If it starts in the n-1 orbital there are n-2 orbitals it can fall into, and so on. So the number of possible transitions is (n-1) + (n-2) + (n-3) ...

This is just the triangle number for n-1

user228700

Yes, correct, OK...

user228700

Damn, my textbook is dumb af.

user228700

Arghfg.

user228700

10:24

@JohnRennie: Can u help?

user228700

:'-(

John Rennie

@Kaumudi help with what. I got the impression you understood the number of lines thing ...

user228700

I did.

John Rennie

I can't help with your textbook being rubbish :-)

user228700

This is a problem related to that, the solution of which is kinda messed up in my book :-|

user228700

10:25

@JohnRennie :-( Yeah.

John Rennie

But if you have a new question then ask away!

user228700

It's related to this number of lines thing.

user228700

This is how the problem goes:

user228700

> "A gas of hydrogen like atoms can absorb radiations of 68 eV. Consequently, the atoms emit radiations of only 3 different wavelengths. All wavelengths are equal of smaller than that of the absorbed photon."

user228700

Oh, wait!

user228700

10:31

Yeah, no, no new insight. False alarm :-|

user228700

I found that whatever orbit that electron was initially in, it jumped to the third orbit.

John Rennie

Well, yes. If there are only three lines that means $n(n-1)/2 = 3$ so $n = 3$

user228700

So it was either in the 1st or 2nd orbit initially.

John Rennie

> All wavelengths are equal or smaller than that of the absorbed photon

Would that be true if it started with n=2?

user228700

Uhhhh.

user228700

10:35

(::Thinking::)

John Rennie

Well if it started at n-2 that means 68eV is the 2->3 transition energy

user228700

How can it possibly have smaller wavelengths? .__.

John Rennie

Suppose it started with n=2 so the 68eV absorbed photon is the 2->3 transition. The atom can then transition 3->1 and emit a photon with an energy greater than 68eV. Yes?

user228700

Ohh. Right .__.

user228700

Dang it.

DanielSank

10:40

@Kaumudi language!

;D

user228700

@DanielSank Oh, really? "af" not allowed? Sigh. OK :-|

user228700

@DanielSank Oh, u were kidding :-P

user228700

@JohnRennie: God, how not to be dumb? Thanks for ur help :-)

John Rennie

Your book does ask the most rubbish questions.

Well, the most uninteresting questions anyway.

user228700

@JohnRennie xD Hey, this one's not so bad, is it?

John Rennie

10:42

I don't think it builds insight into physics ...

user228700

Yeah, no, Idk if it does :-|

user228700

It sort of tests ur "non-dumbness" to the max. extent possible, in all sorts of ways, basically.

John Rennie

You just have to work out the 1->3 transition energy of hydrogen and figure out what value of Z^2 increases this to 68eV. Not bags of fun.

user228700

Yeah, but see, I didn't even figure out if it started out at $n=2$ or $n=1$

user228700

That part is meant to be tricky for idiots like me so yeah...it's not so bad, I guess.

user228700

10:46

2->3 BTW.

user228700

Oh God @JohnRennie!!!

user228700

I've been meaning to ask u for so long and I keep forgetting.

user228700

I'd gone to the kitchen to fish for something to eat and I literally ran back to ask u about it when I remembered :-P

John Rennie

:: John nervously wonders what's coming next ::

user228700

:-P Are u familiar with Raoult's law?

John Rennie

10:55

Yes, I learned it at college though I don't think I ever used it in my research.

user228700

OK I'm still hungry. There's nothing to eat but 2 mins, lemme go check. Sorry! 2 mins!

John Rennie

Raoult's law is effectively the same as Henry's law

user228700

I didn't find anything so I just drank a bunch of water :-(

user228700

@JohnRennie Riight. How so?

John Rennie

Henry's law says the vapour pressure of the solute is proportional to the concentration of the solute.

user228700

11:01

Essentially, yes...

John Rennie

Raoult's law says the vapour pressure of a component of a mixture is proportional to the concentration of the component in that mixture.

user228700

Except, the proportionality constants are different...

John Rennie

Yes, OK, but only because in one case we have a concentration and in the other case a mole fraction.

The two are easily interconverted.

user228700

My textbook has made a mess of this statement:

user228700

4 mins ago, by John Rennie

Raoult's law says the vapour pressure of a component of a mixture is proportional to the concentration of the component in that mixture.

user228700

11:06

In the mixture = In the liquid form, right?

John Rennie

@Kaumudi yes

user228700

They are easily interconverted?

Mew

@JohnRennie thanks for you answer on the GRE meta :)

John Rennie

@Kaumudi converting a concentration in grams per litre to a mole fraction shouldn't unduly tax you.

Mew

Do you consider the new site an experiment in that if successful, physics.SE will allow homework questions again?

John Rennie

11:09

No. I doubt that the PSE will ever allow homework questions.

Mew

ok cool, so rather if successful, may be an alternative avenue for users to post such questions

John Rennie

But if the new site is a success it will relieve some of the controversy around homework. At the moment many of us feel uneasy that good homework questions are going unanswered, and if the new site answers them we can sleep more peacefully.

Mew

cool

John Rennie

@Kaumudi but don't let me divert you away from the question you were going to ask ...

user228700

@JohnRennie xD You did.

user228700

11:14

It can't be done using just one simple formula, I reckon...

user228700

OK, anyhoo, yeah, no, my textbook keeps presenting everything in the worst possible way. So I was really confused about the statement itself.

John Rennie

What statement?

user228700

Raoult's law.

user228700

(:-P)

user228700

(I'm working out some other stuff given in my textbook...)

user228700

11:21

Oh, crap.

user228700

Yeah, I was dealing with completely immiscible liquids.

user228700

That's why nothing was making sense.

user228700

I'm sorry :-| Thank you :-)

Qmechanic

12:05

The mill says moo? :)

2

Mew

12:38

Ok guys

John Rennie

12:56

@Qmechanic the sound was Pink Floyd playing in the background

Qmechanic

13:16

@JohnRennie : Listening to Atom Heart Mother? (cf. cover.)

Loong

The Dark Side of the Moo

Qmechanic

13:41

@Loong : Ha-Ha. Didn't know that bootleg.

heather

hello

Mew

sup

did you receive my PM heather?

heather

@Mew, indeed, good idea removing the wall. I might also add a way for moderators to put a banner on user accounts saying that they have been banned. Finally, I'm not sure private messages are a good thing to include...I actually got a, um, spam note from S007 a couple of days ago. At least moderators can review the wall; private messages are...hard to review.

Mew

heather, yes I understand you got spam that is why I temporarily required all users to be approved

but I have now added measures to reduce the probabiliyt of spam as outlined in the PM

I understand your point though

but I know some users are using the PM system to communicate currently

I think until we have a chat room, the PM is useful

but once a public chat room is established maybe then we can remove the PM system

heather

@Mew, that is true. Yeah, I agree with that idea, then.

Mew

13:48

Potentially even then I will keep the PM system on, but maybe restrict it to moderators only or something

heather

@Mew, or make a moderator-only chatroom, like there is here.

Mew

Yeah with pinging etc.

heather

yeah.

Mew

I think eventually this would be a good meta post

keep or remove the PM system

and what min rep is rewquired etc.

heather

yeah, it would be. Oh, I know some of the site is built on PHP, and I started teaching myself some of the basics.

Mew

13:53

how r u finding it?

heather

@Mew, how to learn PHP? I'm doing the codecademy tutorial...

Mew

14:17

cool

Lucas Gautheron

14:43

Hello guys, I am looking for specific data sets (namely intergalactic or at least rather stellar-nucleosynthesis-independant nuclei abundances), and I was wondering if this would be a valid/appropriate question on stackexchange

user116211

15:05

0

Q: How is the Cauchy-Schwarz Inequality used in Quantum physics?

More specifically when it is applied to complex vector space. My quantum physics lecturer has given a problem asking "Show that the Cauchy-Schwarz inequality applied in complex vector space leads to: " We haven't had any lectures on its relevance to quantum physics so I'm at a bit of a loss...

quantum-mechanics

Mew

Looks like a homework question

user116211

Body sounding different from the title? Homework?

user116211

@Mew yes, at the first sight it seems so to me.

heather

@MAFIA36790, the title seems to me to be on-topic (though maybe too broad, I don't know). The body does seem to be homework like.

Mew

Is it even physics?

heather

15:09

It just got marked as a duplicate with the gold badge + mod hammer

Mew

and the duplicate was asked only 23 hours ago

John Rennie

@Loong you win this weeks ultimate cool dude prize :-)

Loong

\o/

Mew

how old are you @JohnRennie?

Bernard Meurer

@JohnRennie Dejavu?

user116211

15:20

@Mew Older than Pink Floyd.

Bernard Meurer

(How old are you question :P)

I'd watch out if I were you John :D

Mew

huh

John Rennie

@BernardMeurer :-)

@Mew some time ago one of the more scrotomorph members of the site got in an argument with me, asked how old I was, then said I hope you die.

Bernard Meurer

@Mew Everyone someone asks for John's age now I get prepared to fight

@JohnRennie I laughed my ass off that day though :P

@JohnRennie Also you missed me and @DanielSank trying to explain memory leaks to @heather yesterday

Mew

oh lol

that's mean

Bernard Meurer

15:23

I built an example in C that crashed my computer lol

Mew

I won't say that

Bernard Meurer

It was so funny

John Rennie

@Mew It's more silly than mean, because one of the few things I can be absolutely sure about is that I will die. Just, hopefully, not for a while :-)

Mew

Unless you believe in quantum immortality

Bernard Meurer

They're python example was like "oh , maximum recursion depth, take care buddy, we love you" My C example just said "I will f'ing destroy everything you love"

Mew

15:25

but anyway, how old?

Bernard Meurer

@JohnRennie Just in case do I get to inherit the laptop graveyard :P

John Rennie

@Mew 55

Mew

cool

that's pretty young to retire

heather

@BernardMeurer, yes, that was pretty funny =P I <3 Python

John Rennie

@BernardMeurer it's down to the OS not to give up and fall over when an app goes haywire.

heather

15:26

Now, I know you're going to stare at me...but I'm reading about PHP

Bernard Meurer

@heather ::spits on the floor::

@JohnRennie Yep, but the Linux kernel will try really, really hard to not kill a process

heather

@BernardMeurer, and it isn't that bad. It reminds me of JS in syntax

Bernard Meurer

Don't break userspace!

heather

which, of course, is another language you think is terrible

Bernard Meurer

Sorry, my inner Linus came out for a while

@heather PHP is horrible

heather

15:28

@BernardMeurer, it's like JS, and JS isn't that bad

quirky, but not bad

Bernard Meurer

crashworks.org/if_programming_languages_were_vehicles

heather

Note that I will always say Python is better

Bernard Meurer

"PHP is a hose, you usually plug one end into a car exhaust, and the other you stick in through a window and then you sit in the car and turn the engine on."

From the great Mr. Tipling

John Rennie

PHP was designed for short unstructured programs, and that's exactly what many web pages are. So PHP is generally fine for writing web applications.

heather

(::sigh::) why all this JS/PHP bias?

Bernard Meurer

15:30

@heather Look, you might not be able to see this now, which is fine, you've just started, but PHP is horrible. It is one of the worst things I've encountered. Mostly not because it is inherently bad, but because it is a supermarket cart being used as a formula

heather

@BernardMeurer, a supermarket cart being used as a formula...

Bernard Meurer

@JohnRennie The entire Facebook was written in PHP until very little ago, how is that short and unstructured :P

@heather pictures.topspeed.com/IMG/crop/201502/…

That is a formula

heather

@BernardMeurer, I thought you meant mathematical formula, lol

Bernard Meurer

@heather You need to spend less time in the h bar lol

heather

@BernardMeurer, nope, I learn all sorts of stuff here, like how you shouldn't code drunk at 3am because you might end up with a memory leak, which is an especially large problem in C, because C isn't as kind as Python =P

^not that I was planning on doing that, for the record

ACuriousMind

15:34

Not all drunk programming is bad. Watch out for the Ballmer peak

Bernard Meurer

@heather Yeah I never quite planned for drunk coding

it just happens, you know?

@ACuriousMind I can't read that xkcd, I get triggered when I read "Windows ME"

There, now I'm triggered

heather

@ACuriousMind, I'm just going to assume that isn't legit =P

ACuriousMind

@heather Has xkcd ever lied to you?

heather

@ACuriousMind, actually, not in my memory...I'm just suspicious of anything that says alcohol helps you think =P

@BernardMeurer, what's wrong with Windows Millenium Edition?

Bernard Meurer

@heather The surprising thing is

@heather Get out

I won't even finish what I was going to say

heather

15:37

@BernardMeurer, hey, I don't know what's wrong with it, I just want to know so I can never say it again =P

John Rennie

@heather BTW why are you using vim? Doesn't your distro have a graphical editor like Gedit?

Bernard Meurer

@ACuriousMind Does that constitute cursing? Can she get suspended :P

heather

@JohnRennie, I don't know. I'm using a version of vim from github that has syntax highlighting and everything; it's pretty cool. @BernardMeurer suggested it.

Bernard Meurer

@heather dewassoc.com/support/winme/known_me.htm

ACuriousMind

@BernardMeurer I see nothing resembling a swear anywhere. I also don't know what's wrong with Windows ME, its bluescreens had a very soothing blue.

Bernard Meurer

15:39

@JohnRennie Stop corrupting the kid with your graphics

@ACuriousMind Lol, check the link I sent

John Rennie

Try running gedit and see if anything runs. It's a graphical editor and it's a lot easier to use than vim.

Bernard Meurer

@heather Don't listen to him, he's basically selling you crack cocaine right now

You just don't know it yet

ACuriousMind

What's wrong with crack cocaine?

heather

@JohnRennie, nothing runs. Anyway, I like vim.

Bernard Meurer

@ACuriousMind that made me crack

heather

15:41

@ACuriousMind, there are so many things wrong with that statement...=P

John Rennie

@heather help.ubuntu.com/community/gedit

heather

@JohnRennie, sorry, just looking at the screenshots, I like vim better =D

John Rennie

If I were doing any big apps on Linux these days I'd probably use Mono: mono-project.com/docs/about-mono/supported-platforms/linux

Bernard Meurer

@JohnRennie Mono is bloat af :/

@JohnRennie Nowadays if I were to build some multi-platform desktop app I'd just use something like Electron and be happy

John Rennie

@heather just a brief return to this lanuage thing: the biggest app I've worked on is Kodi and that runs to millions of lines. Unless you code ina highly structured way projects this big will run away from you and become completely unmanageable.

Bernard Meurer

15:49

@JohnRennie And gedit is highly structured? :P

John Rennie

Kodi is written in C++ using very highly structured code. But languages like PHP don't lend themselves to structured programming.

heather

@JohnRennie, that makes sense.

Bernard Meurer

Ah, I see where you're going now

John Rennie

So while PHP is great for small projects you'd be certifiably insane to use it for anything really big. That's why Bernard has such a downer on it.

heather

that is good to know. thanks for explaining! =)

Bernard Meurer

15:51

@JohnRennie No, I have a downer on it because I like to pick on people :P

John Rennie

@BernardMeurer I write code for Kodi using Visual Studio which is GOD'S OWN DEVELOPMENT ENVIRONMENT!!!

Bernard Meurer

@JohnRennie VS is really good, but mostly because Microsoft made an amazing debugger

Sadly I don't like the MSVC C/C++ compiler

CLion is very good too

John Rennie

I've always been a hard code C++ hacker, and C in the days before C++ went mainstream. However I have to say I think C# is the best language I've ever used.

Bernard Meurer

Although admittedly GDB is not as good as MSDebugger

@JohnRennie I fooled with C# for a while once, but that was before I even used Linux

So it's been a while so to speak

John Rennie

C# is a modern language and it shows. Stroustrup got every right about it.

Bernard Meurer

15:54

Can you even imagine it Jon? Me without Linux?

John Rennie

And that's why I'd use Mono on Linux.

Bernard Meurer

@JohnRennie I remember liking the fact that, unlike C++, C# didn't try to "beat" C

It tried to build on C and do something different

heather

C#, C++, C...what is even the difference between them?

John Rennie

@BernardMeurer Mono compiles C# to native code. There's no performance hit.

Bernard Meurer

@heather stackoverflow.com/questions/692225/…

@JohnRennie "no performance hit" might be an overstatement

You could say it is minimal though

Ramanujan

15:56

@JohnRennie why you don't use mackbook?_(if you don't mind)_

Bernard Meurer

@Ramanujan Not worth the cost

John Rennie

There's no reason why compiled C# should produce assembler that's any different to compiled C++.

Bernard Meurer

@JohnRennie C++ compiled code is slower than C compiled code (on some cases)

Ramanujan

@BernardMeurer I want to know from John

Bernard Meurer

Back in the day you could compile the Linux Kernel with g++ and gcc. Just using g++ made everything slower

John Rennie

15:58

@Ramanujan there's a bit difference in culture between the Apple and Linux worlds.

Bernard Meurer

@JohnRennie I wish linux was UNIX like macOS is though :/

John Rennie

Apple want you do worh the way they want you to. Apple computers are great if you work the Apple way.

OSX is derived from BSD isn't it?

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