The h Bar

5:00 PM

Yes it could be better for me if u could show that del S /del t = {(u+v)/2}*t ... like we can do for del V/del t =a @JohnRennie

0celo7

@BalarkaSen it's ok

you'll figure it out one day

David Z

@Sanya not necessarily.

Homework-like questions are those where the value to the poster lies in learning how to do them, not in actually getting the answer.

John Rennie

@ffahim Well $v = u + at$ and if we substitute this into your equation we get $s = (u + u + at)t/2$, and this rearranges to $s = ut + at^2/2$.

0celo7

@DavidZ Suppose I have a question of the form described that you don't think is homework. Suppose I add "this is for homework" at the bottom. Does that question become invalidated?

David Z

@0celo7 yep, assuming we believe you about the question being homework (or, well, I suppose it depends on the exact nature of the question, but probably)

ACuriousMind

5:03 PM

@DavidZ That's what the policy post ostensibly says, but I don't think many people actually vote based on that.

David Z

@ACuriousMind yes, which is why we're getting rid of that policy

ffahim

Hang on. S=ut+.5*at^2 was derived from A=(u+v)/2*t @JohnRennie

0celo7

@DavidZ Suppose the poster removes "this is homework" after the question being closed. Should it be reopened?

John Rennie

@ffahim A=(u+v)/2*t is wrong. It's s=(u+v)/2*t

ACuriousMind

@BalarkaSen I'm with you here, the beard is totally fine

John Rennie

5:05 PM

@ffahim A = (v - u)/t

Balarka Sen

@ACM High fives

ffahim

Let it be ... its my phone causing typing problem

ACuriousMind

::strokes beard::

John Rennie

@ACuriousMind when I get old, well older, I'm going to grow a tramp beard as well

I've been practicing the wild staring eyes

ffahim

@JohnRennie .. I type s. But button a was just beside s

John Rennie

5:07 PM

@ffahim Ah OK.

Sanya

@DavidZ the way is always more important ... :| dunno, I know the rule, but ... the distinction doesn't convince me

John Rennie

@ffahim What I'm saying is that start with s=(u+v)/2*t then replace v by u+at and you get s = ut + at^2/2

Student404Mus

@ACuriousMind in the set they posted U only. As my calculations implies U/N which is bounded as mentioned

David Z

@0celo7 I think you're mixing up the question being homework with it being closed. The mere fact that a question is homework doesn't mean it should be closed. (Under the current policy) if it's homework and doesn't show effort or doesn't ask a specific conceptual question, it gets closed, but if it stops being a homework question, it's probably still a bad question and should stay closed.

ffahim

Ohh. U got me wrong.. I need the derivation of S=(U+V)/2*t @JohnRennie

Student404Mus

5:09 PM

@ACuriousMind in solution set sorry

0celo7

@DavidZ No, you just said a good question can be closed as homework because the OP says it is homework.

John Rennie

@ffahim I already did that. It's the average velocity multiplied by the time.

David Z

@0celo7 No I didn't.

Not a good question.

ffahim

As we always keep saying that average velocity=(u+v)/2 ... @JohnRennie but..

It's not necessarily need to be arithmetical average@JohnRennie

It's not something like for acceleration that its very easy to derive from its definition

Student404Mus

@ACuriousMind anyway which form is right, ?

ffahim

5:13 PM

In case if we could proof (u+v)/2 always means average velocity. Then I wouldn't have any problems@JohnRennie

ACuriousMind

@Student404Mus I don't know, but I don't think it makes sense for the total internal energy to be bounded.

ffahim

By average velocity I just know that change in displacement / time taken.. @ACuriousMind

Student404Mus

And they say: for this reason U is bounded, most of particles are found to be in the upper level

@ACuriousMind they say: for this reason U is bounded, most of particles are found to be in the upper level

ACuriousMind

@ffahim Assuming that the acceleration is constant we have $v(t) = at + u$, and from that $a = \frac{v(t) - u}{t}$. Plug that into $s = ut + \frac{1}{2}at^2$ to arrive at the equation you want.

heather

hello

ffahim

5:21 PM

Ohh come'n ... just imagine.. if you r asked what is acceleration then definitely u should know what velocity is... same here ..

@ACuriousMind

ACuriousMind

I'm sorry I have no idea what you just said.

heather

what sorts of quantum computations/algorithms require error correction?

ffahim

It's something like that S=ut+.5at^2 was derived from s =(u+v)/2*t @ACuriousMind

0celo7

@DavidZ So a homework-like question doesn't have to be homework, and homework doesn't have to be homework-like?

ffahim

So unless you can't derive the main one.. how can u say that :-)@ACuriousMind

John Rennie

5:24 PM

@ffahim S=ut+.5at^2 is derived by integrating v = u + at

ACuriousMind

@ffahim If you're saying we need the latter equation to arrive that the former, then that's just wrong. And I'm utterly lost as to what your actual question is.

heather

@0celo7, I think we need to use set notation here =P

ACuriousMind

@heather Just draw a Venn diagram

ffahim

I see...! That's what I was totally unknown about .. I don't have any knowledge about Calculas since I am a Secondary school's student :-)@JohnRennie @ACuriousMind

heather

@ACuriousMind, nah, that's too boring =P

David Z

5:27 PM

@0celo7 there is some difference between homework and homework-like, yes, but I think what you're getting at is: a homework question does not have to be close-worthy.

heather

@ffahim, may I recommend The Cartoon Guide to Calculus by Larry Gonick - an excellent book I'm currently reading.

John Rennie

@ffahim Aha! If you start with a constant acceleration then integrating once gives you $v = u + at$ and integrating again gives $s = ut + at^2/2$.

ffahim

Sure.. is it available in pdf?@heather

user116211

@0celo7 He looks like Castro.

heather

@ffahim, not sure...a few lucky google searches might get you there, but I personally got it from Barnes & Noble, where it was around $20. But if you have a gift card, or a willing parent...

ffahim

5:30 PM

@JohnRennie .. let me have enough concept about calculus... :-). ... I hope now it's okay for me.. ..

Shing

calculus is fun.

ffahim

@heather I will have a look on Google

@Shing .. oh.. :-)

Shing

I still remember when I was in high school, how the plots $f(x)$,$\dot{f(x)}$,$\ddot{f(x)}$ blowed my mind.

user218912

@Shing depends what function tho

0celo7

@DavidZ The terminology does not reflect that.

Shing

5:33 PM

and doing integral by parts, I intentionally do the wrong way, and then accidentally discovered talyor expansion.

those days

wonderful

0celo7

@obe if you get an iPhone, get it in white

user218912

@0celo7 why?

Shing

now my time was pretty much consumed by stupid school rules... damn

David Z

@0celo7 how so? (we could take this up in the alternate room if you prefer)

0celo7

The new black shows dust so horribly

@DavidZ I'm in topology right now, maybe later.

But I haven't participated on the site in a long time, so probably ignore me.

David Z

5:35 PM

@0celo7 gotcha, cool

heather

physics.stackexchange.com/review/reopen/151444

^weirdest change ever...someone want to take a look at it? The guy basically changed the question.

i skipped the review, because I didn't know what you were supposed to do there

ACuriousMind

@heather I rolled back the edit and informed the poster that's not how the edit feature is supposed to work.

heather

@ACuriousMind, thank you.

David Z

@heather That... shouldn't happen. Edits that completely change the question should be rolled back. Although the original question was so garbled I honestly don't know if it was really a drastic change in the first place!

ACuriousMind

(you could've done both of that yourself, but not from review)

heather

5:40 PM

actually, @ACuriousMind, I think you didn't rollback far enough...or is it fine the way it is?

John Rennie

@ACuriousMind you rolled back to the wrong revision ...

Shing

Is there a non-linaer quantum physics?

ACuriousMind

@JohnRennie Arrrgh, today isn't my day

John Rennie

No, I've just rolled it back!!!

ACuriousMind

Already misclicked in several other places :/

heather

5:42 PM

okay, didn't do anything

John Rennie

@heather We get some very strange questions here. You've probably noticed :-)

ACuriousMind

@Shing You'll have to be more specific, but there are non-linear evolution equations in QM, e.g. Gross-Pitaevskii.

heather

@JohnRennie, yeah, I have =)

John Rennie

Like this one. Maybe it's me, but I just can't see what this means:

0

Q: Radial Nodes in Atomic Orbitals

Could anyone explain why we count only the number of radial nodes between the subshells that have the same orbital angular momentum l ? For example, 3p-orbitals have 1 radial node that exists between the 3p- and 2p-orbitals. Shouldn't be there additional radial nodes that exist between the 3p- a...

atomic-physics

user218912

I want a cat.

user116211

5:46 PM

@obe Blue cat?

user218912

I saw a cat dressed up in a blue dress yesterday on the street.

user218912

yes

user218912

grey cat is okay too

heather

@JohnRennie, I edited the question, is it now a little less nuts? Maybe in good form for migration to philosophy.SE?

user116211

Just tame some stray cat.

ACuriousMind

5:48 PM

@heather Don't recommend other sites unless you're familiar with their scope

Of the four recommendationd on a question here to post it at another SE site I saw today, all four were just wrong.

And if that question is on-topic for philosophy, I'd be seriously surprised.

heather

@ACuriousMind, what are those recommendations?

(for migration)

user218912

@MAFIA36790 there was a cat that came to eat food from my bird feeder everyday.

heather

and I didn't recommend anything

user218912

dunno what happened to him but

ACuriousMind

@heather Sorry, forgot the "I saw today"

user218912

5:49 PM

didn't see him for a while

user116211

@obe Male cats go away after a certain period of time.

ACuriousMind

@heather That was kind of a preemptive warning

user218912

@MAFIA36790 really? how come?

ACuriousMind

I don't want a philosophy mod shouting at me why our users want to migrate nonsense there ;)

user116211

@obe Don't know; maybe to start a new family? I had two stray cats who also used to feed on the leftover dog food; they lived with my family for two years; but suddenly they were gone.

user218912

5:52 PM

died mb?

user116211

Noticed one after few months in some far street.

ACuriousMind

@MAFIA36790 Cats don't form "families"

user116211

ohh; I've no idea.

ACuriousMind

They are either territorial individuals or they form packs that control a territory

0celo7

@ACuriousMind Elven lord just told a story about life before typewriters...

How old is this dude

user218912

5:55 PM

do a.i.'s form families? xd

user116211

@obe They serve the humanity only.

ACuriousMind

@0celo7 Did he tell it as first-hand experience or just as an anecdote?

0celo7

@ACuriousMind he taught at the "London college of printers" back in the day

They were still printing by hand with a movable type

user218912

remember this

user218912

John Rennie

6:00 PM

Dear God, no, please. There are enough cat pictures on the Internet already :-)

0celo7

Do you not like cats?

John Rennie

I have no pets, nor feel any great desire to have a pet.

0celo7

What a sad life

John Rennie

I quite admire cats for their athleticism

user116211

I love dogs.

0celo7

6:01 PM

My cat discovered spacetime

user218912

:O

Physics Meta

0

Q: How do I make this question specific enough to be opened?

The title is Methodology used in Climate models This is annoying and I hate to do this, but I am looking for answers from experts working in the field who have spent several years studying this subject. It is a huge topic and It is definitely not easy to get into. Is it by default that such que...

discussion close-reasons

user218912

lol this is a thing "if two guys were on the moon and one killed the other with a rock would that be fucked up or what"

user116211

@PhysicsMeta Again?

0celo7

@JohnRennie I mean, you really owe my cat for all your rep.

user218912

6:12 PM

lol

John Rennie

@0celo7 I'll post a herring to him. What's your address?

Swapnil Das

I have a math question here.

What should be my 'perfect' approach for Q53?

John Rennie

@SwapnilDas All the circles have to pass through a single fixed point. Yes?

Swapnil Das

Umm. Perhaps?

0celo7

@JohnRennie I'll tell you over skype

ACuriousMind

6:16 PM

It's explicitly stated in the exercise text that they do.

John Rennie

Why perhaps?

Swapnil Das

Indian examinations' wordings are never clear.

0celo7

Did Michael J Fox take that picture?

Swapnil Das

So (d)?

John Rennie

@SwapnilDas why (d)? Show your working!

Swapnil Das

6:17 PM

Intuition?

John Rennie

@0celo7 OK. It should only take a few days (at room temperature) for the fish to arrive :-)

Swapnil Das

@JohnRennie does that work?

ACuriousMind

lol, @BernardMeurer gets a laptop and @0celo7 gets a dead fish

John Rennie

@SwapnilDas Honestly, replying with intuition is not taking this seriously.

Swapnil Das

Hmm. Rigor cleans the window through which intuition shines, I know :P

0celo7

6:19 PM

@JohnRennie Proof?

John Rennie

@ACuriousMind he's got a 256GB iPhone 7. The guy must be loaded! Well, he used to be loaded before he bought the phone :-)

user116211

@0celo7 proof?

0celo7

@JohnRennie I make 11 bucks an hour, I assure you I'm not loaded.

Swapnil Das

@JohnRennie are the fixed points, centers?

0celo7

@JohnRennie honestly, the outrageous price is worth being able to see replies in chat

John Rennie

6:24 PM

@SwapnilDas the question says all the circles pass through a single fixed point.I take this to mean:

Swapnil Das

OK. Hands up, I cant proceed any further.

John Rennie

where the red dot is that single fixed point.

Oh!

OK so far?

Whoa what's that

6:25 PM

Yes!

John Rennie

OK, so what do we know about the distance between the centres of the circles and that fixed point?

Swapnil Das

Same!

It lie on their circumference.

0celo7

@JohnRennie the home button isn't even a button

John Rennie

And the locus of all points that are the same distance $r$ from a fixed point is?

0celo7

It simulates clicking

Swapnil Das

6:28 PM

A circle!

That was an excellent explanation! Thanks a ton @JohnRennie

John Rennie

Yes. There's your answer!

0celo7

@JohnRennie a geodesic sphere?

John Rennie

@0celo7 in 2D? :-)

0celo7

Circles are spheres too

Swapnil Das

Rly?

John Rennie

6:31 PM

@0celo7 Oh go back to your balls! :-)

Swapnil Das

Was that topology?

0celo7

I've been too busy to work with balls lately @JohnRennie

But I have a 400 page book just on balls I want to read :)

But I need to check some stuff about space forms first.

@JohnRennie do all old men have bad nose hair?

John Rennie

@0celo7 Most do. I just let mine grow these days. I used to trim it but that got to be a hassle.

0celo7

I guess your girlfriend is already too hairy to notice new hair on you

John Rennie

Apparently when you get really old you get hairy ears, though that hasn't happened to me yet.

0celo7

6:38 PM

I should flag that, but can't.

John Rennie

@0celo7 anyway she gets sheared every spring

Swapnil Das

No one can fight me on the grounds of flying hair ;)

John Rennie

Anyway, I'm off. I have a cold beer calling out to me.

Swapnil Das

Bye!

user116211

@JohnRennie Don't drink too much.

3

Swapnil Das

6:41 PM

@MAFIA36790 why, are you jealous? :P

0celo7

Mafia is a good Muslim, he will not drink.

Swapnil Das

That's flying hair^^

user116211

I don't drink.

user116211

And I'm not muslim either ;(

0celo7

What?

Swapnil Das

6:44 PM

Hindu then?

user116211

I'm not into religions, sorry.

Swapnil Das

Atheist?

user116211

Don't care.

Swapnil Das

Lol. OK.

user116211

> Just to clarify: this is not homework, I'm asking this on behalf of a fellow teacher whose English is a bit tentative and the book ....

user116211

6:45 PM

Typical misinterpretation by taking homework in the literal sense.

heather

actually, i think the term in this case would be agnostic

but anyway

^man, it took me 4 tries to spell "anyway" right. having trouble today =P

0celo7

@MAFIA36790 ...you're not the man I thought you were.

user116211

._.

heather

so, i dunno if anyone else has this problem

user116211

@heather MathJax rendering?

heather

6:48 PM

but sometimes i look at profile images and think they are something

user116211

ohh.

heather

and then I zoom in and realize, wait, that is completely different from what I thought it was

for example: i thought @MAFIA36790 had a profile image of some cloudy day thing

and then, wait: it's a dog

sleeping under blankets

user116211

WTH ;/

heather

i know, it's bad!

or I thought DanielSank's was a dog

and then I realized it was an octopus

user116211

@heather I took it initially as a leopard's face.

heather

6:50 PM

huh

0celo7

Is that a dog?

heather

what?

0celo7

@MAFIA36790

Why is the dog on concrete?

user116211

yes.

ACuriousMind

@heather To not care would actually be apatheistic.

heather

6:51 PM

@ACuriousMind, oh, good to know

user116211

Now, it proves again ACM is AI.

ACuriousMind

One can be agnostic or atheistic and care very much about the existence of gods

user116211

@0celo7 It's my roof; that was a sunny winter morning.

ACuriousMind

@heather Yes, I have that very often

Most images do not scale well

0celo7

@MAFIA36790 what?

heather

6:53 PM

@ACuriousMind, now I am reassured that I'm not completely nuts =P

0celo7

@heather is a candle, right?

heather

@0celo7, lol, no, it's a rocket launch =D

0celo7

Hmm.

ACuriousMind

::zooms in on image::

::squints::

heather

guess I need to pick a better photo =)

ACuriousMind

6:57 PM

I have spent weeks under the impression that's a candle!

But it does appear to be a rocket. Unless you just now changed it to mess with us :P

heather

nope, I'm not that evil

man, I'm laughing so hard right now, I had no idea everyone thought it was a candle!

=D

user116211

@heather, I thought it was diya.

heather

^this one better?

I don't know though...

I like my current photo a lot =)

ACuriousMind

Well, you don't have to change it if you're willing to be mistaken for a candle at first sight ;)

Speaking of profile pictures, I think I'll change mine soon

heather

to what?

0celo7

7:02 PM

@ACuriousMind oh, goodie.

I've been considering changing mine to Hathcock

But it might scare the children :(

heather

but the squashed cat! It is perfect!

=)

@HDE226868, it is an octopus...

ACuriousMind

@HDE226868 it is an octopus...

HDE 226868

Brainfart. I meant falcon.

ACuriousMind

lol

HDE 226868

Then I realized it was an octopus.

0celo7

7:05 PM

What the heck are you taking about

user116211

@0celo7 Falcon turning to Octopus.

HDE 226868

@0celo7 I used to think Daniel Sank's profile picture was a falcon. It is not.

0celo7

It's algae, no?

Did he get banned?

user116211

No.

user116211

Classic Homework:

user116211

7:18 PM

0

Q: How to prove the following Dirac matrices identity?

$\{\gamma^{\mu}, \gamma^{\nu}\}=-2i\sigma^{\mu \nu}$. I have an important exam tomorrow and would really appreciate if someone could help me to prove this. Thanks so much.

homework-and-exercises particle-physics

user116211

> I have an important exam tomorrow and would really appreciate if someone could help me to prove this.

ACuriousMind

@0celo7 No, he took the messages to the other room, don't be paranoid :P

user116211

ACM changed the pic!

Swapnil Das

What's the feeling of getting a 'doctor' right behind your name?

user116211

Now, not a female; phew

ACuriousMind

7:26 PM

@MAFIA36790 "phew"?

user116211

Hmm, you used many female pics in the past.

Swapnil Das

Is that an old Tarzan?

Obliv

@Secret Perhaps. I don't have an interesting answer though. Currently doing a lab report on a photoelectric effect lab. My physics class really irks me because we scratch the surface in many topics in physics and never give a rigorous treatment in any of them. 8^(

Secret

Well most physics labs work like that. (At least for my uni) physics labs mainly train you on how to interpret and treat the data, error propagation, and experimental setups. The theory is secondary to the purpose

ACuriousMind

@MAFIA36790 True, but "phew" indicates that you are somehow...relieved by this one being male

Secret

7:31 PM

[Division by zero] While randomly investigating the dependence of entries in a cayley table in some arbitrary ring with the sequential insertion of axioms, Accidentally found a division by zero semiring. Yes this thing has an additive identity but it does not collapse into the trivial ring

user116211

@ACuriousMind Well, yeh, I like this more than your other avatars.

Swapnil Das

@ACuriousMind :P

Secret

The catch: To make it any bigger will require breaking associativity. I am not going to touch nonassociative algebras until I know enough of it

0celo7

@ACuriousMind I'm not paranoid

ACM looks insane now

Secret

Other organisation thingy which has nothing to do with division by zero:

Everything is easier with matrices:

Suppose one is being asked to compute the associative laws of $(\mathbb{Z}/3,+)$ by hand. First rewrote the cayley table of $(\mathbb{Z}/3,+)$

\begin{array}{|c|c|c|} \hline + &0&1&2\\ 0& \hline 0&1&2\\ \hline 1&1&2&0\\ \hline 2&2&0&1 \\ \hline\end{array}

as follows

$$\begin{pmatrix}0&1&2 \\ 1&2&0 \\ 2&0&1\end{pmatrix}=\begin{pmatrix}0&0&0 \\ 1&1&1 \\ 2&2&2\end{pmatrix}+'\begin{pmatrix}0&1&2 \\ 0&1&2 \\ 0&1&2\end{pmatrix}$$

where $+'$ is operating $+$ entrywise, Then associative law (where $a\in \mathbb{Z/3}$) can be condensed into

heather

7:35 PM

whoa, @ACuriousMind, sorry, but your profile pic looks creepy now

Obliv

@secret uh I'm not good at reading cayley tables, what are the elements of that ring you linked?

@heather I think @acuriousmind looks very handsome now.

what game is that from, btw?

user116211

@Obliv yes.

user116211

I like this avatar.

ACuriousMind

@0celo7 It's Durance, he's not...entirely of a sound mind

Obliv

oh it's pillars of eternity?

ACuriousMind

7:37 PM

@Obliv Pillars of Eternity, cf. above link

Obliv

is it like DnD?

Secret

Obliv: 0,1,a:
a is basically plugged in just to force 0 and 1 to be one sided multiplicative identities (else I have no multiplicative identities in that structure), which is why it is an additive and multiplicative absorber (although it can only be * associative as a multiplicative absorber)

0 and 1 are left multiplicative identities, where 1 is the right multiplicative inverse of 0 and 1 is the left multiplicative inverse of 1

0 is the additive identity

Collapse is checked against the equations 0+0=0 and 0+1=1 by left or right multiplying 1 or 0 to go to other entries of the + cayley table

Obliv

@secret is it closed under addition?

Secret

Yup, for +, 0+0=0, 1+0=0+1=1, 1+1=1, and a+whatever=a

do not be alarmed for 1+1=1, because there is no 2 in this structure (we are not in the reals)

As far I am aware, binary division by zero algebra seems to always break the inductive axiom 1+1=2

Obliv

is it $\mathbb{Z}/2\mathbb{Z}$ or something?

with 0

ACuriousMind

7:42 PM

@Obliv It's from the people who made Baldur's Gate, which was D&D, but it runs on its own system and is set in a different world, so no

It's "like D&D" only in the sense that it's a fantasy roleplaying game

Secret

It is not $\mathbb{Z}/2$ because the * cayley table has a right null subsemigroup in it

Obliv

so it's non-commutative under multiplication?

that's really weird. it's like $0,1$ are semi-identities

Secret

Yup, it is not commutative under multiplication, but commutative under addition

heather

okay, I created a community ad, and I wanted to know what you guys thought of it:

Secret

In particular, in most algebraic systems, 1 has a property that it is its own inverse, thus an involution and also an identity. Therefore making it one sided actually does little to the expression 1*1

Obliv

7:50 PM

@secret so if you divide any element by $0$ in that semiring, do you just get the element?

Secret

You will get 1 (if it is 0 or 1) or a (if it is a)

More explicitily, because 1 is the right inverse of zero, then

0*1=1 (right inverse of 0)
1*1=1 (left identity of 1)
a*1=a (a is annihilating)

So roughly speaking if we identify "multiply by 1 on the right" with /0 we get

"0/0=1"
"1/0=1"
"a/0=a"

heather

if anyone hates it, please let me know

or if anyone dislikes it