The h Bar

user228700

9:00 AM

@JohnRennie Yeah, sorry, this is what I mean.

Mew

Indeed John Rennie's equation is equivalent to negative gradient

RE-arranging John rennies gives m1 = -m2

John Rennie

Well the gradient of the tangent at a point $(x,y)$ on the ellipse is just $dy/dx$

If you start with $a^2x^2 + b^2y^2 = c^2$ then, erm, $a^22xdx+b^22ydy = 0$ ... is that right, hang on I'd grab a pen and paper.

user228700

No, wait.

Mew

It's just a simple oval. Look at it, the fall/run of the top gradient is the same as the rise/run of the bottom gradient

and since rise = -fall

...

John Rennie

@Mew: yes but K wants to show it algebraically

user228700

9:05 AM

I have an equation with $m$ in it!

user228700

I want to go from there.

John Rennie

What equation with $m$ in it?

user228700

47 mins ago, by Kaumudi

Yes, they are! The equation $y=mx ± \sqrt{a^2m^2+b^2}$ gives the equation of two parallel tangents to an ellipse (Notice, same slope!) with diff. y-intercepts.

John Rennie

That's not an equation for $m$

i.e. that doesn't tell you what $m$ is for the tangent at a point $(x,y)$ on the ellipse.

user228700

No, it's not, but I can substitute a point to make it an equation of $m$.

user228700

9:08 AM

And no, not a point on the ellipse.

John Rennie

You've lost me ...

That's the general form of a tangent to the ellipse. To get a particular tangent you have to say which point on the ellipse the tangent passes through.

user228700

If Ik the value of (x,y) to substitute in that equation, I get a quadratic in $m$.

Mew

@JohnRennie, can you please derive an equation for the gradient, m, in terms of the value x

user228700

I mean, I can take any value of $(x,y)$ outside the ellipse and plug it in to get a quadratic in $m$.

John Rennie

Those are the two tangents that equation describes.

user228700

9:12 AM

Yeah.

user228700

We substitute some value of $(x,y)$ that satisfies one of these tangents, doesn't matter which one.

Mew

alright

enuf of this non-sense

I"ll show you how it's done

let's start with the equation

John Rennie

Ah, OK. You're saying that if we choose an arbitrary point $(x,y)$ outside the ellipse then which tangent(s) pass through that point?

Mew

x^2/a^2 + y^2/b^2 = c

user228700

@JohnRennie Yeah.

Mew

9:13 AM

now take the derivative of both sides with respect to x

John Rennie

^ $c^2$

Mew

yeah

who cares we're getting rid of it next step lol

we then get

2x/a^2 + (2y/b^2)*(dy/dx) = 0

therefore, dy/dx = b^2x/(ya^2)

now here we can see that for an elipse centred at 0, for a given x, you can get y or -y

subbing in shows you get a + or - answer of equyal magnitude for dy/dx

if the elipse isn't centred, it still applies but we need an equation for y in terms of x

that is y = +-sqrt(b^2(c^2 - x^2/a^2))

John Rennie

Like so?

Mew

put that into the equation

correct

except that diagram is a bit wonky

sorry if the elipse isn't centred you need more complicated maths

but I"m assuming a centred elipse

John Rennie

@Mew no it isn't. IIUC K is asking about tangents through arbitrary points not points on the x axis

Mew

9:16 AM

you can transform the results later

@JohnRennie, he is asking about why m1/m2 = -1

John Rennie

@Kaumudi: have I understood correctly?

Mew

and this only applies when the tangents have the same x coordinate

sorry I thought he was still asking about why the two tangents are negative of each other

user228700

OK.

Mew

u geddit now?

user228700

I had asked about tangents from arbitrary points before, yes @JohnRennie

user228700

9:19 AM

Secondly, @Mew: I wanted to get the result $m_1/m_2$ not by diff. the equation, but by using the equations I've got.

user228700

Thirdly, I'm female.

user228700

(:-P)

Mew

Sorry you look like a male :-P

user228700

@Mew That's not my picture! ^_^

John Rennie

Using the equation $y=mx ± \sqrt{a^2m^2+b^2}$ ?

Mew

9:21 AM

oo i see

user228700

@JohnRennie Yes!

Mew

@Kaumudi, alright step 1 re-arrange to get m = something

user228700

@Mew John Green, that's who.

Mew

is that your boyfriend or somethinig

step 2 get -m = -something

user228700

@Mew Oh my God. Dude, he's like, 40! I'm 17!

Mew

9:23 AM

lol sooooryyyyyy

it happens u know

user228700

I've never even met him. He's a NYTimes bestselling author.

Mew

some 60 year olds date 20 year olds

especially rich ones like that

user228700

@Mew Yeah, Ik. I reiterate; I haven't even met him!

John Rennie

Can't you just do: $$ \frac{m_1x + \sqrt{a^2m_1^2+b^2}}{m_2x - \sqrt{a^2m_2^2+b^2}} = 1$$

Mew

so you haven't asked his permission to use his photo?

tsk tsk

user228700

9:24 AM

@Mew google.co.in/…

John Rennie

@Mew let's stay on topic guys

Mew

@JohnRennie, I think you made a negative error somewhere

user228700

@JohnRennie Rendering

Mew

I think he loves pizza

@JohnRennie, I think you just fixed it

John Rennie

@Mew Oops, yes, thanks.

Mew

9:25 AM

wait

no you didn't

alos put a -1 instead of 1

wait

yeah

-1 on the RHS too no?

John Rennie

No

Mew

the y-values won't be the same hto

user228700

Why did u switch signs?

Mew

because y of the first tangent is negative of y of the second tangent

oh wait

John Rennie

@Kaumudi the equation is $$y = m_1x \pm \sqrt{a^2m_1^2+b^2}$$

user228700

9:27 AM

Yeah...

John Rennie

where $x$ and $y$ are constants i.e. we're choosing the values of $x$ and $y$ that we want our tangents to go through.

So we have two equations: $$y = m_1x + \sqrt{a^2m_1^2+b^2}$$

and $$y= m_2x - \sqrt{a^2m_2^2+b^2}$$

Mew

but

Bernard Meurer

QUICK LATEX HELP PLOX

Mew

the y coordinates differ

of the two tangents we're interested in

John Rennie

No they don't good grief!!

We want the two tangents that pass theough the same point $(x,y)$

Mew

9:29 AM

we're looking at my picture h

Bernard Meurer

\title{Lógica & realimentação proporcional}
\date{4 de Novembro de 2016}
\author{Bernardo Meurer\\
        \texttt{99999}
        \and
        Maria Adelaide Ambrósio\\
        \texttt{99999}
        \and
        Inês Coelho\\
        \texttt{99999}}
\begin{document}
\maketitle
\newpage
\end{document}

Mew

no

Bernard Meurer

Why does the '&' in my title break things

Mew

we are comparing the gradients of the tangent that passes through y, with the gradient of the tangent that passes through -y

Bernard Meurer

Ah \&

Mew

9:29 AM

I know we are using different equations to model this scenario

but we're modelling the above picture nevertheless

John Rennie

$(x,y)$ is the red dot in my diagram, not the points on the ellipse

user228700

Wtf is happening? :'-(

Mew

oh

John Rennie

Mew is barking up the wrong tree.

Mew

roof roof

John Rennie

9:30 AM

Ignore him, I am the superhuman genius here!! :-)

3 mins ago, by John Rennie

So we have two equations: $$y = m_1x + \sqrt{a^2m_1^2+b^2}$$

user228700

:-)

Mew

:'-)

John Rennie

Did what I said make sense?

user228700

OK, no, wait!

user228700

You know what? My brain is not performing well.

user228700

9:32 AM

It would be best if I came back to this in awhile.

Mew

aw k

John Rennie

OK. I'm stuck here for several hours waiting in for a delivery, so I'll be a round.

Anonymous

@Kaumudi What is the question btw ? :-P

John Rennie

But I suspect it's simpler than you think ...

user228700

Yeah, Ik. That's why I need to come back to this later.

Mew

9:33 AM

@S007, for a given x-value on an elipse, the tangents to that elipse have gradients that are negative of one another. Proove this result using an unconventional formula as a starting point.

user228700

@S007 :-P You're a little late to the party.

John Rennie

OK, I'm going off to check my Facebook posts ...

user228700

@Mew Well, yeah, basically.

Anonymous

@Kaumudi That's true for me in real life too...always ;-)

Mew

@Kaumudi, remember if you have a physics problem to pop into www.physicsproblems.nfshost.com

user116211

9:57 AM

I'm really tired after this mammoth Abstract Algebra test ;/

THE LONE WOLF.

Guys What can i do for moderator's attention for a question??

Mew

what is the question?

John Rennie

What question, and what would you like the mods to do?

Mew

responding is a good start

THE LONE WOLF.

I want reopening of this question... physics.stackexchange.com/questions/289614/…

Mew

10:05 AM

lol

no mod is going to make that happen budd

buddy

don't you know it is "I" not "i"

user116211

@THELONEWOLF. Since you edited, it will go to the review queue.

user116211

You will have to wait.

THE LONE WOLF.

Okay,Thanks @MAFIA36790

user116211

No need for mod attention now.

THE LONE WOLF.

Hey @Kaumudi are you kaumudi harikumar???

Sorry if i m spelling your name incorrectly

user228700

10:29 AM

@THELONEWOLF. Uh, why dyou ask?

Mew

that's very personal lonely wolf

user228700

@Mew: Will u help me solve two equations pls? I'm like, dying.

user228700

It's simple af, but yeah, my brain is dumb af so.

user228700

@JohnRennie: S'thing I thought might interest you...there's a newspaper called "Kerala Kaumudi". How about that? :-P

John Rennie

kaumudi.com

Is that it?

user228700

10:37 AM

Yeah, lol :-P

John Rennie

I wonder how it got that name. Is Kaumudi specifically a girl's name in Tamil Nadu or can it be used more generally?

user228700

@JohnRennie I will get back to u. I'm trying to solve s'thing.

user116211

I need coffee.

user116211

It's really cold here.

Mew

@Kaumudi, yeah what are the questions

@JohnRennie, can I ask a question to the super genius

John Rennie

10:52 AM

If you know one, but if you can't find a super genius I can try to answer :-)

Mew

is the word "exchange" in stackexchange copywright

as in

if i make a site called qandaexchange.com

is this a violation of stackexchange.com?

John Rennie

No

Mew

ok thanks

Dvij

Hi, everyone! Sorry for interrupting your ongoing discussions. I feel this must be something trivial but I am not getting it. Is it always possible to find a flat higher dimensional embedding for a given infinitely differentiable continuous manifold? But I think the super genius can also multi-task. :P

John Rennie

If you called your site stacksexchange you might be headed for a lawsuit, but quandaexchange seems sufficiently different.

Mew

10:53 AM

alright makes sense thanks John

John Rennie

Actually stacksexchange might be unsuitable for other reasons :-)

@Dvij that's the embedding theorems.

Mew

lol

Dvij

okay. thanks!

John Rennie

For Riemannian manifolds the answer is yes and there's even an equation for the minimum dimensionality of the larger manifold.

For pseudo-Riemannian manifolds it's more complicated though I think the answer is still yes.

Google for Nash embedding. Now I think about it, I think only causal pseudo-Riemannian manifolds can be embedded. If a spacetime contains CTCs then it can't be embedded - or something like that.

user228700

@JohnRennie: "Kaumudi" is a Sanskrit word and it means "Moonlight". I reckon there are about 20 people in India w/ my name. I haven't met a single person who has heard of the name prior to meeting me, my whole life, so yeah.

user228700

11:02 AM

I think it's mostly a name for females.

John Rennie

@Kaumudi Aha!

user228700

What? :-P

Mew

moonlight that's a pretty name

user116211

@JohnRennie WTH are you writing John?

John Rennie

I just meant *aha* as in that means the newspaper name is unrelated to people's names.
The Moon tends to be taken as feminine in most mythologies, with the Sun as masculine.

Mew

11:04 AM

Yeah the moon is weak and the sun is strong

user116211

Hmm; the chat room is going weird.

John Rennie

@MAFIA36790 it's a web site for transgender physicists :-)

user116211

Girls, relationship, death; and now these ;/

user228700

@Mew Yeah, thanks :-) My mom was reading a Sanskrit scripture whilst pregnant with me. And in it, there's the word "Kaumudi". Apparently, in the middle of reciting, when she said " Kaumudi", I kicked for the first time :-D

user228700

@MAFIA36790 Duuude. Give us a break.

user116211

11:04 AM

@JohnRennie Aha! Only this left out of the chat room.

user228700

@JohnRennie Yeah :-)

user228700

@MAFIA36790 lol :-P

Mew

Hi @heather

heather

@Mew, hello

Mew

@heather, did you see the meta post about the new site domain name

pls let me know what you think

user116211

11:13 AM

@Mew, Are you looking for new name?

heather

@Mew, just saw it...I'd have to go physics.qandaexchange.com, I think that sounds good; the site background would obviously have a physics style design.

user116211

I would go with @johnR's suggestion.

Mew

@MAFIA36790, yes we want to abandon the nfshost.com

user116211

Learning Physics.

Mew

we're going to go with physics.qanda-exchange.com

user116211

11:14 AM

Very well then.

John Rennie

@MAFIA36790 thank goodness for that, I thought you meant stack sex change.com for a moment.

heather

sounds good to me

Mew

haha

user116211

@JohnRennie This chat is going crazy ;(

heather

I'd design the page with just the physics bit though, just like stack exchange

Mew

11:15 AM

@MAFIA36790, what do you expect after we saw the graph

of how many weeks we hav e

silliness for the max

heather

@Mew, what graph?

user116211

@Mew Huh?

Mew

@heather, you don't want to know

John Rennie

@MAFIA36790 it went crazy a long time ago. As long as some good physics is discussed in between the bouts of lunacy that's fine by me.

Mew

there was a graph yesturday

user116211

11:16 AM

...

Mew

by kaudmaudi i think

you can go look if u want

I think it was a chart of the number of weeks a human life has up to age 90

and it didn't look very big lol

user116211

Anyways, Mosul is separated from the entire country by Iraqi Forces; ISIS' days are numbered.

Mew

na ISIS is a virus

they aren't localised

user116211

@Mew Their hub is in Mosul.

heather

@Mew, what do you think of this meta post I just added?

Mew

11:21 AM

@heather, I like it

fantastic job

heather

Okay, just making sure

Mew

One recommended adjustment if i may

heather

Go ahead =)

Mew

I'd remove quesiton 1

heather

okay...

Mew

11:21 AM

I don't like questions where I might not like the answer

we're going to be talking about both sites on both sites

heather

took care of it

Mew

so I don't think we should ask for permission to do so

cool :)

heather

gotcha

@BernardMeurer, I'm in CDT (in the U.S., I live in Iowa)

Mew

I'm from AUS

heather

cool!

@Mew, okay, what should I be doing?
- what changes need to be made to the ad?
- do I need to write any code for you?

- do I need to help with the background design?

- are there any meta questions, on PSE or on PPQA, that need to be asked?

Mew

11:26 AM

@heather, what would be helpful is a meta post of a to do list on what we need done to the site

then as we find more bugs and think of new improvements we can add them and cross them off as they are developed

Also if you get time asking good quality questions will be helpful

heather

@Mew, yes, that is a good idea

Mew

as many new users will be turned off if there aren't many questions on the site

heather

okay, I will work on those

Mew

The add is good and I will work on changing the url

when that is done you can update the add if necessary with the new site name

I will keep both urls linking to the new address

so users who have favourited the old url will still be ok

heather

okay, that makes sense

Mew

11:28 AM

Once the "to do" list is created, it will be easier to see how much more coding is required and how we can plan to achieve it

are you comfortable coding in php heather?

and javascript, jquery

heather

javascript, yes

php, no

jquery, a bit

Mew

yep jquery is just a javascript shortcut

useful to leanr

the site is based on php and javascript

there is a tutorial for creating plugins somewhere hold on

heather

I'd definitely be willing to learn php

Mew

docs.question2answer.org/plugins/tutorial

it mnight not be necessary

I should be able to do all the coding but if you learned it and wanted to help that would be fine too

heather

sure, okay

Mew

11:32 AM

actually i'll just do all the coding probably easier that way

heather

okay =)

whichever you think is better

Mew

yep

so a meta on technnical issues is good ie the to do list

and also a meta on non-technical to do list

e.g. designing logo, advertising etc.

or ti could be all in one meta, whichever

We also need to construct a privacy policy

and terms and conditions

heather

already started a list of things to do, I'll split it into two parts (different headings)

okay, terms and conditions

legalese...=(

Physics Meta

1

Q: Physics Problem Solving Site

So, some of you may have heard of how a couple of us tried to propose a new physics site for less conceptual questions and more problem solving, the proposal was closed, and we ended up creating a new site outside of the stack exchange network (see here for more details). In this post, I'm going ...

discussion

Mew

cool free advertising ^

heather

11:38 AM

yup =D

@BernardMeurer, UTC is 5 hrs ahead of CST

@Mew, what needs to be done for terms and conditions?

Mew

dunno

whatever SE has I guess

heather

I guess, yeah

Mew

@heather

important question

what's better

physics.qandaexchange.com/ or physics.qanda-exchange.com/

heather

@Mew, hmm...how about the first one? I don't think the hyphen is necessary, and I don't see an embarrassing words hiding in there.

Mew

@heather, thanks heather the new url has been created

it may take time to update and will be available within the next 24-48 hours

both urls should work after that (and the current one should still be working now)

heather

11:54 AM

@Mew, okay, I'm a good ways along on the new meta post, I'll be posting it shortly

@Mew, how's this

Mew

looks really good

good point about QandA being the same as Quesitons

heather

okay

glad I did it right =D

Mew

r u really 14

heather

in that age bracket, yes

Mew

impressive

heather

12:01 PM

you removed the compliment to you in the meta post

Mew

yeah

don't want to make it persona

l

heather

okay, just let it be known that I meant that =)

Sir Cumference

12:21 PM

@obe Answer plz

Mew

@heather, maybe we should delete the physics.SE meta post until the site is more refined

it has -1 vote

user116211

@Mew No.

Mew

@MAFIA36790, why not?

user116211

@Mew One downvote doesn't mean you have to delete the post.

Mew

The site has a new name physics.qandaexchange.com

0celo7

12:28 PM

-1 is hardly a good start

Mew

@MAFIA36790, yes but given the site isn't refined yet, I think users will be more receptive to it later

exactly @0celo7, voting here shows a scale free topology

that means that people tend to vote in the way the users before them have voted (as they are influenced by the existing score)

user116211

@Mew then you should mention it explicitly that it is still in preliminary stage. I don't think deletion of the post is the answer; but that's your call.

Mew

it's heather's call

user116211

Well, okay.

Mew

you could be right

maybe it will turn around

but if it gets to -2 i'd delete before it gets to -10

user116211

12:30 PM

Just wait for a day, at least so that most of the users of the community get the chance to read that.

Bernard Meurer

@heather Ah, that makes sense

so you're UTC-5, I'm UTC+1, but we have DST right now so the time delta is 5 hours (as opposed to 6) between us

Or does CST have DST right now too?

0celo7

12:48 PM

I don't even know when DST is

Bernard Meurer

Here it kicked in a few days ago

user116211

@BernardMeurer Daylight savings?

0celo7

Fucking Benjamin Frankin

First electric charge, now this

user116211

That old US father guy.

user116211

We need Trump to fix all these.

0celo7

12:53 PM

I doubt he'll fix anything

user116211

I like his new tactic to allure Hindu voters.

Bernard Meurer

@MAFIA36790 DST=Daylight Saving Time, yeah

user116211

@BernardMeurer This is bothering.

user116211

Afternoon @JohnRennie.

John Rennie

Why is daylight saving time bothering you?

user116211

12:58 PM

No, not this.

user116211

Daylight-Saving Time May Be Bad for the U.S. Economy

John Rennie

People continuously moan about DST in the UK as well.

Mew

same in Aus

John Rennie

Whether DST is a good or bad thing doesn't have a simple answer. The real question is whether the benefits outweigh the disadvantages.

Personally I don't care either way, but I have a lot of sympathy for the argument that it means schoolchildren don't have to walk to or from school in the dark.

I suspect the main conclusion to be drawn is that people like moaning :-)

Mew

They should just change school times

user116211

1:24 PM

user116211

Do you miss the punch cards @johnR?

user116211

Old days.

ACuriousMind

@DanielSank You cannot always find a Hamiltonian/Lagrangian description of a physical system. The question of whether or not a given equation of motion admits a (global) Hamiltonian description is in general an open question. Relevant physics.SE posts: physics.stackexchange.com/q/282724/50583, physics.stackexchange.com/q/175021/50583, physics.stackexchange.com/q/51510/50583 and some other questions linked there

In particular, I'm rather certain that there are dissipative systems that don't admit a Hamiltonian formulation, because the Hamiltonian formulation of disspative systems usually exploits some special property or symmetry of the system in question

John Rennie

@MAFIA36790 I did use punch cards in my first year at university. That would have been 1980. However as I recall the card punches were replaced with teletypes the following year.

And no, I don't miss using punched cards :-)

user116211

Ah. You are not that old.

user507974

1:48 PM

@ACuriousMind Recent work seems to admit a Lagrangian description for dissipative systems so I wouldn't be surprised to see a Hamiltonian formulation arxiv.org/pdf/1210.2745v2.pdf

ACuriousMind

2:08 PM

@user507974 Ah, well, introducing additional variables is cheating ;)

user507974

@ACuriousMind But does it seem to help with @DanielSank 's question

ACuriousMind

I think there are two different questions: "Does a given equation of motion admit a Hamiltonian formulation?" and "Does every physical system admit a Hamiltonian formulation?" I answered the first, you answered the second, and you're probably right that the second is what he asked.

0celo7

@ACuriousMind Have you heard about perfect maps?

Danu

2:24 PM

Fractals? :D

ACuriousMind

@0celo7 Vaguely

PhilipV

2:53 PM

I have question. I have a state equation of the form AN^3+BN^2+CN+D=0. Where A,B,C,D depend on things like chemical potential, and other physical parameters(k,w). I know that this system has a critical point for values of k,w where N has a double degenerate solution, and then it bifurcates in two branches in (k,w) space. How would I go about finding the critical point?

user228700

3:36 PM

@JohnRennie: Guess I never came back :-P

John Rennie

Well you're the one trying to pass a stressful exam, so you should use this chat in whatever way works best for you.

I use the chat for the pleasure I get in discussing physics, so you shouldn't feel under any obligation :-)

user228700

:-) I just never got back to it.

user228700

Did u get that "delivery" lol :-P

John Rennie

I did. I have a shiny new laptop to play with!

user228700

:-D Enjoy fixing the screen!

user228700

3:41 PM

(Isn't that the one?)

John Rennie

@Kaumudi Yes. It is a surprisingly enjoyable process fixing laptops. It's the satisfaction you get when you turn them on and they work perfectly.

user228700

:-) Well, have fun!

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