The h Bar - 2014-12-01

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Kyle Kanos

03:25

@Danu NB: Numerical Recipes that ChrisWhite promotes there is actually copyrighted information. That is, you cannot legally use the subroutines in any publicly or privately available software without the recipient having a legal license to use NR.

user54412

@KyleKanos fortunately for us all, those authors have such awful, opaque coding style no one would ever want to directly use their code

Kyle Kanos

@ChrisWhite Debatable. I've actually seen their routines in public code (which goes to show no one gives two sh*ts about software licenses)

3 hours later…

David Z

06:14

@KyleKanos this is true. My advisor uses their code

Come to think of it, some of it may have snuck into our released software (!!) so I should check that... sigh more to do

1 hour later…

Danu

07:23

@KyleKanos Luckily, I'm so bad nobody else will ever want to use my code

JamalS

07:46

@KyleKanos: Oh, I didn't know that... So, if I've included it in some software I've written for my computer, that's technically a breach?

Danu

08:00

Hah

The Taylor seires thing I did does give the constant, I was just too stupid to realize @Qmechanic @JamalS

JamalS

08:57

@Danu: All I did in Mathematica was find the solutions, and then take a Taylor series around infinity, so I was surprised it didn't work for you

2 hours later…

Physics Meta

10:28

0

Q: I cannot access my account because I forgot my username, can you send me the email on my profile that is not public?

I contacted stack exchange several times because I had to make a duplicate account and I want to merge them. I have not still received any response after abunch of emails acros about a week. Mt problem is that when I created my google account I misspelled it. I do remember the password, but not...

support user-accounts

David Z

10:55

@JamalS If you bought a code license, or the package that contains the book and a code license (and the code on a CD, IIRC), then you can use it for whatever you want on your own computers. If you haven't bought a code license, then you can't use it.

see this

16

A: Is this scene from the A Team explainable by Physics?

I took a look at the clip and my take on it that the object of the exercise was not to slow down the tank, but to move it sideways and land in the lake a half a mile away. Hannibal says, "rotate the main gun to 82º" which I take to be sticking out sideways. Background Information The facts I co...

Maybe someone feels like turning that into MathJax... I don't, not right now

ACuriousMind

One should think that one of the 16 people voting that up would have bothered to do so...

David Z

Yeah, you'd think. But apparently not :-(

2 hours later…

ACuriousMind

12:47

Footnote from my QFT lecture notes: "Whenever we don't know what to do, and Fourier transformation is not the answer, we insert a $1\!\!1$."

JamalS

13:37

@ACuriousMind: I'm surprised you haven't stabbed your lecturer yet!

ACuriousMind

13:51

I...actually found that note funny :D

And this is not the same one I ranted about

@JamalS Also, stabbing lecturers would look bad on my resume

JamalS

@ACuriousMind: Yes, doesn't show sufficient creativity or ingenuity

Kyle Kanos

14:35

@JamalS As far as I know, purchasing the book does not actually grant you the license. So technically, if you did not purchase the license and are using it, that is a breach. If you have the license, then you can use it in whatever codes you want so long as it is only you using it!

ACuriousMind

14:58

0

Q: Does information paradox in the Many Worlds interpretation cause a problem?

I'm taking a philosophy of time travel class. In one of the lectures, the teacher was discussing problems with the Many Worlds interpretation. He talked about how since anything that can possibly happen happens in some timeline, in one timeline there is a person that comes out of what appears to ...

time information multiverse

Is this on- or off-topic?

How "non-mainstream" is it to ask about quantum interpretations and time travel?

(Pretty non-mainstream, if you ask me...)

Kyle Kanos

Well the non-mainstream closure reason does say, ...specific questions evaluating new theories in the context of established science are usually allowed

This could be construed to fall under that

But I'm with you that it's not really mainstream (mostly due to the time-travel aspect)

Physics Meta

15:24

0

Q: Is this question still too broad?

Wave Equation Variables and Interpretation was put on hold as too broad, but edited the same day. Now it's been on hold for a few days. Is it still too broad?

discussion reopen

1 hour later…

ACuriousMind

16:34

0

Q: How does a bowl of hot water move by itself?

I think everyone should have seen something like a bowl of hot water moving by itself on a flat surface(seems like it is moving by itself but maybe there are some external force applied to it when it moves). What is the explanation of this phenomenon? Hypotheses : Since the bowl is heavy, for ...

everyday-life pressure friction

Do soup bowls move for anyone else?!

I'm quite confused by this post

Jim

Answer: Magnets

ACuriousMind

Yeah, as hot water is known to replace supercooled electromagnets in our next big collider, I should've thought of that.

Jim

Yeah, I've got no idea

maybe the table is at an angle

ACuriousMind

I should think OP tested if a bowl of cold water moves, too, before so boldly stating it about the hot one

Jim

what type of bowl I wonder?

Brandon Enright

16:42

I've seen the effect but only when there is water on the surface and the bowl is hot enough to make steam.

Jim

and how much does it move?

ACuriousMind

Also, the question is formulated as if hot bowls of soups moving around is one of the more usual occurences in their day

Brandon Enright

Then it lifts up on a cushion of steam and slides a bit.

Jim

If it's only a jump, it could be that the expanding glass of the table drags whatever part of the bowl is most stuck to it along for a very short ride

ACuriousMind

What kind of paper-thin bowls do you people eat soup out of? All my bowls are far too heavy for me to imagine them just sliding on the table, water or not

I would be very bewildered by my household items moving around that way

Brandon Enright

16:45

Well when I've seen it happen with a steam cushion the weight of the bowl doesn't mater as much as the seal the bowl makes against the table surface. It needs a pretty smooth lip all the way around to trap the steam under it.

Jim

he posted a picture of the bowl

ACuriousMind

@Jim I see. Looks like mine. Now I need a glass table :D

Also, is your answer there really an answer?

Jim

Partially

Brandon Enright

@ACuriousMind Probably not. There are probably a few days it can happen.

Jim

it does say that what he describes doesn't happen

which is as much as we could say for a bowl sliding across a table of its own will

the rest is of the not-an-answer variety

Brandon Enright

16:52

it's easiest to reproduce by putting a hot bowl or coffee cup or inverted tumbler (pint glass) on a smooth table with isopropyl alcohol on the surface of the table. The alcohol already has a higher vapor pressure and it evaporates at a lower temperature.

Jim

@BrandonEnright I really don't think that is what is happening in this case

Brandon Enright

ah I see the asker included this "I have heard that water may form a thin layer between the bowl and the surface, but l'm not sure whether it is the case or not."

I've only seen sliding on a cushion of vapor.

Kyle Kanos

17:29

@ChrisWhite Uh-oh. I may have spoke too soon:

0

Q: What is this simple mechanism called?

I ofter build with lego and use this mechanism: It simply converts the rotational movement to making the stick between the two blocks go back and forward. What is this mechanism called?

classical-mechanics

ACuriousMind

@KyleKanos: Is that even on-topic? It is even less about physics concepts than the dreaded homework questions.

Kyle Kanos

Chris's comment that I linked to asks if questions about apparatus-identification would be on topic here

1 hour later…

El'endia Starman

18:44

Hey guys, I'm wondering if this question would be a good one to ask on the main site: "Why is the Lane-Emden equation written as $$\frac{1}{\xi^2} \frac{d}{d\xi} \left( \xi^2 \frac{d\theta}{d\xi} \right) = -\theta^n$$ instead of something like $$\frac{d\theta}{d^2\xi} = -\frac{\theta^n \xi}{2}$$?"

ACuriousMind

@El'endiaStarman It could become one, if you told us why you think it should be the latter. It seems to me these two are not equivalent - did you neglect the product rule in resolving the differential on the LHS?

Kyle Kanos

IMO, it's not really a good question. The Lane-Emden equation is the result of taking the polytropic model and inserting it into the hydrostatic equilibrium equation.

El'endia Starman

@ACuriousMind I initially was quite confused as to why you wouldn't just multiply out the $1/\xi^2$ and $\xi^2$. From what I've understood, $d/d\xi$ is acting as a function here (I suppose that all derivatives are functions, but they're usually not written that way). So, why not just take the derivative with respect to $\xi$ and simplify it?

Kyle Kanos

Then some change of variables, obviously.

Note also that you are assuming that $\theta\neq\theta(\xi)$

And how do you get $d\theta/d^2\xi$? Should that be $d^2\theta/d\xi^2$?

ACuriousMind

@El'endiaStarman Derivatives aren't functions, they're derivatives!

El'endia Starman

18:53

@ACuriousMind Then why was it ($d/d\xi$) written as if it was?

Kyle Kanos

$d/d\xi$ is operating on the term in the parenthesis

El'endia Starman

Okay, so then, $\frac{d}{d\xi}(\xi^2)$, for example, should be $2\xi$, right?

Kyle Kanos

Yes

El'endia Starman

Hmm. Maybe I forgot to take into account the product rule.

$$\frac{d}{d\xi} \left( \xi^2 \frac{d\theta}{d\xi} \right) = 2\xi \frac{d\theta}{d\xi} + \xi^2 \frac{d\theta}{d^2\xi}$$

Then multiply by $1/\xi^2$ and you get...

$$\frac{2}{\xi} \frac{d\theta}{d\xi} + \frac{d\theta}{d^2\xi}$$

Kyle Kanos

You're doing your differentials wrong:

$$\frac{d}{d\xi}\left(\xi^2\frac{d\theta}{d\xi}\right)=2\xi\frac{d\theta}{d\xi} + \xi^2 \frac{d^2\theta}{d\xi^2}$$

David Z

19:02

@El'endiaStarman I'd echo what ACuriousMind said, that it could be a pretty good question if you explain why you think it should be the "other" way in the question

But all this chat discussion may be leading you to an even better question

El'endia Starman

@KyleKanos Ahh, an error in notation. $\frac{d^2\theta}{d\xi^2}$. Excellent. Thanks for the correction.

@DavidZ I did my math wrong. Which kinda sucks considering I'm a math major. :P

David Z

heh, well to be fair mathematicians actually don't do much math :-P

(by which I mean they don't do as much raw manipulation of symbols as one might think)

El'endia Starman

Okay, then that means that one could write the Lane-Emden equation as: $$\frac{d^2\theta}{d\xi^2} = -\frac{2}{\xi} \frac{d\theta}{d\xi} - \theta^n,$$ correct?

Which, by the way, I would say is much more clearly a second order ODE, and poses less risk of confusion.

Kyle Kanos

That is a correct formulation

El'endia Starman

I mean, really, $\theta'' = -\frac{2}{\xi}\theta' - \theta^n$.

Kyle Kanos

19:09

For $n=1$, this formulation is the way to go because it can be solved with power series

El'endia Starman

@KyleKanos Hmm. Could you explain, please?

Kyle Kanos

Let $\theta(\xi)=\sum_m a_m\xi^m$ where $a_m$ are the coefficients. Sticking this into the Lane-Emden eq'n leads to a recursion relation for the $a_m$'s.

With enough work, you'll find $\theta(\xi)={\rm sinc}(\xi)$

El'endia Starman

@KyleKanos Coefficients of what?

Kyle Kanos

Of the power series

$\theta(\xi)=\sum_ma_m\xi^m=a_0\xi^0+a_1\xi^1+a_2\xi^2+\cdots=a_0+a_1\xi+a_2\xi^‌2+\cdots$

David Z

"method of Frobenius" I think that was?

Kyle Kanos

19:19

Google-fu says you are correct:

Frobenius method

In mathematics, the Method of Frobenius, named after Ferdinand Georg Frobenius, is a way to find an infinite series solution for a second-order ordinary differential equation of the form with and in the vicinity of the regular singular point . We can divide by to obtain a differential equation of the form which will not be solvable with regular power series methods if either p(z)/z or q(z)/z2 are not analytic at z = 0. The Frobenius method enables us to create a power series solution to such a differential equation, provided that p(z) and q(z) are themselves analytic at 0 or, being analytic...

El'endia Starman

@KyleKanos What are the $a_m$?

Or how do we find them?

Kyle Kanos

By applying boundary conditions for some

The remaining you actually just take derivatives and set $\xi=0$ to find the remaining values

Eventually you'll find a recursion relation between the $a_{m+2}$ and $a_m$ coefficients

David Z

@El'endiaStarman arbitrary coefficients - you just plug into the equation and differentiate. So on the left side you get $2 a_2 + 6 a_3 \xi + 12 a_4 \xi^2 + \ldots$

El'endia Starman

19:48

@KyleKanos Okay. Boundary conditions were mentioned. For the problem I have to solve with numerical methods, one boundary condition is $\theta(0) = 1$ and the other is $\left( \frac{d\theta}{d\xi} \right)_{\xi=0} = 0$.

I don't quite understand the last one though. Is it saying that the derivative of $\theta$ at $\xi=0$ is $0$?

Kyle Kanos

Yes, that's what it's saying

El'endia Starman

Okay. I think some of my difficulty in really getting this, understanding it, is that $\xi$ is just something weird.

Kyle Kanos

Don't think of it that way

It's a coordinate

El'endia Starman

So I don't have a good mental picture.

Kyle Kanos

Well, a scaled coordinate: $r=\alpha\xi$

where $\alpha$ depends on some constants (e.g., central density, Newton's universal G)

El'endia Starman

19:52

And $$\alpha = \left( \frac{n+1}{4\pi G} \kappa \rho_c^{(1-n)/n} \right)^{1/2}.$$

Kyle Kanos

Those look like the constants I was thinking of

El'endia Starman

What's the purpose of $n$?

@KyleKanos I'm currently in a Numerical Analysis class taught by a professor who does lots of physics work. Hence, his problems tend to be physics based, and in particular, this one is in the context of stellar structure. How the pressure/density varies inside a star, I believe.

I just copied that straight from the problem statement.

Kyle Kanos

$n$ is just the power of the polytrope:

Polytropic process

A polytropic process is a thermodynamic process that obeys the relation: where p is the pressure, v is specific volume, n, the polytropic index, is any real number, and C is a constant. The polytropic process equation is particularly useful for characterizing expansion and compression processes which include heat transfer. This equation can accurately characterize a very wide range of thermodynamic processes, that range from n=0 to n= which covers, n=0 (isobaric), n=1 (isothermal), n=γ (isentropic), n= (isochoric) processes and all values of n in between. Hence the equation is polytropic in the...

It relates the pressure and volume to some constant

(in this case, $v=\rho/\langle m\rangle$)

El'endia Starman

> The following derivation is taken from Christians.[1]

Lol.

Kyle Kanos

Well, to be fair, the Christians did a lot for humanity: hospitals, education, peer review, Pasteurization, printing press, and so on. In this case, however, it's a persons surname :)

Phonon

20:04

@ACuriousMind heyy :D I just realised you've started a blog already, great! you finally got around to it ha?

ACuriousMind

@Phonon Hah, I meant to tell you when you joined chat today :D

Kyle Kanos

I have a blog

I just don't use it

ACuriousMind

Seems you stalk my profile regularly enough that there's no need ;)

Kyle Kanos

^ OoOoOoOoOoOoOoOo

Phonon

@ACuriousMind lol

Kyle Kanos

20:05

Actually, I have two blogs

Phonon

@ACuriousMind really happy you finally did it ;D

Kyle Kanos

My academic one and the Fortran in a C World one that I link in my profile. Neither is used

Phonon

@ACuriousMind I ll have a read at the two posts already later tonight, after work

@KyleKanos had no idea, you've stopped blogging then?

@ACuriousMind Gauging thoughts :pppp

ACuriousMind

@Phonon A Curious Mind, was, unfortunately, already taken^^

Phonon

haha :D

Kyle Kanos

20:10

@Phonon Yeah, I just ran out of ideas to write about so I stopped

Phonon

@KyleKanos :(

Kyle Kanos

I'm over it. It's not a big deal, somethings I'm just not good at

Jim

Blaspheme! You're the best at everything!

Kyle Kanos

Like the intuition for the Rubik's cube. I solve it rather rigidly (piece A goes there, piece B goes there) rather than trying to solve it smoothly (e.g., if I move this piece here, then rotate, I've got these 3 adjacent pieces together)

Phonon

20:26

@ACuriousMind one can use latex in comments? (your blog)

ACuriousMind

@Phonon Uhh...I guess so? You have to write latex directly after the initial dollar, though.

Phonon

@ACuriousMind kk, testing it with a first comment

Kyle Kanos

This is one of my favorite problems in physics:

-1

Q: A Special Case About Projectile Motion

There is a common fact about projectile motion. Let me remind you with an example: Assume that you are aiming at a hanging banana on a tree. If there were no gravitational force, the bullet would follow a linear path at a time "t". But since it does exist such kind of force, a parabolic path will...

homework-and-exercises newtonian-mechanics newtonian-gravity projectile free-fall

Chris Okyen

HEllo

Jim

20:41

Hello

Chris Okyen

I want to see as far as I can around my hand with a mirror for armwrestling. I want to be be able to see both arund the side and somewhat over the top of my hand . would a homemade parabolic mirror lke this be able wrap around very far on a hand? youtube.com/watch?v=_8sd9UgjXLE

Jim

That sounds like a lot of effort just for armwrestling

Phonon

@ACuriousMind done, lol it says "waiting moderation" :D

Kyle Kanos

1

Q: Use mirror to see every angle around me

An applied physics question: How can I setup a mirror all around an arm wrestling table so I can see all angles of what I'm doing? The arm wresting table looks like this With the wrestlers on table looks like

visible-light reflection

Chris Okyen

@KyleKanos Their is no longer answers

There was at one point

Don

Kyle Kanos

20:46

It was deleted because it wasn't really an answer

Chris Okyen

Oh

@Jim Yep but is worth it if it is works. Can't think a simpler way with little money spent and being as simple.

Kyle Kanos

I just don't get the "why" aspect

Jim

@KyleKanos Bravo! you referred him to his own question. That is a special kind of kindness right there

Kyle Kanos

It wasn't meant for him

It was meant as reference

(e.g., for you)

Chris Okyen

How can I approximate if given parabola mirror would reflect ( wrap around ) around my hand enough

Jim

20:49

@ChrisOkyen a simpler way for what?

Or rather, why?

Chris Okyen

The why - Arm wrestling is a lot of form and you can't always see what your fingers are doing without moving your head which is annoying and can put you in bad positon

Jim

Mount a camera and watch the video like a football team does when they practice?

otherwise, wouldn't this be not allowed by pro arm wrestling regs?

Chris Okyen

I am doing it during practice

If I took a bendable surface and rigged a reflecive material or painted a reflective material on it , would that work?

Kyle Kanos

It could work, but you'd be the one who could tell, not us :/

Jim

Camera is the way to go then. That way you see your form while you're concentrating properly. You aren't looking at a mirror and distracting yourself (which might make your form worsen compare to when you aren't watching the mirror)

Chris Okyen

20:54

We basically are stopping when the person I am pulling wants to show me my form or his form when we are practicing

Jim

Right, but if you capture it on camera, you can do the wrestle and go back after to watch your form. Just like the football teams do it

Chris Okyen

@Jim Your are right I just thought it would spend less time being done with it during instead of going back. But with a camera it is recorded so you can review :)

Phonon

@ACuriousMind btw send me an email to that address, so I can tell you bout some typos here and there (don't wanna spam the chat here).

2 hours later…

Danu

22:30

Rereading (the debates on) the questions of 't Hooft is so fascinating. physics.stackexchange.com/users/11205/g-t-hooft

Those were the days...

@DavidZ Lol.

Phonon

@Danu idd, same for Peter Shor's

1 hour later…

ACuriousMind

23:41

@Phonon Sry for delayed reply, I will - tomorrow. For now, I'm going to bed.

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