The h Bar - 2016-04-26 (page 2 of 2)

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7:02 PM

$\lvert S(t)\rangle $ would be more $e^{i\hat H t}\sum c_n \vert n \rangle$

can anyone tell me why minimum and maximum tension are here in a string?

user image

hey guys, whats the scaling law for VdW interactions

would the most right way to describe it as Surface area, number of atoms, length of chains, etc

7:17 PM

@DeNiSkA what is constraining the string into that shape?

@JohnRennie you can take tension acting at ends and a pulse is generated

Van der Waals force

In physical chemistry, the van der Waals forces (or van der Waals' interaction), named after Dutch scientist Johannes Diderik van der Waals, are the residual attractive or repulsive forces between molecules or atomic groups that do not arise from a covalent bond, or electrostatic interaction of ions or of ionic groups with one another or with neutral molecules. The resulting van der Waals forces can be attractive or repulsive. The term includes: force between permanent dipoles (Keesom force) force between a permanent dipole and a corresponding induced dipole (Debye force) force between inst...

@DeNiSkA Aha, so we're looking at a soliton propagating along the string. Yes?

yup

@DeNiSkA well now we've sorted that out I have absolutely no idea what the answer is. Sorry ...

ah, sounds bad! but thanks

7:24 PM

@DeNiSkA Since the wave propagates at a given speed, you can look at how far a given piece of the string has to move in a given time.

Presumably, that's just the derivative of the wave or something.

@JohnRennie i meant a non-distance scaling law (basically stuff that probably makes up A), but thanks'

btw guys, is there a good book for learning diffusion

@user507974 Not a single book. The material is spread out over several books.

Any thoughts on Poincare duality and why it should be obvious?

@barrycarter just too much material?

@user507974 That was my pathetic attempt at humor.

7:29 PM

@barrycarter you got several knee slaps from the people around me

@user507974 Woohoo! Deep in the heart of Texas?

@barrycarter nah(after thinking for 2 minutes)! i can't get you

@DeNiSkA Well, the place where the wave starts has the greatest y-direction derivative, right?

yes

@DeNiSkA And the top of the wave has 0 y-direction derivative, right?

7:31 PM

yes

@DeNiSkA And I have no idea what I'm talking about, right? ;)

haha :|

@DeNiSkA That was of my attempt at giving you hint, but I'm not sure it's really useful. Ultimately, consider what happens to a given point on a string as the wave passes.

@JohnRennie : Permission to speak? Arnold gave some good references, but when you actually read them it's all fairy tales and mathematical handwaving that totally ignores what a gravitational field actually is. And then his "dry facts" add up to electrons and positrons pop into existence spontaneously like worms from mud. It's just not enough.

I'd say no permission to speak

7:32 PM

@Slereah Well that was unexpected...

nhs.uk/Conditions/Burns-and-scalds/Pages/Treatment.aspx

@barrycarter "Ultimately, consider what happens to a given point on a string as the wave passes." <----------- it is easy to say but hard to do!

@DeNiSkA Yes, that's why I said it :P OK, focus on a point on the string that's 5 inches from the left end of the string. Now, as the wave hits it, what is its y position (this actually isn't easy, but since I'm not doing the math, it's fun)

@DeNiSkA Perhaps more specifically, what's the velocity of that specific point on the string.

@DeNiSkA I might mean acceleration (force = tension)

@barrycarter this one i need to think

@DeNiSkA Yes, I like to delegate my thinking to others.

ahm ahm...

7:38 PM

@JohnRennie Come back, I have so much love to give.

@DeNiSkA You know two things about a given point on the string: 1) it goes from y=0 to y=1 and back to y=0, 2) unless the string is stretchy, a given point remains a constant distance from one end of the string.

I love giving hints that imply I know the answer, when I don't.

haha :|

@barrycarter this doesn't hellp

@DeNiSkA Can you write a formula for the x,y position of a given bit of string as the wave passes?

$y=\dfrac{\sqrt{8}}{\pi}\cos(\pi x+ \pi t)$

this is the actual question

4

Q: Transverse simple harmonic wave travelling in a string

This question came in my exam. My Attempt: I thought that the tension will not vary in the string because since we know that velocity of wave in a string is given by $$v=(T/m)^{1/2} $$ where $v$ is the velocity of the progressive wave in string and $T$ is tension in the string and $m$ is ma...

homework-and-exercises waves velocity harmonic-oscillator string

@DeNiSkA Oh, OK, that's probably better than what I was saying. I meant x(t) and y(t) not y(x)

@DeNiSkA Oh shiny, I might've figured it out. You know that x(t)^2 + y(t)^2 = 5^2 for a point 5 inches from the left end of the string, right?

So anyway

You folks remember that answer by a rando dude on tachyonic fields?

7:45 PM

@barrycarter hmm! 5?

He said that the behaviour of a PDE with respect to causality depended on its principal part

@DeNiSkA I just picked that number at quasi-random.

Does anyone know what theorem that is

@barrycarter oh! then OK

@Slereah I think that was ValterMoretti in a comment to an answer by Qmechanic, not some rando dude :P

7:46 PM

@DeNiSkA And you know that y = sin(x) [or some variation] as the wave passes, right?

ya

We all are a rando dude, @ACuriousMind

And no, I don't know any PDE theorems

@Slereah I might be a rad dude, but I'm certainly not rando!

@DeNiSkA So you can now find x(t) and y(t), compute the change in velocity and thus the acceleration, which is proportional to force.

why don't we have a PDE expert on the chat

Can't we hire people

a retinue of chat people

7:48 PM

Well, I think @yuggib knows a thing or two about PDEs...

@barrycarter i have to test this out on pen+paper

@DeNiSkA I thought that's what you were doing?

microsoft paint is a beautiful application

2

That was just a surreal comment.

Is that theorem one of the two things he knows

7:50 PM

hehe

i am plotting on wolfram

@DeNiSkA Whom are you plotting against?

0

A: Question/Doubt about Time Dilation Symmetry in Special Relativity

This may or may not be helpful. The diagram above shows the following in the planet frame: The solid blue line is Earth, which remains 8 light years distant from the planet at all times. I've drawn dots for t=-8 to t=10. The solid green line is the planet, which remains at x=0 at all t...

Could someone look over that answer?

@barrycarter i am plotting against time

2

Damn, that was pretty clever.

@ACuriousMind Hello dear ACuriousMind. May I ask you a request?

ooh, can I get this!

@lucas Yes, you can ask a request.

8:06 PM

@NeuroFuzzy Are you ACriousMind?

@lucas Did you mean: "May I send a request"?

@barrycarter My English isn't good.

@lucas I think @NeuroFuzzy was indicating that, most likely, you have the cognitive and physical ability to send a request.

@lucas In that case, I'll stop.

@lucas I generally not see any need to ask if you can ask a question. Just ask!

I was going to ask if it was necessary to ask if you could ask, but I guess it isn't.

Sᴋᴜʟʟ ᴘᴇᴛʀᴏʟ

8:09 PM

Ask a way!

@ACuriousMind May I ask you to see my question and its answer and comments and then judge about?

0

A: Injectivity of Physical Functions

Since David Z wants us to write a trivial comment's worth of a remark as an answer: 1) Plenty of such functions exist. The radial distance of a planet as a function of angle is one. In general all periodic functions are non-injective. 2) No, since it's not true. I am not even sure how one woul...

@lucas Are you sure you don't mean just a function instead of an injective function? For example, many things have a length of 5 meters.

@lucas Uh, no. I already "judge" (i.e. vote and comment on) all questions that I see where I want to do so.

However, nothing has a length of both 5 meters and 6 meters at the same time.

@barrycarter Yes many things have same length. But are those things functions of their length?

8:13 PM

@lucas Let's use one from your example. Many materials have the same resistance, right?

@barrycarter I don't know.

@lucas OK, would you agree many different systems can have the same internal energy?

@barrycarter yes

@lucas So, the function converting systems to internal energy isn't injective.

You can have two systems with the same amount of energy.

@barrycarter What you say isn't a physical function. It is a mathematical function.

8:16 PM

@lucas How do you define "physical function"?

Easier example of non-injective function in physics: The motion of a vibrating string. Since it (the position of one point on the string as a function of time) is periodic in time, it cannot be injective.

@ACuriousMind Please see comments below the answer.

I have no idea where you see the difference between being "determined" by time and being a "function" of time.

@lucas Functions, like sets and relations, are inherently mathematical constructs.

@barrycarter For example, when we say he is a donkey, our sentence is correct grammatically, but we know any human is donkey.

8:19 PM

The position is a function of time - if I feed it a certain time as input, it spits out a certain position as output. That's what a function is.

@barrycarter $$\ddot y= -\sqrt{8} \pi \cos(\pi x+ \pi t)$$
and this equation tells me at yellow point tension is maximum and at orange it is zero!
which is totally opposite to the correct answer (*i think i am going wrong somewhere*)
picture............

user image

@DeNiSkA I wasn't suggesting you use the string equation from the question you quoted. I was suggesting you derive it from first principles.

@lucas OK, but in this case we're talking about a function that maps a system to its internal energy.

@barrycarter huh! i tried your equation still weird same result

@ACuriousMind We measure the velocity by measuring distance and time. But the velocity isn't a function of distance and time.

@DeNiSkA There should be no pi's and weird stuff from my equation. Show me what you did?

@lucas v(s,t) = s/t (so yes, it is)

8:22 PM

@barrycarter Only for linear motion

@ACuriousMind May I ask another question? May you explain this sentence for me: "Since David Z wants us to write a trivial comment's worth of a remark as an answer"

@ACuriousMind I was speaking infinitesimally :)

@ACuriousMind Did he flout me?

42 mins ago, by barrycarter

@DeNiSkA Can you write a formula for the x,y position of a given bit of string as the wave passes?

@lucas I'm not sure what you're saying. Whenever I have an expression a = b*c/d, a is a function of b,c and d. So when I write x = sin(wt), I have written the position as a function of time (and angular frequency), and as a function of time, it is not injective.

8:24 PM

@barrycarter hey but x is not a function of time

@ACuriousMind Is position depend on time?

Yes

That what solving equations of motion is all about - getting the position as a function of time.

@ACuriousMind No. It is measured by time.

What?

@ACuriousMind It is a mathematical function of time not physical.

8:26 PM

There is no such thing as a "physical function"

A function is a purely mathematical object

@ACuriousMind Anyway, please tell me if he flouted me.

@DeNiSkA I am considering the flat string as the x axis, so any point on the string has an x value, yes?

yes

@DeNiSkA And this x value changes as the wave passes, right?

@ACuriousMind Don't you want to answer me?

8:29 PM

@barrycarter for wave it changes but for particle on the string it is same

@lucas What? I answered you - there is no such thing as a "physical function", and I have no idea what you are talking about.

@DeNiSkA OK, consider the part of the string that is 5 inches from the left end. It's initial x value is 5, right?

@ACuriousMind Please read my question's answer and tell me if he flouted me in his first sentence?

OK

@ACuriousMind My English isn't good.

8:30 PM

@DeNiSkA As the wave starts passing, that points y value increases, correct?

100%

and after sometime it decreases

@DeNiSkA But the distance from the origin remains at 5, right?

yes

@DeNiSkA So there's no way the y value could change without the x value changing too

Unless the string stretches

@ACuriousMind OK. Thank you very much because of your attention. Good luck.

8:33 PM

x(t)^2 + y(t)^2 = 5^2 and y(t) = sin(x(t)) for example

@lucas I'm not sure what you mean by "flout" ("flout" generally means to show disregard and contempt for rules)

He anti-flouted you.

@barrycarter but that stretching $\to 0$

@DeNiSkA I assumed this was a rigid string?

The first sentence is trying to say that he considers the answer to your question so blatantly obvious that it is not worth writing an answer.

8:33 PM

@ACuriousMind abuse

OK

@lucas I would not call it "abuse" to say that a question is trivial.

Especially when it is :P

@DeNiSkA It's a little counter-intuitive, but if a given piece of string's y value increases, it's x value has to decrease to compensate.

@ACuriousMind May I ask another question?

Only if you from now on ask your questions directly instead of asking whether you can ask them.

8:36 PM

@barrycarter haha! sounds really bad

@DeNiSkA Why?

@ACuriousMind May you tell me your opinion about me?

@barrycarter because no such thing happens

@DeNiSkA Such a thing must happen. The string gets "jerked back" by the wave.

@lucas I don't know you well enough to have an opinion about you

8:38 PM

@ACuriousMind You saw my question and chat me for a while.

@ACuriousMind What do you think about me?

why are you taking string and wave separate things
string = wave and vice versa

@lucas Well, all I can say from that is that you seem confused about something concerning functions, but that I don't know exactly what.

@DeNiSkA Not exactly, though. OK, if you have a piece of rope/string handy, mark one spot on the rope and then pass a wave through it. Notice the spot goes closer to the origin briefly.

@DeNiSkA I'm talking about a single fixed point on the string, which is a little different from the string as a whole.

@ACuriousMind What do you think about my age, knowledge, etc?

@barrycarter "Notice the spot goes closer to the origin briefly" <------- for this i need a super slow motion camera

8:40 PM

@lucas Uh, I have no idea how old you are or what you know?

@DeNiSkA Not really, you could pass the wave slowly. I should say that the spot gets closer in x value to the origin.

@ACuriousMind Please tell me what do you guess?

@lucas May I guess?

@barrycarter Yes.

@lucas I would guess that you are fairly young and just learning physics, and having difficulty communicating because your English skills are (admittedly) not that great.

8:43 PM

@barrycarter What means fairly young?

@lucas 14 or 15, maybe. Early high school?

@barrycarter Thank you dear barrycarter.

@lucas Well, tell us?

@barrycarter May I ask another question?

@lucas Stop doing that, it's annoying. But yes.

8:45 PM

@barrycarter Do you neglect me because of your approach about me? Do you think my question is stupid because I am a high school student?

@lucas I actually didn't see your question and don't have an opinion about whether it's stupid. In general, I used your comments to determine your age, not vice versa.

@lucas However, do keep in mind that homework questions are frowned upon here, so you might try another place to ask.

user image

@barrycarter This isn't my homework. I am not a student. I am near 30 years old.

@DeNiSkA Well, OK, I was suggesting you mark a spot in the middle of the string, not at one end.

@lucas OK, how much physics have you studied?

@barrycarter i think this pic will show you that x is not decreasing

8:49 PM

@DeNiSkA Actually, when the wave runs off the end of the string, the other end of the string will have a decrease in x, since its length can't change.

@barrycarter I am a graduate of mechanic engineering.

(pines briefly for Ocelot)

@lucas Well, are their books in your own language about physics. Engineering really isn't physics.

@barrycarter What I studied is a lot, but what I understood is a little bit.

@barrycarter but that is $very^{very^{very^{very^{.....}}}}$ negligible

@lucas Hmmm. See, the issue here is that you speak a different language, and don't appear to know much about physics, so the questions you ask could be both difficult to read and not make much sense. That's probably why we won't be able to help you much.

@DeNiSkA Umm, no. Because x[t]^2 + y[t]^2 is fixed, remember?

@DeNiSkA Any point on the string is constrained to a fixed distance from the origin.

8:53 PM

@barrycarter oh

@DeNiSkA Does that make sense now?

ya

I should probably leave while I have one victory to my name :)

:D

@barrycarter I don't need my questions answers. I left the science since 7 years ago. I want to help the others. My questions are useful for thinking people, but they bother people only have studied and don't like thinking.

8:56 PM

@lucas You want to help others by asking interesting questions?

@barrycarter Interesting and forgotten and maybe useful.

@lucas But you don't really want answers to the questions you ask?

(did anyone else just get bumped for a fraction of a second?)

@barrycarter What do you mean by "getting bumped"?

@ACuriousMind a message saying that I had to log in to post. with edit/retry/something

@barrycarter If anyone answer them, I like to see the answer, but I don't need.

8:59 PM

@lucas Are you asking the questions to get answers?

@barrycarter No. Maybe I want to give a start point for interesting people.

@lucas Then you probably shouldn't be posting here. Stackexchange is for people to ask questions that have definitive answers.

@lucas Consider writing a blog or visiting other sites like quora.com which encourage this sort of thing.

@lucas That is a very strange thing to say, especially given that the one question I've seen from you did not make much sense.

You can't just declare that other people should find your questions interesting. You'll have to let them decide from themselves.

@barrycarter If I leave this web site, do you get happy?

@lucas Maybe a little bit, but the point is that you'll find what you're looking for.

9:03 PM

hullo,
$F=ma$ this equation doesn't give any physical meaning similarly $OPL=\mu x$ (where $\mu$ is refractive index and $x$ is path length) doesn't give any physical meaning or sense! i want to know what does optical path length acutally mean?

@ramsay For F=ma, you could argue that's the definition of force.

f=ma i know what it means, but i need OPL=$\mu x$

@ramsay Are your two questions related? How does force affect optical path length?

no they both are not related

@ramsay OPL means the distance light travels.

9:05 PM

@barrycarter Thank you very much because of your time and attention. My questions are finished. Do you have any question?

@lucas No, not really. I wish you the best of luck.

@barrycarter if it means distance light travels then it should be a simple $x$ but why we write $\mu x$

@barrycarter Thanks.

@ramsay Because light travels slower outside of a vacuum.

so

9:07 PM

@ramsay So it travels less distance in a given amount of time.

news.harvard.edu/gazette/1999/02.18/light.html

OK, that didn't convert, but it basically shows you can slow light down to 38 mph or so.

@ramsay OK, clarify what you're asking?

ok sir,
tell me how to do this :
in young's double slit experiment if a thin sheet of thickness t and RI $\mu$ is introduced find change in optical path

@ramsay OK, I have no idea. Maybe @ACuriousMind will help.

Off for a bit.

@ACuriousMind will you?

@ramsay Not right now, sorry

ok i will wait

2 hours later…

Alfred Centauri

11:40 PM

@lucas "I want to help the others" - It's been my observation that those that genuinely help others simply do it and that the phrase "I want to help the others" is almost always used by those with something else in mind.

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Apr '1626

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