Hello.

sup

is that question on-topic?

IMHO, no, it's off-topic

There is a tag called "specific-reference"

so I am still wondering

physics.stackexchange.com/review/suggested-edits/162846

> Comment: fixed grammer, corrected spelling & improved formatting

oh the irony

@ACuriousMind Someone wearing a shirt "I came, I saw, I ate"

"GREEK fest 2016"

How does that even happen

@AccidentalFourierTransform That's an easy-to-make typo, pretty sure I've been a grammer nazi myself :P

@0celo7 wat

What's confusing?

EVERYTHING

Ah, well.

You need to ask more specific questions.

2 downvotes in 20 seconds

That was fast.

@ACuriousMind I reckon I can generalize its results, wanted to see if someone had scooped me =P

@YashasSamaga Hm, well... the thing is, specific-reference is for the type of questions where a perfect answer would include a journal reference. In this case, the OP already has a journal reference. There's kind of nothing left to ask for - at least, nothing we could provide which the OP hasn't already found.

Oddly enough if the question were made less precise, e.g. "I'm looking for an old paper by P. C. Peters about different speeds of light", it might be on topic...?

I'm still not sure.

@help There the liquid is considered to be a body. And the force due to other liquid particles is already taken into consideration by using the formula for pressure at a depth h from surface.

well, IMHO if a question could be answered by someone with good google skills and no knowledge of physics, the question is off-topic

@EmilioPisanty Looks like you're safe ;)

@YashasSamaga so if i contract a spring my energy would be converted into elastic potential energy

?

could someone simplify this for me please: $$\frac{\left(\frac12n\right)!}{\sqrt{\pi}}$$

@MartianCactus yes

and when i let it go, that elastic PE will be converted into kinetic energy and heat

yes

then what will that kinetic energy be converted into?

heat

if you have an ideal spring

and what will that heat convert into?

it will keep oscillating

more kinetic energy?

noo

@MartianCactus Heat is lost into atmosphere

@MartianCactus This question cannot be answered.

how can energy be lost?

wouldnt that mean energy is destroyed?

it isn't lost

@MartianCactus Not in that sense. I mean it just goes/flows into the atmosphere.

no

oh

it would increase the temperature of the atmosphere

and that heat energy will stay in the atmosphere

needn't be

a windmill might convert it into electricity

oh

so the heat energy might be converted to the flow of wind

and that flow of wind into electricity

kinda

theres no end to it

flow of wind is also heat in some way

yea there is no end to it

but if you go deeper, theres only kinetic and PE

flow of wind is kinetic energy

heat is kinetic enrgy

so when the pebble hits the ground

that kinetic energy becomes elastic potential energy for a while

hey

and then it bounces back off as kinetic energy

oh do I have enough reputation to chat then?

what does pebble has to do with the heat in the air?

@MartianCactus Kinetic energy and potential energy are only book keeping tools which sum up to the total energy. Total energy also includes internal energy of bodies.

@user35276 you are chatting :/

and each time it hits the ground heat and sound energy is emitted

@user35276 Yes, chat is unlocked with 20 rep.

which goes into the atmosphere

lol I know, I didn't realise I had rep

there IS no end

strictly speaking, there is an end

internal energy?

entropy keeps increasing when energy transfers happen

@user35276 Huh? You didn't know you have 26 rep on space.SE? Chat reputation is counted network-wide, you can chat in physics.SE's chats even if you don't have a physics.SE account at all, for instance

@MartianCactus I don't think you need to know the details

isnt entropy the amount of disorder of energy?

disorder of energy? I'm not sure about that

but entropy is indeed a measure of disorderliness

can someone help me undersand why i am under the impression that when taking a cross product and we use a 3 x 3 matrix to calculate determinants that the middle vector column.. or usually y.. that it counts as a negative.. am i imagining this because it doesn't seem to be consistent everywhere..

@user35276 That is how you calculate a determinant of a matrix.

yes and the middle column always comes out with a negative contribution yes?

not always

@user35276 Because you are using a right handed coordinate system which follows some specific rules.

you have the negative sign outside

your inside value could be negative

in that case, they will cancel out to yield a positive quantity

i have an example here F = qv x B

and the the derivative of the y velocity component gives a negative velocity of the x component

what does the derivative got to do with the force equation (called the Lorentz equation if you had electric field considered along with the magnetic field)

oh its an example where E = 0 and F = mdv/dt = qv x B

@user35276 That makes no sense

There are rules to be followed when you differentiate vector products

@anonymous what is wrong with it?

so dv_y/dt = q/m[v x B]_x = [qBv_y]/m

@YashasSamaga Derivative of a velocity cannot give a velocity

so dv_y/dt = q/m[v x B]_x = -[qBv_x]/m

F = ma = mdv/dt is what he wrote

@YashasSamaga I am talking about the previous sentence he/she wrote

"and the the derivative of the y velocity component gives a negative velocity of the x component"

sorry.. that is the negative I'm asking about

@anonymous that is possible in periodic motion :P derivate of one component of the velocity gives the other

yes sorry i was trying to explain that equation

usually the sign is altered too

@YashasSamaga Derivative w.r.t what?

time

so i was asking that the negative is just coming from the fact that it is the middle column we are doing the determinant from

of e_y

@YashasSamaga Give me an example. I can't understand which situation you are talking about

but it just seems that this practice doesn't apply in every case.. sometimes i would make my y direction of the determinant negative but it was wrong

This : dv_y/dt = q/m[v x B]_x = -[qBv_x]/m ?

yes that equation there.. its the negative at the end that I'm asking about

i understand taking the determinant and i was aware of this practice of making the middle component a negative determinant, but i don't know why we do this and when we do this as it doesn't seem to be done in every case

i might call my tutor over the phone, i do distance learning but ive never called him before lol

Just use the product rule of differentiation. $$\frac{d}{dt}(\mathbf{u} \times \mathbf{v}) = \frac{d\mathbf{u}}{dt} \times \mathbf{v} + \mathbf{u} \times \frac{d\mathbf{v}}{dt}$$

i will call my tutor but thanks so much for helping, i didn't expect to get help from chatters, glad h bar is a good place :]

Good luck

It is easier than you think :)

Hi, I would like to ask for a bit of help, regarding plots of x-rays. I am not sure how on-topic it is, but I can't find any information on it. I have a wavelength-intensity plot of x-rays and I would like to "remove" the characteristic radiation from the curve. What program could I use for this?

@anonymous $\vec{v} = sin(t)\hat{i} + cos(t)\hat{j}$

\hat i and \hat j

oops, thanks

@YashasSamaga What about it? Derivative of that gives accn

@YashasSamaga "entropy keeps increasing when energy transfers happen" what do you mean by that?

@anonymous a case where derivative of one component of velocity gives the other (with the sign altered)

@MartianCactus strictly speaking, what I said is not right

entropy can decrease during an energy transfer

but the net entropy always increases

@YashasSamaga Hmm. The derivative one gives the other numerically. However what that actually gives is $a_x$ rather than $v_y$. Take care of the units.

and how does entropy increase?

bear with me im still in high school

$dS = \frac{dq}{T}$

@YashasSamaga hi/ interesting, can you cite that paper re SE voting patterns plz

@vzn it wasn't about SE voting patterns, I was talking about a research paper which took samples of blog post and news articles

they termed it "irrational herd behaviour"

@MartianCactus if you don't have the mathjax plugin enabled, I meant dS = dq/T

where dS is the differential change in entropy

dq is the differential amount of heat change

and T is the temperature

i do have it

@YashasSamaga ok still interested in the citation. the effect is already well studied wrt hyperlinking to "popular" sites... not really a previously unknown phenomenon & maybe not really accurate to call it "irrational" although definitely similar to herd behavior

oh

@vzn John Bohannon,“Why You Shouldn’t Trust Internet Comments.”©2013 by American Association for the Advancement of Science.

what is "the differential amount of heat change"

Commentsthatreceivedfakepositivevotesfromthe researcherswere32%morelikelytoreceivemore positivevotescomparedwithacontrol,theteam reports.Andthosecommentswerenomorelikely thanthecontroltobedown-votedbythenextviewer toseethem.Bytheendofthestudy,positively manipulatedcommentsgotanoverallboostofabout 25%.However,thesamedidnotholdtruefor negativemanipulation.Theratingsofcommentsthat gotafakedownvotewereusuallynegatedbyanup votebythenextusertoseethem.

copy paste doesn't work huh

wat ^

@YashasSamaga thx. fyi wrt hyperlinking the phenomenon is known as "preferential linking" or "preferential attachment" & there are some physics-like models etc en.wikipedia.org/wiki/Preferential_attachment suspect/ seems likely to me that voting follows similar patterns etc; as others have pointed out it shows up on SE in the "hot network questions"...

Comments that received fake positive votes from the researchers were 32% more likely to receive more positive votes compared with a control, the team reports. And those comments were no more likely than the control to be down-voted by the next viewer to see them. By the end of the study, positively manipulated comments got an overall boost of about 25%.

However,the same did not hold true for negative manipulation.The ratings of comments that got a fake downvote were usually negated by an up vote by the next user to see them.

I added the spaces now

@MartianCactus differential means a tiny amount

infinitesimally small amount

If you haven't started calculus yet, then what I wrote is approximately equal to

delta S = delta q / T

@ACuriousMind How would one go about proving that the "high school" definition of Riemann integral in terms of mesh size $\to 0$ agrees with the usual one in terms of Darbeaux sums?

adifference in heat / temperature..

so there will be more difference in heat

@0celo7 I have no idea what definition you mean "in terms of mesh size"

as theres more heat being added

that divided by teperature

oh cool

I also don't know what a Darbeaux sum is, I define Riemann integrals in terms of Riemann sums

@ACuriousMind $\int f =\lim_{\Delta x\to 0} \sum f(c_i)\Delta x_i$

@ACuriousMind Pleb

but you said it would have an end

*Darboux

about what we talked about the pebble

That's the "mesh size" definition

Suitably generalized to rectanges in $\Bbb R^n$, ofc.

@0celo7 What up dawg

then how is increase in entropy = ending of energy transfer?

@ACuriousMind I would like to report mod abuse

$\Delta x_i$ is then replaced by the measure of the subrectange

and the "mesh size" is the largest diameter in the partition

@YashasSamaga i will be back in some time so write the answer in chat plaese and then i will scroll up

thanks

@ACuriousMind You actually define the Riemann integral using tagged partitions and not upper/lower sums?

@MartianCactus en.wikipedia.org/wiki/Heat_death_of_the_universe

@0celo7 Okay, your "mesh size" is what I call a Riemann sum.

@ACuriousMind My god, that definition is terrible. There's a reason why all modern analysts don't use it

Can I see your lecture notes from analysis?

I've never seen Riemann sums treated properly

@0celo7 No, because a) that was before I had a tablet and b) we didn't really do Riemann integrals as far as I recall. I also recall some upper/lower sum business from school, but if you're looking to converse at your usual nitpicky standard about Riemann integrals, I'm not the guy to talk to.

"usual nitpicky standard"

What does that even mean?

@0celo7 He's calling you a twat

90% certain

@ACuriousMind Are you calling me a cunt?

@BernardoMeurer The standard procedure is to either raise a custom flag or contact SE staff, but what's the matter?

@ACuriousMind A PSE moderator came to my house and broke my wardrobe door. I have substantial proof

@ArtOfCode Where were you during the time this crime occurred?

@0celo7 No, I'm saying my knowledge about rigorous Riemann integrals is no up for the close scrutiny you tend to take to everything.

"the close scrutiny to tend to take to everything"?

@BernardoMeurer Contact your local law enforcement agencies.

@dmckee Social science?

He also drank some of my beer

@BernardoMeurer I know your handwriting you know.

And ACM's.

@0celo7 So you know that's not my handwriting

It looks like it tbh

@ACuriousMind I do not take close scrutiny to everything.

That's such a broad claim.

The nice thing about it is that it's self-demonstrating - you're right now scrutinizing it ;P

@ACuriousMind $S A V A G E$

@BernardoMeurer good luck in analysis 2

@0celo7 Hm?

I mean, I will need it

I'm not helping

That would require me to scrutinize things

@0celo7 I never complained about your scrutiny

I appreciate it, it's a quality imo

ACM doesn't like it and I want him to like me

@ACuriousMind Like him

isn't that question more fit for chem SE?

@YashasSamaga If you think so, flag and move on.

I don't want a declined flag lol

Flags aren't a game you're trying to win, they're how you help keeping the site in order.

I won the flag game and the mods told me to stop :(

I got 6 declined flags in one day :d

3 of which were because the person edited and fixed the question

too late to retract

so now I have started commenting before flagging

@YashasSamaga in which case it's not a problem? Disputed flags are a bit more of an issue.

But yeah, just flag if you think it needs special handling. If you get a specific message or multiple declined/disputed flags of the same type, then change up a bit, but don't let one or two stop you.

I just don't want to get my flaging rights removed (even if it is temporary)

@EmilioPisanty Huh? No, it's the declined ones that count towards the autoflag ban. He's not talking about VLQ flags being disputed because someone edited, but close flags being declined because the close review completed with 3:0 Leave Open.

@ACuriousMind yeah, that one's a problem

I guess

dunno if an intervening edit gets counted

@EmilioPisanty I have a job interview tomorrow to be a baby at a lab. Tips?

Wear makeup

I'm not sure if my official title is "baby" but I'm fairly certain that's what the job entitles

@BernardoMeurer uh.... don't act like a fool?

@EmilioPisanty Okay, I'm not getting this job

@0celo7 I always do

@BernardoMeurer "baby" is the term they use in front of you

as likely as not, the internal term they use is "slave"

(speaking from experience)

Lol, yes, exactly

If you're getting paid, you're not a slave

no, seriously, if you're an undergrad then they're mostly looking at your attitude

@0celo7 Ancient Roman and Greek slaves were paid.

@ACuriousMind then they weren't slaves

show you're eager to learn and serious about what you do

pretty much just that, I think

as far as essentials go

and no beer

unless they were legally owned

I don't think @BernardoMeurer is owned by the lab

@EmilioPisanty Sounds solid, thanks

@0celo7 Not yet.

@0celo7 I was born in a lab

@AccidentalFourierTransform We both know that's impossible

@BernardoMeurer this an actual lab, or a theory "lab"?

That would explain everything

if it's an actual lab, some amount of not-tripping-and-ruining-everything also comes handy

@EmilioPisanty An actual lab, they do research on microprocessor architecture and methods for efficient multithreading

Also a lot of using FPGA's as coprocessors for media decoding

like HEVC and stuff

Sounds like GDP

@BernardoMeurer as in clean-room we-do-our-own-lithography lab?

or just a bunch of guys with computers?

@EmilioPisanty No clue, haven't been to the lab yet, I don't have the needed credentials to enter it

Geez

It's in a building of it's own

There are more research groups there though

@BernardoMeurer so yeah, if there's experimental equipment, don't knock over anything expensive

i.e. don't knock over anything, just to be safe

@EmilioPisanty So I can't get drunk and use the turbomolecular pump?

@BernardoMeurer do you have steady hands?

@0celo7 Yes

@BernardoMeurer would advise against at the job-interview stage

@EmilioPisanty Will do my best :P

@EmilioPisanty being sober is hard for him

but yeah, just keep your wits about you, (do) ask smart questions, and keep it cool

@0celo7 It's the real struggle

@EmilioPisanty Do I wear my Einstein socks or my Windows 95 Icons sock?

These are GDP people, they don't care about Einstein or his Evidence

Dude, Killer Mike is really fat oh my god

All Americans are really fat

@ACuriousMind are people actually upset about the national anthem thing?

@0celo7 What national anthem thing?

At the tennis match

Someone played the first stanza

Where?

a tennis match

here

Ah. Well, that's pretty much confusing present-day Germany for Nazi Germany so...I guess that is something people get upset about, yes.