The h Bar

Pᴀᴜʟsᴛᴇʀ2

00:09

0

Q: Would adding dimples to a car reduce drag?

This golf ball is dimpled to reduce drag and increase range. Would adding dimples to a race car perform better?

Question for Physics.SE?

tpg2114

@Pᴀᴜʟsᴛᴇʀ2 It's really more of an engineering question, so not really. Plus, that user is currently suspended here.

Pᴀᴜʟsᴛᴇʀ2

@tpg2114 - LOL (on the suspension). Yah, I know Muze.

I KNOW EE.SE wouldn't take it, lol

Thanks ... I'll close it down on Mech and leave it that way.

tpg2114

Engineering.SE might take it

I don't know their scope terribly well though, I am not active there

Although, looks like that user is suspended on Engineering.SE also.

So... maybe just treat it on your site as needed and don't worry about migrating

Pᴀᴜʟsᴛᴇʀ2

@tpg2114 - Oh, Engineering ... still, I'm not fond of the question. It really serves no purpose other than it's Muze.

Yah, already on hold.

TY!

tpg2114

No worries! Thanks for checking!

danielunderwood

00:17

So where is the line between physics SE and engineering SE? When a question comes down more to numerical values than concepts?

tpg2114

@danielunderwood That's a... good questions. We've talked about it quite a bit on meta.

More recent examples of the conversation:

4

Q: What is engineering and what is experimental design?

So we have a close reason for engineering questions because selection of materials for building that ultralight in the garage are off-topic. But where is the line drawn between engineering and experimental design? The question I'm thinking of specifically is: Pressure applied to flat plate With...

discussion close-reasons

4

Q: Math and engineering questions: where do we draw the line?

I was reading through some old meta discussions recently, and I was interested in some of the points made, but they never quite seemed to be implemented or discussed further. They also seemed to be buried among the various conversations about physics overflow and the various controversies surroun...

discussion scope questions

4

Q: Engineering, what does Phys.SE deem as not Physics?

Right so I posted a question about something (not really important because the response is the important part). One of my responses (who I believe flagged the article) complained that it was not Physics because it was engineering. Now I know a few engineers and there is no doubt that what they ...

discussion scope questions specific-question

Long story short -- I think it's hard to really say what is or is not acceptable as engineering, it's kind of a "I know it if I see it." For instance, asking why dimples reduce drag is on-topic. Asking if dimples will reduce drag on a race car, off-topic. Many "engineering" questions can be rewritten to focus on physical principles instead

I guess our FAQ on it is here:

19

Q: Are engineering questions appropriate for this site?

Often we get questions of the form "How to build this particular device?" or "Why was this design implemented in such-and-such product?" Are these questions appropriate for Physics Stackexchange?

discussion faq scope

danielunderwood

00:32

Ahh interesting...I've seen a few questions that seemed to lean on the engineering side in the past, but never thought to flag them

Although in the flag options we have engineering as an off-topic reason, but the only options I have for moving are Math.SE and Physics Meta...or maybe that's just related to my rep on certain sites

> This question appears to be about engineering, which is the application of scientific knowledge to construct a solution to solve a specific problem. As such, it is off topic for this site, which deals with the science, whether theoretical or experimental, of how the natural world works. For more information, see this meta post.

tpg2114

Yeah -- we don't have a migration path to Engineering.SE at the moment

It's not great to flood beta sites with migration questions. But, we can move things there if the person asking the question wants us to. Or, they can go ask their question there (even better)

danielunderwood

Ahh I didn't realize engineering was in beta...there are so many beta sites around

tpg2114

I probably should participate more over there, since I am actually an engineer... but I like to pretend I am a scientist instead.

vzn

03:17

@bolbteppa !!! very nice/ substantial find(s). surely you must concede these new refs (thx! much!) use nearly the same language/ math ideas as Tenev + Horstemeyer. and dont see the language of strings. and am noting the use of dimensional analysis. etc!

oh! look at that! lol! 2nd ref Izabel cites Tenev + Horstemyer [5]

VARUN.N RAO

04:22

Can there be a self sufficient standing wave in a three dimensional medium. Like can there be a circular self protruding wave or whirl at the center of the dimensional medium??

If not what properties of the medium would help it?

thermomagnetic condensed boson

11:31

can any moderator tell me why this question physics.stackexchange.com/questions/483733/… is not a duplicate of this one? physics.stackexchange.com/questions/54975/…

I mentioned it, but my comment was totally ignored (no upvote, no comment), while kzhou's suggestion for another duplicate received 3 votes but it isn't exactly the same question. While in my case, it is exactly the same question. Just curious, what's going on?

ACuriousMind

@thermomagneticcondensedboson 1. The close vote review has not been completed, so it's too early to say whether people think it is or is not a duplicate. 2. The auto-generated "Possible duplicate" comments generated when one casts a dupe close vote gets upvotes when another close vote is cast for the same duplicate target. Since voters can only choose a single dupe target, it's to be expected that one of these comments will get more upvotes than another.

thermomagnetic condensed boson

12:38

I see, thanks @ACuriousMind. It's now 4 votes to 0, and kzhou's link contains only 2 short answers (with low votes) compared to my link (many more answers, for over 80 upvotes). Still, it looks like as if nobody had checked my link. The question is a duplicate, regardless of opinions of reviewers.

And what's more, kzhou's link points to a duplicate (of my link!) because it was posted on august 2013, vs Feb. 2013. So I'll report his link as a duplicate :)

user280247

14:52

for pure substances there is a state function following f(p,v,t,m)=0

user280247

for closed systems, internal energy is written: U(T,V)

user280247

why isnt it a function of pressure?

ayc

15:24

Guys I just finished high school and I wish to study Griffiths introduction to quantum mechanics.Is it necessary for me to read griffiths introduction to elecrodynamics to understand his q.mech book? Can I start directly his Q.Mech book?.....I have AP level knowledge in Physics and math

@JohnRennie Help!

John Rennie

@ayc I've got Griffiths' book somewhere but I've never read it so I can't really comment. However I'd be surprised if you needed to learn electrodynamics first.

In your place I'd give it a try and see how I got on.

ayc

@JohnRennie SO,I wont have any big problem directly going for griffiths intro to quantum mech?

John Rennie

@ayc I would guess you'll cope fine with it.

ayc

@JohnRennie Thank you for your help..I'll jump right in then!

knzhou

16:03

@thermomagneticcondensedboson I just remembered we've had similar questions a million times and searched for one with a similar title.

Stuff like EPR always has piles of candidate duplicates. You got less votes just because you happened to arrive later, that's all.

In any case now both are marked, so it's fine.

Sir Cumference

16:44

@ayc A linear algebra background and some knowledge of probability would probably be important beforehand

dmckee

17:46

@JohnRennie Most undergrad quantum books assume that you've had the introductory E&M that is usually done in the big-tome 1 year intro course. I've never worked through Griffith's QM in toto, but I don't recall needed upper-division E&M for it.

@danielunderwood Keep in mind that experimental physics is explicitly on-topic. We've had a little trouble in the past with people who are heavily focuses on what I would call "black-board theory" not recognizing experimental questions for what they are.

dsm

18:13

wandering my way into QFT now, and confusing myself when interpreting the "conventional" many body normalization. to begin with, how should I think about the following:

$$\langle\mathbf{r_1},\mathbf{r_2}|\mathbf{r'_1},\mathbf{r'_2}\rangle = \delta(\mathbf{r_1}-\mathbf{r'_1})\delta(\mathbf{r_2}-\mathbf{r'_2})\pm\delta(\mathbf{r_1}-\mathbf{r'_2})\delta(\mathbf{r_2}-\mathbf{r'_1})$$

if I can get a feel for the two body one, I suspect it won't be too much of a leap to understand the following for the n-body normalization:

$$\langle\mathbf{r}_1\cdots\mathbf{r}_n|\mathbf{r'}_1\cdots\mathbf{r'}_n\rangle = \sum_{\text{all perms}}(\pm 1)^P\prod_{a=1}^{n}\delta(\mathbf{r}_a-\mathbf{r'}_{\Pi(a)})$$

jacob1729

what decides between $\pm$ in those formulas @dsm

dsm

oh, right, + for bosons and - for fermions

jacob1729

Are those just slater determinants then?

Slater determinant

In quantum mechanics, a Slater determinant is an expression that describes the wave function of a multi-fermionic system that satisfies anti-symmetry requirements, and consequently the Pauli principle, by changing sign upon exchange of two electrons (or other fermions). Only a small subset of all possible fermionic wave functions can be written as a single Slater determinant, but those form an important and useful subset because of their simplicity. The Slater determinant arises from the consideration of a wave function for a collection of electrons, each with a wave function known as the spin...

dsm

18:30

alright great, looks like this is the piece I've been looking for, thanks. not sure why the book jumps right in with no motivation like that, I guess just assumed. however, I still have a confusion of the two particle normalization. integrating that over both of the primed coordinates, wouldn't we want a normalization of 1? instead, that gets us $1\pm1 = 0,2$

jacob1729

That's the wavefunction. You still need to mod square it before the integration.

I think the normalization is off though (usually there's a $1/\sqrt{N}$ in front). Maybe they've normalised it to integrate to the number of particles.

dsm

say we put the $1/\sqrt{2}$ in front and square it out, for fermions wouldn't that still get $(1+1-2)/2 = 0$?

likewise for bosons, getting $(1+1+2)/2 = 2$

jacob1729

No, the negative terms are zero

They've got factors like $\delta(x-a)\delta(x-b)$ that get integrated over x

dsm

ohhh, of course, alright thanks!

jacob1729

Hm, maybe that's not quite right. I'm a bit confused by this, in other contexts they would be disappearing. $\delta$'s are funny beasts.

Oh no it is, because that's $\delta(a-b)=0$ right. Okay, I'm not losing it.

dsm