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12:20 AM
Heeyyy
Quick, stupid question: Why don't the oceans spill into the mantle? Aren't there cracks between the tectonic plates?
 
12:32 AM
@tpg2114 well... it's asking about a mathematical model, not asking for a software program, which makes it not a clear case of software recommendation IMO. But I also think it's iffy for other reasons, as described above.
@Walkerneo Only tiny ones, and they're already filled with lava
Should exist? I don't think so...
cc @freiheit
 
user54412
12:54 AM
@Walkerneo the oceans would cool the magma and solidify it - it's like a natural clotting mechanism
 
user54412
also, the water is presumably less dense than rock, melted or not
 
user54412
1:35 AM
@DavidZaslavsky I don't suppose there's a way for me to suggest edits any more?
 
user54412
I sometimes would rather suggest an edit the author can review instead of unilaterally changing things
 
3:50 AM
@ChrisWhite If you have over 2000 rep, then no, you can't suggest edits.
Of course most suggested edits are not reviewed by the post authors anyway
In a situation like that I usually leave a comment suggesting the change.
or if it's a really minor edit, like just a spelling correction or a retagging, I'll just do it
Also in cases where the OP is new to SE and is having trouble formulating the post effectively, I'll sometimes just edit and post a comment asking them to review it. Remember that edits can always be rolled back.
 
 
8 hours later…
11:23 AM
3
Q: Wrinkling paint - soluto/thermocapillarity - is it due to the primer or solvent

drNThis is really a one-and-a-half part question. I know that when paint is mixed with a solvent or used with a primer, it sometimes wrinkles. As I understand, a key physical phenomena here is a non-uniform evaporation of the primer or solvent that gives rise to different evaporation rates at the p...

 
Solvents, etc.. - Doesn't it suite your FAQ..?
 
^^ To me, this seems OK on Physics.SE. Anyone feel otherwise? (I can migrate to Chem, but only if it's not on topic)
@CrazyBuddy both FAQs
 
Another one..? NICE :-)
 
 
3 hours later…
2:29 PM
Anybody around to help with a math question?
 
@tpg2114 am I around? Yes. Can I help with math? Probably not, but ask away :)
 
So I have this huge mass-spring system to represent a solid material
And if I look at the x-component of the force vector for a given (i,j) indexed mass
I get something along the lines of:
 
@tpg2114 Me too... (around) - Umm... But, I'll fill in the audience :-)
 
$F = a*f_1(x,y)^{1/2} + b*f_2(x,y)^{1/2} + c*f_3(x,y)^{1/2} + d*f_4(x,y)^{1/2} - B$
2
 
@tpg2114 Now, that's rude & nasty ;-)
 
2:34 PM
Where the f functions are the dL of the spring (so it's really ||x_{i,j}-x_{i-1,j}|| for f_1
Etc
With me so far?
 
@tpg2114 dL?
 
Change in length of the spring
\Delta L
 
yeah, so four springs?
@tpg2114 no I got that
 
Right, each mass (i,j) is connected to it's 4 neighbors
But it's not Cartesian aligned which makes it annoying because no terms drop out of the || || term
 
gtg
 
2:36 PM
So I've been trying to get it into a form Ax = b
But those 1/2 powers are making it hard
 
In a Cartesian system they would go away
 
Okay, I've starred it... Someone will have enough patience to read along the transcript :-)
 
Maybe I need to find the force in the {\xi, \eta} generalized system and then convert that back into physical {x,y} coordinates
Rather than converting the strains back to physical coordinates and trying to find the force
 
 
3 hours later…
5:34 PM
1
Q: General solution of $\frac{d^{2}u}{d\rho^{2}}=\frac{l\left(l+1\right)}{\rho^{2}}u$

Peter4075In his discussion of the radial wave function of hydrogen Griffiths (Introduction to Quantum Mechanics, 2nd ed, p.146) gives the general solution of$$\frac{d^{2}u}{d\rho^{2}}=\frac{l\left(l+1\right)}{\rho^{2}}u$$ as$$u\left(\rho\right)=C\rho^{l+1}+D\rho^{-l}.$$ Why is this? There's not even a...

Is it just me or should that go to Math?
 
5:47 PM
@DavidZaslavsky I agree, it's just math since the tricks to go from one form to the next don't depend on any physical intuition/reasoning
It's just algebra
 
@DavidZaslavsky I don't know if should exist, or not... I'm a bicyclist and bicycles.se user (and mod), and I don't know the physics.se tags very well at all. Bicycles (and motorcycles) are a somewhat interesting dynamic system that's commonly misunderstood. There's research into bicycle dynamics as recent as 2 years ago. Also, the tag has 3 followers (one of which is me).
 
@DavidZaslavsky @freiheit I would vote no (as a cyclist too!) just because we don't have tags like even though there's a lot of interesting vehicle problems in physics.
Although we do have which should be killed under that argument
 
@DavidZaslavsky probably not.
 
I think i'd personally be better off if there were no tag, because reading the questions and answers makes me want to go try to clear up misconceptions about bicycles/bicycling.
 
@freiheit Migrate all to Bicycles.se and watch them tear 'em apart!
 
6:01 PM
@Manishearth Then I'd want to go and clear up all the physics misconceptions... :/
 
@tpg2114 go, young grasshopper, you have my blessings!
 
I have my hands full flagging OT posts here :)
 
Oh damn checks for flags
Y U MAKE WORK FOR ME?
( j/k, we want people to flag)
 
Because I don't know anything about QM or BH so I won't get to a high enough rep to do anything myself any time soon
 
@Manishearth Most would be off-topic or not-constructive on bicycles.se, since most don't really correspond to practical bicycling problems
 
6:04 PM
@tpg2114 give HW hints. Answer fluid mech qs. Answer simple conceptual mechanics stuff. Answer simple relativity/QM qs. How do you think I got my rep here?
@freiheit I know ;-)
 
@tpg2114 Can we just agree on clearing up the reality misconceptions?
 
Maimon has now voted down my accepted answer to physics.stackexchange.com/questions/45598/… twice now. The first time was a week or two ago. I commented back that his answer was superficial and likely wrong. We went back and forth a couple of times in comments. Now those comments have vanished. But I now see that he has voted down the answer again and said that it was "bad physics." This is just ad hominem. ...
Since according to his profile Maimon no longer participates in this site, should he have the ability to vote past, accepted answers down?
 
@MarkWayne hang in there, will reply in a mo'
 
@freiheit The physics of cycling along with racing/training/performance are my hobby... :) Which is how I end up doing things like: dicklanevelodrome.com/topic/2010/06/… and dicklanevelodrome.com/node/1037
@Manishearth I jump on as many fluids questions as I can, even though I think since I didn't put pictures on this one I'm not going to get votes even though I have the same answer but earlier ;)
2
Q: Why does smoke go out the window of the car - and what if there's wind blowing instead of the car moving?

JoeWhen driving a car while smoking with the window open (safety and legal issues aside), I've noticed that the smoke tends to go outside the window. Why does the smoke go outside? If the car is standing still and there is wind blowing at the same velocity the car was going - will the smoke behave...

 
@tpg2114 Next time add more pretty pictures! Some of my posts took a loong time to get votes, though I expected em to get lots.
 
6:12 PM
@tpg2114 I skimmed that first one, and I think the physics of track sprints are simpler than long outdoor rides. :)
 
On the other hand, this gets lots of votes, just because of the frog :O
 
@tpg2114 (I think I'm fastest in cool humid conditions because warm humid weather limits heat dissipation and significantly limits my power output)
 
@MarkWayne As long as his profile isn't deleted, he can continue to participate, regardless of what his profile says
 
Hi @Mansihearth -- thanks. I should have mentioned that the main point is that he voted down the answer twice. The first time Maimon and I had an exchange in comments, they were all deleted but the down-vote remained. Then he commented and down-voted again two days ago. It's not a big deal for me -- annoying, for sure -- but it seems he's abusing a site he no longer cares to participate in.
 
@MarkWayne same answer? He can't.
one sec, looking
 
6:14 PM
@Manishearth -- the original comments were deleted. Then he down-voted again.
 
@MarkWayne he can't. He can just leave another comment saying so
There is only one downvote on the post
What happened here was that he probably didn't like his original comment being deleted, so he re-commented
 
Doesn't seem that way to me -- if you look at my stats for the past week you'll see -2. I don't know where the other -1 came from.
 
Also, that's not an ad-hominem attack
@MarkWayne hmm, looking
@MarkWayne Each downvote costs you 2 rep
The -1 in his comment just means "I downvoted", since the post score goes down by 1
 
@Manishearth - if someone says your answer is "bad science" you're not going to take it personally? (Thanks for explaining the -2)
 
@MarkWayne Well, it's close, and I might take it personally in some cases, but it's not an ad hominem attack :/ He commented on your answer (albeit in an abrasive manner), and called it "bad". He didn't call you "bad"
Though I shall edit out the bad science part
(Next time, if you feel someone is offending you, flag the comment as "rude/offensive"
You may not always find a mod in chat
> An ad hominem (Latin for "to the man"), short for argumentum ad hominem, is an argument made personally against an opponent instead of against their argument
Here, the argument was made against the opponent's argument, so yeah, it's not ad hominem
Though having your work called "bad" can be taken personally :/
@tpg2114 do you think this question is off topic?
3
Q: Wrinkling paint - soluto/thermocapillarity - is it due to the primer or solvent

drNThis is really a one-and-a-half part question. I know that when paint is mixed with a solvent or used with a primer, it sometimes wrinkles. As I understand, a key physical phenomena here is a non-uniform evaporation of the primer or solvent that gives rise to different evaporation rates at the p...

 
6:28 PM
@Manishearth - that's some serious hair-splitting you've got going on there. I'll leave it alone but if you condone this sort of "abrasive" behavior, I think you don't have to wonder much why more people aren't participating. Maimon's point, that I've heard him voice on several occasions, is something like, "I'm too busy to be polite." That's just ridiculous. That sort of attitude is counterproductive. Maimon is due no special deference. He is consistently offensive.
 
@MarkWayne No, not hair splitting. We have to allow people to disagree with posts. That's a given. If someone says "your post is wrong", yes, people take it personally. But not allowing it introduces the risk of wrong posts not being recognized/corrected
@MarkWayne Yeah, that is his point. Which is counterproductive, and he's been banned for it X times
@MarkWayne And we give him no special deference, even though the community has asked for it a few times
@MarkWayne Basically, as long a "this answer is wrong" comment explains why it is wrong (and only attacks the answer, not the answerer), then it's fine. But yeah, such comments can be more abrasive than necessary (Ron's generally are), and they should be flagged+edited
But, while you may feel like taking a "this is wrong" comment personally, it's better if you don't.
 
@Manishearth I was under the impression that he'd been suspended twice. From what I've seen of his posts, whenever there is a disagreement, he goes postal. But thanks for letting me know that it has been "X" times that he was suspended (I take "banned" to mean "permanent"). If X >> 1, then it begs the question: if some folks don't learn, how many suspensions are allowed before a permanent ban is applied?
 
6:46 PM
@MarkWayne Well, I can't comment on the details on the suspensions I'm afraid
But the general policy is 7 days, then 30 days, then 1 year, then pretty much permanent
(btw, there is no such thing as a permaban here, but we can ban for 10years and all which take care of the issue effectively)
 
@Manishearth His last suspension was for 30 days. So he's got two left.
 
@MarkWayne Well, we can always extend the period if we want. the 7/30/1y thing is just a rule of thumb (though the software suggests it)
 
@Manishearth Thanks for explaining. Just one last comment. Maimon was contributing mostly good answers (though without, as far as I can tell any original work -- basically, he just regurgitates the literature or textbook). But now that he doesn't do that anymore, what's the point of allowing any counterproductive responses from him at all?
 
@MarkWayne Well, that sounds a bit dictator-y. "Contribute or thou shalt be banned!"
AFAICT, his response on your question isn't really counterproductive since he did mention why he feels that it is wrong
@CrazyBuddy Btw, the folks at my lab recommended ANSYS, but they said that they hadn't modeled the quadrotor on a comp at that time.
 
7:06 PM
@Manishearth I'll look at that question, hang on... Had to drive home from the lab
 
@tpg2114 kk :)
take your time
 
I think it could be on topic if it asked about the physics, but as it is I'm not sure what it's really asking. What is a "solvent problem" or a "primer" problem?
If it asked about the effects on surface tension due to differential evaporation, or the creating of wrinkles due to differences in surface tension and the physics behind that then it's on topic
 
he's probably asking if the issue stems from the solvent or the primer
 
@Manishearth That's not my statement. If a user is just going to abrasively comment on previous answers, why should that be tolerated? I'm done - thanks. See ya.
 
I'll ask for clarifications
To see if we can get it more on-topic/clear
 
7:11 PM
@MarkWayne it is not. Another comment like that, and he gets a warning (most probably)
But in itself the comment isn't that bad.
 
7:28 PM
@Manishearth Then..?
What are you saying? - They hadn't modeled on a computer?
 
@CrazyBuddy nope
 
Looks like I've missed some discussion...
Okay, I'll check in later...
5 days vacation - smoked away :-(
gtg to colg. again..!
 
 
1 hour later…
8:57 PM
I want to be the first to get the Marshal badge... :)
 
9:16 PM
@Dilaton I've told you before, chat is not the place for that. Cast flags when you see behavior you think is inappropriate.
 
Okay, I'm sorry if I'm becoming part of the problem. I'm still fairly new to the site and apparently I need to develop thicker skin. As a working physicist though, with not a lot of time on my hands, who participates here for the fun of it, such interactions as I've recently had with Maimon make it less fun. And, here, I really will bow out of this chat.
2
 
@MarkWayne Understood... just remember, any time you see inappropriate behavior on the site, flag it. If you do, the moderators will deal with it.
2
 
9:36 PM
@DavidZaslavsky when I see something bad and flag it or say so by a comment, all my flags get declined and the comments get deleted whereas the bad things keep going. For example what is wrong when I say this is a good answer, +1, there is nothing wrong with the PHYSICS therein? Why does this always get deleted whereas comments just nonconstructively stating that the poster does for whatever personal reasons (?) not BELIEVE in magnetic monopoles
 
@Dilaton I'm looking at your flagging history, and it does not support that
 
are allowed to stay there @Manishearth ?
 
If you just post a comment to say "+1 this is correct" or something like that, it's not constructive. It doesn't add anything that an upvote wouldn't. While we don't proactively delete such comments, it will get deleted if it's flagged or if we're clearing comments on the post for another reason.
But a comment which states a disagreement with supporting evidence is constructive, and will often be left alone.
 
What is constructive about "show us a magnetic monople Ron" then? This is just a snarky nonconstructive comment and such things obviously are allowed to stay there.
Ok, got to go now, I should just learn to no longer get too involved in things here, it negatively affects my "real life" ... :-/
 
OK
That comment could be better phrased, but it is constructive. It's stating a specific objection to the content of the answer.
 
10:07 PM
Ok @DavidZaslavsky there is just one last thing I am too curious about: would such a snarky comment be constructive too, if say Ron Maimon or I had said such a thing below another post? I unfortunately got recently the impression that everything what some people do or say is constructive and allowed by definition, while (even slightly and politely formulated) disagreeing with them is not allowed or at least badly received. This happend in particular on meta.
 
 
2 hours later…
11:51 PM
@DavidZaslavsky: It's not so obviously constructive. It looks to me like Sklivvz is simply nitpicking the first sentence in Maimon's answer. If you read to the end, Maimon makes it quite clear what he is saying.
 
@Dilaton no, the constructiveness of a comment has very little to do with who posted it.
@user1504 that's just because the first sentence also happens to be the main point of the answer.
By the way, what is this weird thing with calling people by their last name?
 
I'm actually a grade school gymnastics teacher.
Just habit, I suppose.
 

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