@ACuriousMind At least they call the zero modes of the operator $\frac{1}{2}\rho^\beta\rho_\alpha D_\beta$ conformal Killing spinors and not superconformal ....

@alarge Why do we have to calculate the forces from A to B and from B to C? Is one of them the force that we are looking for and the other one the torque?

@0celo7 I swear I've read superconformal somewhere for something

@MaryStar They are both forces.

Why do we have to calculate these forces, the force from A to B and the force from B to C? I haven't understood that... @alarge

@MaryStar Because you were asked to calculate the forces on the cylinder?

Yes, but why do find the forces at these specific points? @alarge

@MaryStar You find the forces between all those points and sum them up. The only forces you have are due to hydrostatics, i.e. anything that is submerged.

Why do we not find for example the force from C to A? @alarge

@MaryStar Because it is zero?

Why is it zero? @alarge

@MaryStar Because that part is not submerged/

@MaryStar You calculate the torque in the same way.

No, because the signs of the forces are different. What you could do for the force calculation is an integral from pi/2 to pi as the other parts cancel out.

The torque's not going to cancel out in the same way, though.

@MaryStar There's supposed to be a cross product there, but yes.

The axis of rotation is the axis that passes throuh O, right? So, aren't the forces perpendicular to this axis? @alarge

What's the direction of the force, though?

Hmm... I don't know... @alarge

Isn't the direction of the force downwards? @alarge

@MaryStar Well obviously it can't be downwards or the force calculation you did in the first step would have just yielded zeros.

Ok... Which is then the direction of the force? @alarge

Is it maybe perpendicular to the surface of the cylinder? @alarge

If you understood the force calculation in the first step, you should have this covered. If not, I suggest you hit the books.

Sure, but the appropriate way to see whether someone there wants to help is not to post a question that is known to be blatantly off topic.

@skillpatrol It's not about theoretical vs. applied. It's about the level of the question, which is surely not research-level math, and MO is explicitly for research-level math.

@alarge The integral means that the force is perpendicular to the surface of the cylinder so the force is parallel to the axis of rotation. Is this correct? @alarge

Ok, I agree with both of you...desperate times call for desperate measures from most students, in my experience.

@MaryStar The force is greatest at the bottom, right? If it were perpendicular, this would lift the cylinder up.

This is not a homework site, and I don't think there's any benefit in me just telling you the right answer anyway. I suggest you look up similar problems in your books and go through them meticulously enough that you feel you've understood the basic principles.

Do you rewrite your notes @MaryStar?

I already gave you a hint: you must've figured out the direction in the first part of the problem. Whatever your guess, it must be consistent with that.

@skillpatrol What do you mean?

Rewrite them neatly and organize them to the way you think about the question.

Since at the integral the angle is from A to B, doesn't this mean that the force $F_{AB}$ is downwards? @alarge

@MaryStar The only thing the integral means is that you are summing all the forces between A and B.

Do we have a policy that says one should not post incomplete answers (where it is obvious that they aren't finished posts) and then repeatedly edit them until they are the full answer one intended to write all along? It annoys me, but it would be nice if I could point to something more than "This annoys me".

@skillpatrol In my notes there aren't many examples.

@MaryStar when a prof puts no examples in the notes that means go and make your own

or go to the library and find examples in other textbooks

while I stand here and "wave my hands around"

I want to understand first the examples that the prof gave to us but some of them are not complete and I have some difficultis to understand them... @skillpatrol

stop him and ask questions

Next time...Now it is too late... @skillpatrol

@ACuriousMind there's a policy that you shouldn't edit an answer an excessive number of times. I don't think it defines "excessive" exactly, but there are auto-generated flags for more than 10 edits.

@MaryStar I know :(

But posting an incomplete answer is not itself against the rules AFAIK. It does open the poster up to downvotes while they edit, though.

and the earliest answers get the most votes...

And those votes will be - or should be - negative, if the answer doesn't constitute an answer on its own.

...there's always a race at the math site to get the first answer up...

speed typing and LaTeXing comes in-handy

even in the chatroom

@MaryStar Well, if you read Wikipedia on Buoyancy, that (with the pictures there), should answer many of your questions.

great idea^

@skillpatrol We're not racing here for answers (in my impression, at least), and I hope we never will be.

I hope so too pal @ACuriousMind

@MaryStar Yes

So, the vector of the force is parallel to the axis of rotation, right? @alarge

If this is correct, that would mean that the torque is zero, isn't it? @alarge

Can anybody tell me why this question has 7 upvotes? People actually think it's a good question to compute and compare random values and be surprised they don't match?

no idea

cargo cult science?

@ACuriousMind : Probably because it became a HNQ.

@Qmechanic Isn't there a threshhold that a question needs to have some upvotes to become a HNQ?

(If not, there should be)

@ACuriousMind HNQ?

@ACuriousMind SCFT = superconformal field theory, cf. Ch. 4.7 in BBS.

@0celo7 Hot Network Question.

@ACuriousMind Have you ever encountered a problem in Skyrim where your character is stuck walking slow, like you've been hit by ice spikes? All of my characters have this problem. They walk just fine, then they slow down to a crawl, then speed up again.

It's not a save game issue since they all have it.

@0celo7 Hm...you're not just walking instead of running, are you? :D

Other than that, I've no idea what it could be

@ACuriousMind No. Even the sprinting is in slow motion.

Weird. Never happened to me

Getting hit by ice spikes and recovering did nothing.

Time to reinstall.

Crap.

Actually.

I should boot it on a flash drive.

@0celo7 You can't play most games from a flash drive, USB is too slow for that.

@ACuriousMind I meant I should put the save on a flash drive.

I'll play the game from the disk.

Dunno why I said it like that.

Ah, that makes more sense.

Oh, that won't work.

The save has the DLC, and the disk doesn't.

I have to move all of the saves, start the game from the disk and start a new character.

GG Bethesda.

Huh? Why can't you re-install with the DLC?

@ACuriousMind I don't want to reinstall if it's a save game issue.

Which it shouldn't be, unless one save infected the others.

If that's the case, I'm fucked. I was well on my way to having a char with 100 for all skills.

@0celo7 Type Player.getav speedmult into the console. If it is not 100, set it to 100 with Player.forceav speedmult 100.

@ACuriousMind Xbox

Well, then I got nothing :/

I now have to sit through the whole Alduin attack.

If I ascertain there is no issue with the disk, then I'll delete the install of Skyrim from my HDD and reinstall it. Furthermore, I will reinstall the DLC and wipe the cache.

If that doesn't work, I'll buy Pokemon Omega Ruby and cry in a corner with my Pokemons because I lost 400+ hours of work.

I could have learned string theory in the time it took me to play Skyrim.

You have got to be fucking kidding me. @ACuriousMind The game crashed during the Alduin attack.

Didn't even save.

lol...it seems it doesn't like you :P

I don't want to deal with this now.

@just_curious You a fan of @ACuriousMind?

I think "just curious" and "A Curious Mind" have different means :)

imgur.com/gallery/ibKBg

@ACuriousMind I took my hard drive out of the console, made a brand new profile on a USB drive, made a brand new save game, and the glitch is still there.

HOW?

That passes 0 tests of logical continuity.

@ACuriousMind Found the problem.

It took some intensive calculations.

The left stick on my controller is broken.

...so, nothing to do with Skyrim at all?

Nope.

The major hint was that I had the same problem in GTA when I fired it up.

The stick has different degrees of tilt and somehow it can't distinguish full tilt and less-than-full tilt.

That controller has been with me since 2011.

Hell, 2010.

That was the controller I got with my Xbox, and it finally bit the dust :/

Well, all things come to an end ;)

Yeah, like my gaming career.

That was my only working controller.

I can continue with this one, but without RB (right bumper), i.e. fus ro dah.

@ACuriousMind Do you know if controller surgery is a thing?

Perhaps I have to clear some epithelial cells from the stuff under the stick.

@0celo7 No idea, I've never heard of someone fixing a broken controller

oooh thanks I had only seen the edited video

You're very welcome :-)

@ACuriousMind Copious amounts of compressed air in various cavities failed to yield any measurable results.

I think I have to open this thing up.

@0celo7 Sure, if you know what you're doing. If it's just dust there should be no problem, if it's a broken potentiometer then it might be a bit work involved. With Xbox controllers being so widespread though you can probably find pictures explaining the whole process so you don't even need to know what you're doing to fix it.

Hey, say I have the hamiltonian $\hat{H}=-J\sum \hat{Z}_i \hat{Z}_{i+1}$. Does the time evolution of, say, $\langle \hat{Z}_i\rangle$ sound interesting to anyone?

For part of a uni project

erm, sorry, that's zero, the real hamiltonian I'm looking at is $\hat{H}=-J\sum \hat{Z}_i \hat{Z}_{i+1}-\sum h \hat{X}_i$

That's the Ising model, isn't it?

Yep. I'm kind of interested in finding the expectation value time evolution but I'm not sure if it's possible.

@alarge I just went out to the garage to perform the surgery. The controller has proprietary screws.

@NeuroFuzzy When writing $\langle \dot{} \rangle$, we usually mean the expectation value on the vacuum. Why would that evolve in time at all?

@ACuriousMind NRQM, he means the Ehrenfest-style average I think.

@0celo7 Are you sure they're not just torx? Lot of electronics have those.

Oh, I don't mean on the groundstate nor on an eigenstate, nor on the first hamiltonian (which would not time evolve)

@alarge It's a torx but with a pin in the middle.

And yeah NRQM expectation

I can't fit my tools into the socket!

'Kay, on which state are we taking this expectation value, then?

Usual QM also only knows expectation values with respect to certain states, if I remember correctly ;)

@0celo7 It's probably standard anyway, so you can just buy a driver for a dollar or whatever.

Just an arbitrary time evolving state.

@alarge i.gyazo.com/d53b66a9875c26f2f25308d012a15cdc.png

Sorry for bad quality, iPhones are bad at close-ups.

Aha, it's a tamper-resistant torx plus.

@NeuroFuzzy ...so, what you want to do is solve the Heisenberg equation of motion for $Z_i$?

Because that would be equivalent to knowing $\langle Z_i \rangle (t)$ on an arbitrary state, I think.

Equivalent?? You can't go back from the expectation value to Z_i. I mostly just think it would be fun to accompany the standard results of the ising model (which I'm presenting) with pictures/graphs of the actual states. And in the pictures/graphs the hilbert space will be too huge to do anything that involving a value for each state.

@NeuroFuzzy Remember ACM's golden rule: if you don't know how two things are related, they are equivalent.

@NeuroFuzzy IHm, you're right, solving Heisenberg is strictly stronger.

Am I being dumb? What's wrong with the Ehrenfest theorem?

@skillpatrol "You can't fire me, my title is permanent member! He said: Here's the difference between a permanent member and a temporary member: A temporary member has a contract. So I was fired." :D

@0celo7 I found my set of tamper resistant torx drivers years ago in the $1.00 bargain bin at a autoparts store. No dead appliance has been safe from me since.

@NeuroFuzzy :D

@0celo7 Nothing, it's the thing to look at, you're right.

@0celo7 well it's just that because Z and X (which are sigma^z and sigma^x) things like $\langle [Z_i,X_i]Z_{i+1}\rangle$ pop up :(

@NeuroFuzzy Ah, I'm not acquainted with this particular Hamiltonian.

There's no relationship between $Z_i$ and $X_i$?

If not $Z_i = X_i$, then this is not the Ising model, or am I going completely crazy now?

Oh they're just pauli matrices. So that term is $2i\langle Y_iZ_{i+1}\rangle$

maybe that does separate. Anyways I guess it's interesting enough to try to solve for a fun extra in the presentation

@dmckee If I have to actually replace something, it'll stay broken. I have experience with electronics, but I don't have any parts or a soldering iron available. Also, I highly doubt I could repair this and put it back together without ruining something else or making it not fit back together.

@0celo7 Oh ... I think I'm about one for eight in actually repairing the things.

:D

When you find some component charred so badly you can't read the labels anymore and the manufacturer isn't making a schematic available you're sorta out of luck.

I doubt I fucked up my Xbox controller that badly.

It's like "Well, I'm pretty sure that a thyristor should go there, but I have no idea what its spec should be..."

Yeah that product I posted isn't trivial to calculate :(

Whatever. I'll just simulate the whole $2^20$ dim. hilbert space.

@NeuroFuzzy What are you simulating?

Which is the energy balance equation for incompressible flow?

I just want some pretty pictures/animations of the time evolution of that hamiltonian. And also a 2D analog.

ic

@ACuriousMind @alarge Do you maybe know which is the energy balance equation for incompressible flow?