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12:03 AM
Oh god
That book uses category theory for meta-theories of physics
the horror
 
It is interesting, though
An actual attempt to define the correspondence rules of general relativity
Define GR from raw measurements
"A semantic analysis of model and symmetry diffeomorphism in modern spacetime theories"
Some GR metaphysics in case GR wasn't terrible enough
 
 
11 hours later…
11:32 AM
Is something wrong with this answer?
0
A: Reason behind the inclusion and exclusion of current sources while using Ampere's law for the total magnetic field and total current

KnightIf we have some steady current flowing in space (hypothetical model) with density $\mathbf J$ then the magnetic field at any point in space is $$\mathbf B = \frac{\mu_0}{4\pi} \int \frac {\mathbf J \times \mathcal {\hat r} }{\mathcal {r}^2} dV$$ and we know that the the curl of magnetic field cre...

 
@Slereah I can't help but be curious - what book? "The category of physical theories" sounds intriguing.
 
OP doesn't seem to give any attention to it, and I don't know why
 
@ACuriousMind Semantical aspects of spacetime theories
It is extremely abstract business
 
Well, that's what it sounded like :P
 
Although it's not great, as it's not a book
just a collection of papers
and I think some of those ideas would be best explained in a full book
I'll have to find out where such a book is
 
11:48 AM
What should I do with post in a chat room so that I can visit them in future when I need them?
 
There's a "favorite" button on each chatroom
in the upper right corner
it's a tiny star
 
@JohanLiebert Open the room menu, click "create new bookmark"
 
@ACuriousMind wow! Thanks! It worked.
Now the question is where to find the saved bookmark!
 
@JohanLiebert Go to the tab "conversations" on your chat user profile.
 
@ACuriousMind thanks!
 
12:26 PM
0
Q: Is MathJax in titles OK?

RuslanThere have been some discussions of this topic on other metas, and it seemed to me that the consensus is to avoid MathJax in titles if possible, for various reasons like improving the look of the posts in HNQ and other places where MathJax is not enabled, as well as avoiding the interference of M...

 
12:45 PM
Hello everybody!
I have a very basic question about direction of force on Atwood machine.
You probably know that acceleration is on the side of the greater mass on Atwood Machine, the acceleration is downward.
$\vec{F}_{net}=M\vec{a}$
$M\vec{a}=\vec{T}+M\vec{g} \text{ (2)}$
$M\vec{a}=T(\hat{y})+Mg(-\hat{y})$
My question is, should I re-write $M\vec{a}$ as $Ma(\hat{y})$ (not related to direction of the acceleration) or $Ma(-\hat{y})$ (same direction as the acceleration)?
Which is the equivalent?
I'm not sure I did ask my question clearly :D
 
1:14 PM
Is this comment a spam or something? It seems to have a upvote but when clicking it it redirects to a page without any relevant information.
 
@JohanLiebert It's five years old. It probably went somewhere relevant when it was posted, but now it's obsolete.
See the follow-up comment:
 
@ACuriousMind oh! Thanks.
I have flagged the comment as :"It's no longer needed."
 
@JohanLiebert What’s the size of your waist?
 
1:31 PM
@Knight never measured it. Why?
 
@JohanLiebert Well you must be wearing trousers? Don’t you?
 
@Knight yes.
 
What size of waist do you buy?
28 , 30 , 32 or 38 ?
 
I don't know if it's as per convention but the label there says 34.
 
Oho!
 
1:34 PM
...why is this relevant to anything?
 
What happened?
 
I’m making a picture of him
@JohanLiebert You have got more width than me
 
@Knight Really if you want to see me I can send you a pic.
 
@JohanLiebert Send it, do it fast
 
@Knight did you see it?
 
1:37 PM
What I couldn’t see it?
 
Never mind I am going to send you a pic on Facebook
 
All right!
 
2:06 PM
ACM what’s your office timing? From what o’clock to what o’clock do you work?
 
2:43 PM
@ACuriousMind Why my edit for this question got rejected?
0
Q: Wrong mathematical conclusion from electromagnetism

sarthakNow suppose a magnetic field is increasing inwards and we keep a loop of radius $r_B$ as shown in picture. then the magnetic field at point A is given $E_A=(r_B/2)*(dB/dt)$ Now if i keep a small loop of radius $r_s$ such that $r_s<r_B$ as shown in figure . Note small loop is kept independent ...

 
@Knight Click on the 'rejected edit' in your activity history to see the rejection reason(s).
@Knight Usually around 9-to-5, but we don't have mandatory times.
 
@ACuriousMind This edit conflicted with... , what does that mean?
 
It means someone else with direct editing privileges (in this case the OP) edited the post after you submitted the suggestion.
 
@ACuriousMind hi.
 
hello
 
2:52 PM
@ACuriousMind So do you wake up at 8:55 AM or earlier
I have starred your Hello bcoz first you have acted so friendly :-)
 
@Knight Depends - why?
 
@ACuriousMind I’m entering into adulthood and you may find it surprising but i’m Impressed from you (became inspired from you much before) and therefor wants to learn from you
You buy your own food (that algae noodles example), you wash your own clothes, you do everything on your own
 
@Knight I'm flattered but I don't think you should model your sleep cycle after mine :P
@Knight ...most adults do that.
 
@ACuriousMind My cousin works in your country, Heidelberg Cements and he got shift duties so he cannot ever get enough sleep
 
yeah, shift work mostly sucks
 
3:00 PM
@ACuriousMind No, most adults just live two years or more without family and then they marry and then their wives do everything. You’re 26 and you’re working on your own from 16
@ACuriousMind He says Night Shift is no less than a jail
 
@Knight This is a cultural difference - that is definitely not the norm here anymore. The average age for marriage is above 30.
It is also not the norm anymore that housework is strictly gendered :P
 
@ACuriousMind Would you do housework after marriage :D ?
 
My lifestyle is not exceptional, it's pretty average for my age and career.
@Knight Probably? Unless I somehow am married to someone who actively delights in doing all of it, I'd consider it only fair.
 
@ACuriousMind May god give you someone who will love you and you will love her and you two make a best of couples 💕
@ACuriousMind I think 9-5 duty time is called General Duty, isn’t it?
 
I think most people just call it 9-to-5
 
3:08 PM
At what time do you wake up on normal duty days?
 
The wheels just keep on turning
The drummer begins to drum
I don't know which way I'm going
But I know which way I've come
 
3:39 PM
I don't know what to learn :(
I'm hungry for knowledge but I don't know what to learn or read.
 
3:49 PM
Hello dear users. I'm familiar with the equations from the young's two slits experiment. In that experiment the light is located in the middle of the two slits. Is there a more general equations for a light that could be in someplace (it does not have to be in the middle)?
 
4:22 PM
1
Q: How can I compute the adjoint representation of SU(2)

Antonio Cobos LuqueI am trying to figure out how the famous three matrices are obtained: \begin{equation} T_1=\begin{pmatrix} 0&0&0\\0&0&-i\\0&i&0\end{pmatrix} \end{equation} \begin{equation} T_2=\begin{pmatrix} 0&0&i\\0&0&0\\-i&0&0\end{pmatrix} \end{equation} \begin{equation} T_2=\begin{pmatrix} 0&-i&0\\i&0&0\\0&0&...

The OP hasn't used $ sign for mathjax. How does that work?
 
\begin{equation} is one of the ways to enter math mode in (La)TeX, just like dollar signs.
 
Oh! So it's the tough way of doing the same thing.
 
@JohanLiebert In MathJax, yes, but in real LaTeX it actually has slightly different semantics and should be preferred over dollar signs, cf. e.g. tex.stackexchange.com/q/40492/71716
 
4:43 PM
How come there is no case of covid-19 in small countries? Whereas there are many cases in USA, S. Korea, Japan, etc.
One reason that I can think of is that they aren't able to even check whether there citizens are infected or not. Might be the case for India?
At 1:51 reversed the direction of time!
 
5:08 PM
@JohanLiebert IMO it's cooler with strobe lights, then you can actually see it in person too. The people filming that wouldn't have gotten to see those cool effects except through the camera.
does look pretty cool either way though.
 
I got that result while searching for "resonance experiment water" on YouTube. At the same time I found this too:
I think this is how flat-earthers are born.
For more fun look at the comment section!
 
@JohanLiebert Oh man thanks for sharing. I love the out there videos like this. I usually can't make it through them, but it's usually pretty entertaining to read the comments and stuff.
Lol one guy mentions that it isn't science. Someone else asks them "how does it rain then?" So they give the basic overview, and then the other persons reason for rain was "Sound at different keys. Also light at different frequency.
It rains cause Sun light vibrates water in the atmosphere to a higher vibrator rate then condenses as clouds and then further into droplets of water called rain. It is the very process of vibration. So yes water can be manipulated buy vibration and intention."
The real reason has 2 thumbs up, the mumbo jumbo is at +8. Youtube comments are something else.
 
5:27 PM
@JMac it's much more entertaining to look at this:
 
5:45 PM
That's pretty cool. I was expecting the comments to be full of people saying "IT'S FAKE" or something based on our conversation though lol\
 
@JMac do you know any explanation for this?
 
@JohanLiebert Are you in your knighty ?
 
@JohanLiebert You can look up pendulum wave, one example: sciencedemonstrations.fas.harvard.edu/presentations/…
 
 
1 hour later…
7:03 PM
For a flat Earther, he earned my respect
At least he really did the scientific method, sadly costing his life in the process
 
1
Q: Wrong mathematical conclusion from electromagnetism

sarthakNow suppose a magnetic field is increasing inwards and we keep a loop of radius $r_B$ as shown in picture. $∮E•dl= dφ/dt$ which means $E(2πr_B)=(π(r_B)²)*(dB/dt)$(as electric field is uniform) Now it is questioned that why electric field induced should be uniform.The only answer to this that i ...

 
@JohanLiebert That's cool
 
It is writing 43 min ago by Wolphram Johnny But I cannot see anything by him. Do you see something? By him
 
7:17 PM
@Secret I don't know what to say but if he ever so slightly believed in the science this might not have happened to him.
 
7:52 PM
Is there a precise way to simulate bicep and tricep contractions without using shock electrodes? Something like an exo-suit or something like that? Or I hook up a wire across each muscle (bicep and tricep) and I then pull the wires (using a DC motor or something like that).
 

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