The h Bar

DanielSank

12:45 AM

@heather Just wrote an answer about quantum annealing.

heather

@DanielSank, thanks! Quite interesting, and clears up a misconception I had.

DanielSank

What was that?

heather

That the system had to be in its lowest energy state.

user228700

Hi everyone :-)

heather

Hello =)

user228700

12:50 AM

Is anybody interested to discuss about the method to calculate the center of mass of various systems? Yes, Ik, very boring stuff :-P Still, anybody?

heather

@DanielSank I was looking at the user who asked that question's other questions, and I saw a comment from you about the term quantum wire not being standard terminology. Is there a standard term for it? I was watching a video series and that was the term used, which is why I'm curious.

@Kamudi, interested but unable to add anything to a conversation about it =p

user228700

1:11 AM

@heather :-P Are u sure tho?

heather

@Kamudi, am I sure about what?

user228700

@heather That u won't be able to add anything to the conversation?

heather

@Kamudi, pretty sure, unless you are okay with me googling everything every five seconds and coming up with the same questions as you.

user228700

@heather Haha, alright then.

heather

@Kamudi, what is your question? (curiosity)

user228700

1:14 AM

Well, I'm trying to get my head around the procedure to find the center of mass of bodies such as a hemispherical ring/disc/sphere etc. and although I understand what we're trying to do, I'm not sure if I understand the math properly enough to maybe try to do it for similar systems on my own.

heather

@Kamudi, well, here is a math.se question

here is a physics.se question

I'm thinking you might have better luck on math chat, though I'm not sure. Let me know if those question/answers helped, if they did, I'll keep searching.

user228700

Hey, thanks! :-D The answer in the first link is a bit difficult to understand, since there is no diagram :/ I'm waiting for Sir (:-P) John Rennie or MAFIA to show up...I'm certain that they'll be able to help me.

heather

@SirCumference, come to the assistance of us! Okay, of Kamudi, but still! You are called by the power of Pie (pi, whatever)!

Sir Cumference

@heather Huh?

heather

@SirCumference, sorry, that was me being weird. But anyway, @Kamudi had a question for you...

Sir Cumference

1:23 AM

What's the question?

Not sure if I can answer it, my level of physics is not up to par with the others in the chat

But hit me anyways

user228700

@heather Thanks! :-P I was reading the answers on those links but I wasn't able understand much.

heather

@Kamudi, yep. Good luck with your question.

user228700

@SirCumference: Hi :-) I was just waiting to see if anybody would like to discuss the procedure of finding the center of mass of systems with uniform mass distribution, like a hemispherical sphere/ring etc.

Sir Cumference

Probably wouldn't be your guy then.

user228700

OK :-)

Sir Cumference

1:27 AM

Unless you have a question about astrophysics, I don't know how much use I am. :/

user228700

@SirCumference OK, I'll keep this in mind :-P

heather

@Kamudi, sorry!

user228700

@heather: Don't be sorry! I just have to wait for JR/MAF to show up. Don't bother urself; they'll be able to help me out quickly. Thanks! You're very kind :-)

heather

@Kamudi, well good luck with your question!

vzn

@DanielSank something to blog about for sure! :) :P

0celo7

2:14 AM

Trying not to destroy @SirCumference

Sir Cumference

2:33 AM

@0celo7 Of course you are. Look, if there's something other than GR you want to discuss regarding astrophysics — say, orbital mechanics, I can fare better.

Do you even like astronomy?

DanielSank

2:48 AM

@vzn Hmm, not sure.

user218912

3:08 AM

@0celo7 lol

user218912

@SirCumference ummm, how can you know orbital mechanics but not how to compute the center of mass for a uniform mass distribution?

Sir Cumference

@bl00 Well, guess my knowledge is too limited...

user218912

read tong's notes...

Sir Cumference

No!

I don't have time for that

user218912

your loss.

user218912

3:10 AM

if you read them then people here will take you seriously.

Sir Cumference

Eh, I can be taken seriously...

I have a good knowledge of astrophysics

user218912

superficial knowledge, because you're missing the understanding part which comes with the math/physics.

user218912

which ur skipping

Sir Cumference

Untrue, I'm using calculations all the time

Mostly for classes, but that counts for something, right?

user218912

I mean you're missing the level of math/physics required for the advanced astrophysics you're "learning".

user218912

3:13 AM

so learn it...

user218912

tong's notes

user218912

they're not even that long

user218912

you could finish it in 3 days (the first one)

Sir Cumference

To an extent. But a good chunk of the astrophysics questions I typically encounter don't require much more than basic calculus and high school physics

Such as calculating how hot a planet will be given its albedo and the flux from the star

Or figuring out how bright a star will be from a given distance

user218912

but that's like high school physics, plug into a formula and you're done.

Sir Cumference

3:14 AM

Or predicting when the Moon will be full or new

@bl00 It's more advanced math. But what then are you trying to show me?

user218912

there is a lot of astrophysics inaccessible to you right now.

user218912

because you're missing the physics and math to learn it.

user218912

so learn it!

Sir Cumference

Well where should I go to learn the math?

I'm already taking math classes

user218912

how much do you know right now?

Sir Cumference

3:17 AM

Calc 1, taking calc 2

user218912

do you like watching lectures or reading notes?

Sir Cumference

Eh, I can work with them

user218912

pick 1 because I have resources for both.

Sir Cumference

Uh, is there any point though? Because I'll be learning it in college anyways

user218912

is there a point to learning astrophysics then?

user218912

3:19 AM

you'll learn it eventually in school.

Sir Cumference

Well, won't the math classes become incredibly boring if I know it all?

user218912

no.

Sir Cumference

That's what happened when I took precalc, goddamn that class

user218912

you'll be able to get more out of the lectures

Sir Cumference

All right, which one worked better for you? Lectures or notes?

user218912

3:20 AM

instead of trying to take notes and understand everything you can relax and just observe and get different perspectives on things you already know.

user218912

learn them better maybe.

user218912

well everyone is different.

Sir Cumference

I suppose

user218912

I liked notes

Sir Cumference

And how long would it take to read all of Tong's notes, if I do it casually?

user218912

3:21 AM

depends on you

user218912

the more you read the more experienced you will be and the faster you can learn.

Sir Cumference

You think > one month?

user218912

but assuming it's your first time doing it seriously... maybe 3 weeks if you read a few pages everyday?

user218912

for dynamics and relativity

Sir Cumference

Yeah but I'll need to learn all the math first

Topology, analysis, etc.

user218912

3:22 AM

no dynamics and relativity only requires calc 1-2 and basic calc 3.

user218912

you'll manage.

Sir Cumference

GR?

user218912

no?

user218912

his dynamics and relativity notes are advanced newtonian mechanics and special relativity

Sir Cumference

I mean does GR only require calc 1-3?

Oh, that makes more sense

user218912

3:24 AM

GR requires calc 1-3, and linear algebra

user218912

basic GR, not what 0celo7 is doing.

Sir Cumference

and topology?

user218912

no

user218912

not for physicists GR.

user228700

Do u guys know anything about projectile motion BTW?

user218912

3:26 AM

if you want to learn GR, just learn calc 1-3, and basic linear algebra like inner product and matrix algebra. and then read a book like schutz or hartle. @SirCumference

Sir Cumference

Ugh...

All right, I'll get started

Can you send me some calc lectures?

user218912

notes or lectures?

Sir Cumference

Both

user218912

it's better you use notes because

user218912

there aren't that many lectures on astrophysics or GR and stuff like that.

user218912

3:27 AM

so don't depend on lectures

user218912

get experienced with reading

user228700

@DHMO: Hi. Do u know about projectile motion?

DHMO

@Kaumudi yes

Sir Cumference

As long as it's linear

user218912

@SirCumference the best notes I know for calc 1-2 are these and for calc 3 these which are a continuation of the first.

user218912

3:28 AM

the fastest and best way to learn advanced calc 1-3. ^

Sir Cumference

This looks like set theory

user218912

it has some basics

user218912

you need to know the basics at least

Sir Cumference

Fun

Set theory notes? (that don't involve calc)

user218912

even for astrophysics

user218912

3:29 AM

what?

user218912

did you even look?

user228700

@DHMO Hang on, I'll think about the problem some more...

Sir Cumference

Oh nvm

DHMO

@Kaumudi ok

Sir Cumference

Wait, why on Earth am I learning basic set theory at the same time as calc?

user218912

3:30 AM

because basic calc requires basic set theory.

user218912

to learn it properly

Sir Cumference

201 pages...

Jesus

user218912

you need to know that stuff

Sir Cumference

I know

user218912

would you rather read a calc textbook that is like 700 pages?

Sir Cumference

3:31 AM

But Jesus

@bl00 What about "Jesus" made you think I would?

user218912

this is the easiest way, those notes have everything you need in the least amount of pages.

user218912

from my experience at least

Sir Cumference

Well looks like this is the tl;dr version...

How long did this take you to read?

user218912

I read these in grade 10 and it took me about 3 weeks for the first one, and 3-5 weeks for the second one.

user218912

I was in no rush though.

user218912

3:33 AM

didn't even like physics that time. xD

user218912

I also made my friend read it

user218912

and it took him the same time basically.

user218912

and you probably know more math than he did at the time.

Sir Cumference

All righty then

user218912

read tong's notes at the same time. :D (dynamics and relativity)

user218912

3:35 AM

once you're done those notes and calc1-3 notes, you can read tong's classical dynamics and electromagnetism.

user218912

THEN you're finally ready for real GR and quantum mechanics.

user218912

and actually understand stuff

Sir Cumference

@bl00 wtf

I can't do that

How is that linear?

user218912

linear?

Sir Cumference

As in, going in order?

user218912

3:36 AM

because you know enough math for dynamics and relativity.

user218912

physics goes beside math.

user218912

so like, learn them together

Sir Cumference

I think I'll try to read this first

user218912

or you could do that

Sir Cumference

If I can get around to it, God help me

user218912

3:37 AM

yes you can.

user218912

it's called prioritizing.

Sir Cumference

Prioritizing over everything else I have?

user218912

not everything

user218912

but some things like answering astro questions in se.

user218912

you can put aside for now

Sir Cumference

3:38 AM

But it's so fun...

Plus people need help, or are just curious

It's my calling

user218912

you need more help.

user218912

so help yourself first.

user218912

then help others.

Sir Cumference

If you say so

@bl00 Is all this contained in a textbook by chance?

user218912

which?

Sir Cumference

3:41 AM

The calc 1-2 notes

user218912

there are textbooks too ofc.

Sir Cumference

But, ones as good as this?

user218912

I don't know of any

Sir Cumference

You said this is the best learning method you've found

user218912

yes

Sir Cumference

3:42 AM

Sigh :/

user218912

I tried a lot of books.

user218912

but I found these notes really good

Sir Cumference

For me it's much easier to read all of it on paper

user218912

lots of examples and explanations

Sir Cumference

But I don't have 200 pages of paper to waste

user218912

3:42 AM

oh

Sir Cumference

printing it all out

user218912

then go to a local printing office and get it binded for like 10 bucks.

user218912

I did that.

user218912

and it's not a waste...

Sir Cumference

Of paper?

200 pages

That's like 4 stacks

user218912

3:43 AM

yes, filling them with useful information that you will read from.

user218912

how is that a waste?

Sir Cumference

And all the ink

And all the time I'd spent waiting

And I'd need to find some super-stapler that can hold all of those papers together

user218912

binded

Sir Cumference

Fine, I get your point

user218912

so like coil bound.

user218912

3:44 AM

if I remember correctly mine was like 1 inch thick of letter size paper.

user218912

not that big.

user218912

instead of complaining so much just get reading.

user218912

alright good luck, gonna sleep bye.

user228700

4:03 AM

@DHMO: U here?

DHMO

@Kaumudi yes

user116211

@SirCumference You will get set theory as intro in every sub-field.

user116211

@SirCumference :( This is quite less. You can't get a monograph of 50 pages.

user116211

You can go with Apostol if you like.

user116211

But I warn you this is not a book of 50 pages.

user116211

4:15 AM

It has two volumes.

user116211

VolII has an intro on vector calculus and multi-variable calculus. But won't prefer it.

user116211

There are other good books for multivariable calculus. However, that's another story.

user116211

For a quick look/revision, you can check Paul's notes online.

0celo7

4:51 AM

@SirCumference Not one bit.

@MAFIA36790 Why do you need analytic geometry?

user116211

Because we have vector analysis and coordinate geometry in our first semester syllabus.

user116211

I have found a book by Hilbert too; have to check.

0celo7

Huh?

Don't you know PhD level math already?

user116211

@0celo7 But I have to revise and do problems on the topic :(

user116211

It's INSANELY dull and boring.

0celo7

4:57 AM

I don't know analytic geometry

what even is thta

user116211

Basically Euclidean Geometry.

user116211

His axioms and all that.

0celo7

sounds hard

I can recommend diff geo books...

user116211

Then vector equations of plane, skew lines; distance between planes and all that. Then there is conic sections. Then polar coordinates; rotations ; phew...

user116211

@0celo7 ._.

user116211

4:59 AM

@0celo7 I will be eager to read a diff.geo book with you.... but later as I'm quite busy and my hands are not free.

0celo7

@knzhou do you know analysis?

user116211

functional, real, complex, numerical?

user116211

I guess functional analysis.

0celo7

no real

I think there's a mistake (AGAIN) on my analysis homework

I think a function needs to be $C^1$ for something

just differentiable is not good enough.

John Rennie

@DanielSank That is a great video :-)

user228700

5:09 AM

Morning sir :-)

John Rennie

Morning :-)

@DanielSank be a little cautious about using the swearword sod as it can be a derogatory term for someone who engages in sodomy and is therefore not PC. I note the video (wisely) didn't discuss that meaning.

user228700

5:33 AM

@JohnRennie: Very quick question? Smiles sheepishly again (:-P)

John Rennie

Go ahead ...

zed111

Hi @JohnRennie I know critical current in a superconducting wire is given by I=2\pi a B_c/mu_0 in SI units. I am unable to prove the relation in CGS units I = 5 a H_c where H_c is in gauss.

user228700

Let's consider a bomb of some sort travelling along a parabola near Earth, under the influence of gravity alone.

zed111

Ref: open.edu/openlearn/science-maths-technology/…

John Rennie

@Kaumudi OK

user228700

5:36 AM

It explodes at the topmost point into two fragments of masses $m$ and $3m$. The lighter mass just falls vertically down after the explosion where as the heavier one keeps moving.

John Rennie

OK

user228700

I'm supposed to find out how much farther the heavier fragment goes before reaching the ground, as compared to the center of mass (COM) of the system.

DHMO

find the velocity of the heavier mass at that point using conservation of momentum

note that the horizontal component is not zero

but I don't know what the velocity of the lighter mass is at that point

user228700

Since the force that caused the explosion is purely internal to the system, I note that all this explosion doesn't affect the motion of the COM.

John Rennie

@Kaumudi OK. Are you told where in the trajectory the bomb explodes, or do you have to provide a general solution?

user228700

5:39 AM

Hang on, I'm not done yet...

user228700

3 mins ago, by Kaumudi

It explodes at the topmost point into two fragments of masses $m$ and $3m$. The lighter mass just falls vertically down after the explosion where as the heavier one keeps moving.

user228700

^ topmost BTW.

John Rennie

@Kaumudi OK, that makes it easier.

user228700

Yes. But that's not my question...

user228700

At the topmost point, let's consider the COM riiight before the bomb explodes; it has a velocity 0 m/s in the vertical direction and some velocity $V$ in the horizontal direction.

John Rennie

5:42 AM

OK

user228700

And then, an infinitesimal time after it explodes, since nothing happened to the COM, it still has the same parameters of velocity and acceleration, correct?

DanielSank

@JohnRennie so glad you thought so.

user228700

@DanielSank: 'Ello good sir! (:-P)

John Rennie

@Kaumudi Yes

user228700

OK, now let's consider the heavier mass. Practically nothing changed for it too, correct?

John Rennie

5:46 AM

> Practically nothing changed for it too, correct

Well, an explosion happened to it ...

DHMO

explosions cause the kinetic energy to increase

user228700

:-P No, I mean for its parameters of velocity, mostly...riiight, that would've changed.

user228700

OK, I think I understand now.

user228700

Y'all know how dumb I can be :-P

user228700

Thank you :-)

DanielSank

5:49 AM

@Kaumudi 'Ello guv'!

user228700

:-P "guv' "? So u're allowed to address females with that word?

rob

@Kaumudi I'm having the strangest feeling of deja vu ... I think either I or someone in my department used that "topmost explosion, lighter fragment falls" as an exam problem in the last couple of years

user228700

@rob Are u a prof. at IIT?

rob

Not IIT, nope. I wonder how the chain of borrowing goes.

user228700

This was a problem in JEE, a few years ago...

DHMO

5:52 AM

@rob it explodes

rob

@Kaumudi JEE?

user228700