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00:00 - 18:0018:00 - 23:00

12:58 AM
@JohnRennie Well, one could go read about Calabi-Yau space an modular functions right away. Not that it would do much good, though ;D
 
 
3 hours later…
3:54 AM
@123 hi
@AvyanshKatiyar I remember you from PSS room
 
123
@Az
@Azmuth , Hi
 
yo
hi
 
123
Yo
Thanks a lot for all the members of this page. To clear me many many confusions.
If you have time can i ask something. Then i have class.
 
ask
 
123
How can i tell students or this is also in my mind. Like if did experiment we can measure some parameters which decide how to solve the problem. like using energy or newton's law
sometimes we can use both.
Share few examples where we can't use newton's law only energy.
 
4:00 AM
You need questions where we can use Newton's Laws and when we use conservation principles?
 
123
Yes @Azmuth
 
Time period of swinging of a pendulum and time period of rotation of pendulum, both questions, they require different approach
 
123
@Azmuth Okay, but both are mechanical pendulum. We can calculate total time, gravity etc.. for both is there any constraint in solving equations?
 
No, for the first question. Find the time period of pendulum when length is $l$ and gravity be $g$.
 
123
Yes.. $T = 2\pi \sqrt{\frac{\ell}{g}}$
 
4:06 AM
Second question. When rotating a pendulum in circular motion, $l$ be the length, find the angle after which the bob will fall on the ground completing 1/4th rotation.
yep
@123 This doesn't requires energy.
 
123
@Azmuth ok.. first question. It is fine don't required energy
 
yep
 
123
Pls explain rotating pendulum more. what is the problem in it?
 
A pendulum is made to rotate in vertical plane.
Let me show you pictures, (I'd have to search them...)
 
123
Sure..
 
4:10 AM
It is a three page solution, so, I'm adding solution too for your easy reference.
 
123
Thanks i see the solution
pls share
 
just 1 second
uploading.
Download that, because, I've a lot of other files, I may delete later after you download it.
 
i wonder how long it's been since i've used long division
we get taught all these techniques in school, but in reality there's a calculator everywhere
 
@SirCumference Personally, I've never seen a question in 5 years that requires that....
I used looong division, looong ago!
@123 hey! You there?
 
123
yes
 
4:19 AM
download it, I'll delete the link after you downloaded this.
 
123
Downloading
Ok its done
 
start from vertical circular motion section.
 
123
Yes this what is am looking.
They start from work-energy theorem. What is the harm in this to start with newton's law
Also free body diagram.
 
It won't work! You can't get the same results using Newton's Laws.
 
123
OoKay... Is there anything where someone try to solve with newton's law but did not get the result. So, I can compare and see the problem. If possible.
 
4:25 AM
Do you want to know why Newton's Laws are unapplicable here?
 
123
Yes if possible.
 
Because We want to know the angle $\theta$ when $F$ becomes $0$, this means corresponding $a$ should also be 0. We (using elementary mathematical functions) can't calculate $a$ as a function of $\theta$ or $t$ for a body in circular motion when direction of gravity is changing every instant of time with respect to body
That's the same problem keeping us from calculating exact time period of a simple pendulum.
 
123
we have to use calculus here.
right
 
No, you can't. Even calculus is insufficient here, because we lack functions.
 
123
O i see.
A conical pendulum consists of a weight (or bob) fixed on the end of a string or rod suspended from a pivot. Its construction is similar to an ordinary pendulum; however, instead of swinging back and forth, the bob of a conical pendulum moves at a constant speed in a circle with the string (or rod) tracing out a cone. The conical pendulum was first studied by the English scientist Robert Hooke around 1660 as a model for the orbital motion of planets. In 1673 Dutch scientist Christiaan Huygens calculated its period, using his new concept of centrifugal force in his book Horologium Oscillatorium...
 
4:30 AM
No, this isn't conical pendulum either.
 
123
pls see this derivation in this. They didn't use energy for time period.
Okay.
 
You don't need energy for time period.
 
123
This is different?
This one is vertical circular motion. right
what you shred.
 
yes
It is just like a normal pendulum that doesn't oscillates and moves in circular motion.
 
123
Yes i read this problem. Thanks a lot @Azmuth , if possible pls just share the name of problem where newton's law insufficient. I will read myself.
 
4:36 AM
@123 If you want to know if Newton's Laws be replaced with energy, yes, it can be, it's usually Lagrangian Mechanics or Hamiltonian Mechanics (usually taught after Newtonian Mechanics).
 
123
few problem where only energy idea works not newton's law
 
Okay.,,
 
123
I read lagrangian mechanics LM but did not solve too many problem using this. I read functionals to understand LM.
By the way thank you so much @Azmuth for your time. I have class after that i will be available. May be also available on phone.
 
@123 finding escape velocity of planets.
 
 
6 hours later…
10:56 AM
If someone doesn't mind clearing up a small point of confusion, if I write $$A^{[\mu}B^{\nu]},$$ does this notation simply highlight that the resulting tensor is antisymmetric on the $\mu$ and $\nu$ indices or does this mean $A^\mu B^\nu$ is a tensor that we are "antisymmetrising" (in other words we are constructing a new object), i.e.: $$A^{[\mu}B^{\nu]}\equiv\frac{1}{2}(A^\mu B^\nu-B^\nu A^\mu)?$$
 
this is the first time I have experienced daylight saving time - my time was automatically moved back one hour on Sunday.
 
And actually as an additional point, if the second case is true, and we have a tensor that is already antisymmetric, does $A^{[\mu} B^{\nu]}=A^\mu B^\nu$?
i.e. the "antisymmetrisation" does nothing to a tensor that is already antisymmetric
 
here yellow or red maple leaves can be found on the ground everywhere.
 
11:34 AM
@Charlie The second case is true, and yes.
 
@ACuriousMind great, ty!
 
@CaptainBohemian it doesn't take long to get used to the daylight savings time feeling of "it's dark outside already?" Or "it's still light out?" :-)
 
@skullpatrol It's only afternoon 12: 44 here now.
 
I meant at sun rise or sun set.
 
11:49 AM
@skullpatrol sunset but I still feel normal because it's autumn. I guess when winter comes I may feel abnormal that the sun sets at 3 pm - I am not sure because I come to a country with daylight saving time the first time.
here everywhere is red zone but there is still a parade - so absurd!
 
Are you talking about social distancing?
 
no, red zone is the place where the Covid-19 pandemic is very serious.
 
Having a parade isn't a good idea for social distancing either, imho
We shouldn't make too big of a fisimatenten about this.
 
@skullpatrol "Fisimatenten" is plural.
 
ok, thnx for the correction, pal
Either way, different countries are officially effected differently by the COVID-19 pandemic. For example China's economy is posting positive gains in their stock market.
And their president has announced:
yesterday, by skullpatrol
yup, our nation is not only healthy, but we are getting ready to go to war
Meanwhile, in the United States, Texas voters are the first to rush out to vote for
...
@FadedGiant plural only? says Wikipedia
I guess I shouldn't use colloquial words anyway.
 
12:39 PM
After 9:00
Minutes
why does the centrifugal force reaches maximum value when the when the other two small masses have completed π/2 angular displacement
 
it's not fun that all courses are conducted remotely.
 
Are you taking any combinatorics courses?
 
1:06 PM
@skullpatrol no, combinatorics is not used commonly in my physics studies.
 
1:24 PM
 
1:36 PM
0
Q: How should mass specific impulse calculated for relativistic exhaust?

uhohThis answer says: Optimising for Isp only is problematic, as it's simply: $$I_{sp} = \frac{v_e}{g}$$ Which is the same as optimising for exhaust velocity. With no constraints on thrust, particle accelerations can achieve velocities arbitrarily close to the speed of light (The LHC is 3 m/s close)...

needs some special relativity...
@PrateekMourya see the original question that is linked in:
9
Q: Should nonsensical question titles be edited?

LudoRecently this question was posted: Is the ISS a tennis racket? I think the question itself is good, but the question title by itself does not make any sense without context. This makes it useless in search results, which harms the value of the question. I didn't want to go as far as to editing ...

 
 
2 hours later…
3:19 PM
@uhoh No. There is a reason that they are non sensical
 
3:41 PM
Reporting to H BAR : Secret Mission XKCD867-2 beta
 
4:27 PM
-1
Q: Eigen value of Orbital Operator

KieranI read a literature, which discusses the orbital operator. $L^2$ and $L_z$ are square of the orbital operator and the z-axis component of orbital operator respectively. |l> and |m> are the eigen vectors for the $L^2$ and $L_z$ operators respectively. $L_+$ is defined as $L_+=L_x+iL_y$. The litera...

 
123
4:44 PM
Hi all. Yo
 
Seems mean to downvote questions that new users accidentally post on the meta site, they're almost always closed immediately.
 
One of the few cases where I can actually see the argument for why it's mean, but then again it doesn't matter as much since votes on meta carry no reputation and no automatic bans
 
123
What is meta in stackexchange?
 
Where you ask questions about the site itself.
Or discuss it.
 
123
Means in chatrooms
 
4:47 PM
@123 "Meta" is the name for the part of the site where we have conversations about how the site works and what the community thinks about it. Ours is Physics Meta.
 
123
Okayee.. I see
 
Chat is just...chat. It serves no special purpose ;)
 
123
Thanks a lot this group guys for given my questions attention. And satisfy my points and confusions.
Specially energy and inelastic collision tipics
How to properly study LM. I read functional analysis, tenors and basics of LM. is there any special way to learn?
 
LM=Lagrangian Mechanics?
 
123
Yes
 
4:53 PM
I wouldn't say functional/tensor analysis are two topics you immediately need to learn Lagrangian mechanics :P
 
123
I did not solve many problems of LM what is the special benefit of LM over Newton Ian mechanics.
 
We'd also need to know what you mean by "special way to learn"
 
123
I learnt functional and tensor analysis
 
You can derive Newtonian mechanics from Lagrangian mechanics
 
@123 see physics.stackexchange.com/q/15899/50583 and physics.stackexchange.com/q/89035/50583 for discussions of the merits of Lagrangian and Hamiltonian mechanics and the action principle more generally.
 
123
4:56 PM
LM is difficult to learn and it is scalar based
 
"scalar based"?
 
123
Means energy
Based on functionals
 
@123 You can get the gist of LM in a week or two if you try
 
123
Which is still not fully drvelopef
 
You only need some very basic knowledge of functional analysis
 
123
4:58 PM
What is the benefit of LM over NM
 
Makes your life 100x easier in a good number of situations
Only con is it's not applicable to every situation
 
I believe a lot of the appeal comes from the fact that you can use generalised coordinates very easily
 
123
Aah. Okay. At which situation LM don't work
 
Which often greatly simplify calculations in situations where regular cartesian coordinates aren't that useful
 
@123 It's mainly used for reducing problems with constraint forces, in which case we need fewer coordinates. E.g. when dealing with a pendulum, the constraint force means we can just deal with the angle rather than both the $x$ and $y$ coordinates
That's a simple example but in a very good case you can turn e.g. a 100 coordinate system into a 6 coordinate system
 
123
5:02 PM
If we don't use Cartesian coord the actual motion can not be specified. Likrle in configuration spacr
 
Why would the actual motion "not be specified" in general coordinates?
 
Sure it can, if I know the angle of the pendulum I can quickly figure out the Cartesian position
 
123
That's good news
Is there any good book to learn LM efficiently and productively
 
A lot of people use Goldstein's classical mechanics
 
no, all physics is unproductive ;P
 
5:04 PM
:O
 
There's also the fact that QM is based on the Hamiltonian formalism and QFT is apparently based on the Lagrangian formalism
@123 I used Taylor's Classical Mechanics, great book
 
The path integral formulation is done in the Lagrangian formalism
 
You only need 1-3 chapters to learn about LM, depending on how much detail you want
 
I haven't actually spent time consciously learning classical mechanics in a while
 
123
@ACuriousMind Hahaha... Why
;p
 
5:07 PM
He is a purist
 
123
What about Lie Algebra. It is mandatory to understand QFT , LM, HM
 
qft yes, lm hm no
well, I guess depends what you think constitutes "understand"
 
123
and Is Lie Algebra is hard to understand or not?
 
LM and HM probably won't require any new math
 
It's quite a broad topic, physics texts tend to introduce Lie theory in a very non-formal and frankly unsatisfying way
 
123
5:10 PM
@SirCumference That's GooD news for me. It means i can understand LM and HM without learn of new math
 
@123 well it's abstract algebra :P
depends how comfortable you are with that stuff
 
"A Lie group is a continuous group of matrices"
 
123
@SirCumference I am comfortable with math. it is always very hectic to completely understand mew math, need to learn and work on thing
until it is understandable.
 
You can do physics at pretty much arbitrary levels of rigor. Lie algebras and groups are everywhere - rotations in ordinary space are a Lie group after all - but whether it is "mandatory" to look at them as such is pretty much unanswerable
Personally I find that the more rigorous concepts often organize the physics in a way I find both more elegant and better to understand, but essentially everyone seems to have a different level of rigor they are most comfortable operating at
 
123
@ACuriousMind Lie algebra i found scary. What you guys think???
 
5:14 PM
as a similar example, manifolds show up quite a bit in high level classical mechanics and GR
but whether you need to know about them for most purposes depends how deep you want to go
 
I was surprised more of a point wasn't made of introducing manifolds at undergraduate level, seeing as basically everything in physics is done on them :P
 
123
@ACuriousMind My problem is that until i understand the topic in complete rigor in physics way also how the math work to explain the phenomenon. The questions bothered me.
 
@Charlie introducing manifolds the right way requires an explanation of diffeomorphisms and such tho
at that point you might as well throw in two analysis class requirements for the major, to teach about things like measure
 
You could always introduce it at the "physics" level of formality, a lot of information can be learned about manifolds by drawing nice pictures :P
 
they tried to do that with vector spaces in my undergrad QM class
did not work well imo :P
 
5:18 PM
The Lie groups/algebras are just a formalization of what we mean really mean by transformations that can be parametrized. I don't find that scary. But trying to understand physics - the math or the actual physics - "completely" is often just impossible.
Knowledge doesn't come in neat little boxes you can exhaust, it's more like an infinitely deep branching tree - there's almost always something more you can do, some other way to look at it, some approximation made at a prior level you can lift to introduce more complexity.
 
123
I think one life is insufficient to understand Math and Physics :O . . .
 
It is.
 
I think just a conceptual introduction to manifolds, charts, coordinates and tangent spaces is within reach of undergraduates, especially since things GR are pretty standard to teach at that level
 
123
But how the peoples like Gauss, Euler have such level of understanding. Don't know
 
Then again I'm sure you could make a similar argument for most things in mathematical physics, at a certain point you have to leave some of them out
 
5:20 PM
The wonder of the written word is that as a society we can accumulate more knowledge than fits in a single mind. That's a feature, not a bug!
 
@123 Euler was around several hundred years ago
 
@123 They lived at a time when there was much less knowledge accumulated. The increasing specialization as time passes is due to the increasing depth of what is known.
 
It's hard to say what exactly makes "great" scientists like Euler and Gauss great, but I'm very against the proposition that they are magical people born with magical brains
 
if you go back a bit further, then in Newton's day it would only take a year or two to be fully caught up on all known physics and math
 
That's also true, I've heard people say Hilbert was the last mathematician to know all of mathematics, but there was a lot less mathematics around back then
not to say it's not impressive :P
 
5:22 PM
Though to be fair, Euler was the kind of guy to publish way faster than any other mathematician in history. Even after losing his eyesight he said "now I have fewer distractions" and increased in productivity
 
Yeah, hard to say
 
123
@ACuriousMind But they developed many many areas. their contribution unforgettable. specially Reimann and lagrange.
 
I know Terry Tao was similar, just published papers like a printer
is*
 
there might be a hundred Eulers today and we don't look at them as such because they work in specialized areas only other specialists can really understand
 
iirc Euler was publishing something like one paper per week
 
123
5:24 PM
@SirCumference Oooooh I see. You means several centuries ago knowledge is very short.
 
@123 yep, contrast a year's worth of knowledge back then to more than a lifetime's worth today
 
Also worth noting they lived in a world without the endless time-sink that is the internet
might as well do some math :P
 
123
I thought they are more genius than today.
 
the polymaths of old were also often independently wealthy and didn't have to spend any time doing what they didn't want to do :P
 
@123 well to give them credit, they came up with ideas no one in the history of our species had ever done before
but nowadays things are a lot more complicated and in depth
 
123
5:27 PM
Why the euler's today don't share their knowledge to the people.
This is the era of internet youtube. Why they don't contribute.
 
Not everyone who is capable of being a world-leading mathematician is interested in being one
 
@123 I don't know why you think they don't. The arXiv is full of knowledge freely shared, for example.
 
123
It is way more better to publish the book only to teach their books on internet via videos.
 
well i'm inclined to agree journal paywalls are anti-science
if people are funding research through taxes, they deserve access to the results
 
@SirCumference I'm not sure that's the point being made here :P
 
5:29 PM
@ACuriousMind not sharing knowledge
maybe it's just an excuse for me to rant about something i hate tho
 
heh, I get you
 
123
People should have videos to teach the complete book.
 
What is "the complete book"?
 
Not everyone who is a great scientist is a great teacher/lecturer (and I personally detest videos as a form of transmission of scientific knowledge and would much rather read a book/article)
well, "detest" is a too strong word perhaps, they have their uses
 
i think especially for math, higher level materials translate themselves better to books
 
123
5:32 PM
Whatever book to be teach in universities, schools. That should be in video format also their solution.
 
What do you think of the Feynman lectures/book form of the lectures?
 
a lot of symbols that need to be read
which is why you'll rarely find videos on that stuff
 
123
People understand way more better in videos rather reading.
 
@123 depends on the person
 
That certainly depends on the people
 
5:33 PM
i'm totally the opposite
 
I don't know whether it's considered a useful classification but I've heard of "visual learners" vs "auditory learners" etc.
 
123
Think about student's. They are always learning new topic everyday.
 
i mean i'm a student and again, i've only been able to learn via text
more straight to the point than videos imo
 
123
how can possibly they learn the topic just only with book. Which is entirely new knowledge for them.
They don't fully understand book. They have a lot of lot of questions.
 
you can't ask a video questions, either :P
 
5:35 PM
Yeah I only really feel like I learn anything when I'm able to apply it. I basically have to read books and then do problems on the topic to actually learn. Videos aren't as easy to flip back to previous references and stuff, it's just not the same. Good visuals can be nice, but book format is definitely more helpful for me.
 
i judge the quality of a book by how well it can predict my questions and answer them in advance
which is why i love Taylor and Griffiths, they're really good at that
but in worst case I just use the internet to ask online
 
123
@ACuriousMind But can't ask question with books.
There is always a way of communication with the teacher.
This is not possible with the book
 
we have a whole stack exchange for physics questions :P
 
@SirCumference and a chat room :-)
 
indeed
 
5:36 PM
I've never realised that your name is in italics John
 
@123 Again, I don't know how you're communicating with a youtube video, either. This isn't an argument for books vs. video, this is an argument for active interaction instead of passive consumption
 
123
I am not saying leave the book. I am saying only book reading is weak procedure to learn new topics. There must be video format with super high teachers specially authors.
 
That's something I liked about school. If you had a question, there was at least someone who was supposed to know the topic well who you could ask for help or advice or clarification or whatever.
 
"There must be video format with super high teachers" is almost never the case
 
I don't think the teacher should be high :P
 
5:38 PM
@JMac in a good school maybe
in my QM class the professor and TA were completely MIA
 
@Charlie room owner's names are in italics.
 
Books often have the advantage that every word can be written carefully with intention, in unscripted videos there's often a lot of fumbling for the correct phrase etc.
ah @JohnRennie
 
123
Everyone knows what i meant to say :p
 
@ACuriousMind There was a (pretty believable) rumor my calculus teacher sold pot and he was pretty on the ball :P
 
123
my english is bad.
 
5:40 PM
again i really think it's a matter of how the person learns
some profs are under the impression that in person learning is strictly better than textbook learning, and therefore make attendance mandatory
i hate that crap
 
Yeah same
 
123
@Charlie Yes. But i am not saying book reading is not helpful, it is second stage. Video is the first and more understandable stage for the students.
 
That just encourages people to think up creative ways to fake attendence :P
cf. my chemistry degree
@123 Again you're assuming that everyone has the exact same experience of learning as you
 
I had one class where I was super glad it wasn't mandatory. I realized in the first few weeks that the lectures overlapped completely. Half of each lecture was covering the second half of the previous lecture, and the other half covered the first half of the next lecture. It was first class in the morning so I slept in for every second class instead of going.
 
123
@Charlie If it is not . why youtube has million trillion views. It is proved. In my point of view. Every video should have to enough to cover the entire topic.
 
5:43 PM
@JMac i've had 9am QM lectures with mandatory attendance, completely ruined the rest of my study day
who is supposed to learn physics first thing in the morning?
 
123
Specially i am in favor of using AI in classroom and as well in videos.
To teach the topic.
 
AI is probably a lot stupider than you think it is :P
 
@123 you're bringing up several different things, we've went from videos > in person > AI :P
 
Yeah mornings were bad for me too. One of the only mandatory attendance classes at my school was a 3 hour lecture once a week from 6:30 PM to 9:30 PM. That was pretty weird.
 
@123 Youtube has lots of views, a lot of those videos aren't attempting to teach anything. This isn't really evidence that video learning is inherently better for all people than books
 
5:44 PM
^, especially for math which has a lot of symbols
 
123
@SirCumference :O
 
My organic chemistry professor once did a count up of all the people in the lecture and compared it to the attendance list, reported those names that weren't in the class :P
 
123
No that's not the case
 
people stopped signing eachother in for a bit after that
 
123
I teach my student's with animations , creating simulations and i found it is more useful and easy to learn by students
 
5:46 PM
But not every single student though, that's the point
 
First year for a mechanics of materials class, some people really struggled with it, but it clicked easily for other people. Supposedly one day after a midterm when me, several of my friends, and apparently a bunch of other people skipped class, the prof read off the names of everyone with over an 80% on the midterm to show they paid attention in class. Many of the people weren't there lol
 
123
@Charlie :P i am not against book reading ;-)
 
I'd rather read a good set of lecture notes than an unscripted video lecture, especially the ones where they write by hand the lecture as they go
 
@JMac that can't be ethical
that's how you make some students the enemies of others
as far as envy goes at least
or how you make some students feel like idiots in front of the class. you'll know someone who wasn't mentioned is getting ≤70s on the exams
 
123
I found most of the lectures insufficient and useless in youtube. Why Book authors and other professors not on the youtube.
 
5:50 PM
because books might just be a better medium for math?
 
It was definitely stupid. I think the prof was frustrated because people weren't learning it as fast as he wanted; but really it was the type of class that was confusing unless it clicked for you. We had a similar thing happen second year when a professor thought we were all stupid because the average on a midterm was like 25% since he didn't prepare us for the type of questions asked.
 
123
@SirCumference I am not against books. But it is second requirement.
 
@123 again you're assuming everyone learns the same way as you :P
for me books are the only way I can learn
 
123
@SirCumference :O No,
 
and in math, there are a lot of symbols and notation, so videos really wouldn't help
 
123
5:52 PM
But this is the easiest and fastest way to learn anthing.
 
@123 to whom?
definitely not to me
 
123
@SirCumference Use AI to animation and create. AI has no boundary no limitations.
 
@Charlie Back in school we had a substitute teacher who tried to get an attendance list (since he didn't really know us) and got a list with a bunch of names hormone-addled not-quite-adults find funny (the esteemed Enis, Peter was present 3 times). He stuffed the list into his bag (still a bit sticking out) and ran out to complain to the head master about this...and one of the pupils he went past plucked the list out of the bag without him noticing.
He came back after 10 minutes and just sat down at the front desk, said nothing and ignored us for the rest of the lesson.
 
@123 AI has no limitations in theory, but it's definitely not a substitute for human teachers yet
 
123
@SirCumference Yes i want that teacher on internet in videos. That's my point
Because book didn't clear topics in depth. There is always unanswered question.
 
5:57 PM
Videos also leave unanswered questions, no medium can predict and answer every question you'll have on a topic (besides a psychic medium:P). I find books a lot better, because I can go back and forth through the information at my own pace without having to time my learning with a video.
 
depends on the textbook
a talented teacher will predict those questions and answer them in advance
most of my favorite books do that
 
"Unanswered question" here can also just mean you didn't fully understand what the book/video was trying to tell you. Misunderstandings happen, nothing can prevent that entirely.
 
123
@SirCumference Agreed. But that good teacher should on internet friends.
 
@123 i think you need a verb in that second sentence, not sure what you're saying
 
123
Which verb?
 
5:59 PM
@ACuriousMind That's a big reason I like books. If I still have questions or confusions I can read specific parts and go between them easy, digesting it at my own pace. Videos are harder to navigate, and you have to basically go at the pace of whoever is explaining it.
 
123
Good teacher?
 
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