The h Bar

Just win baby

01:50

8

0celo7

@ACuriousMind ok the witches quest was pretty horrifying

or...ladies

Secret

02:18

The integral with many logs

Dawood ibn Kareem

03:53

@Justwinbaby So you've worked for IBM too then?

Las Vegas Raiders

04:04

I'm not quite sure that is the path that physicists imagined for the "heat death of the universe." :P

Epari shalini

04:29

Does anybody know what is the main annihilation mode of neutrinos? I am very new to this area and am unable to find the information

sorry i meant neutralinos

The info i found said that the branching ratios are model dependent, so most people use b-bbar and w+w- as representative of their results

i want to look at mssm only

Mann

06:50

0

Q: Why Lagrange multiplier is dependent upon time only?

Book : Classical Mechanics - An Introduction by Dieter Strauch pg 63,64,66,84,85 Consider a particle under motion with a constraint $G=0$ . The Virtual displacement $\delta \vec r$ will then be oriented tangential to the sheet described by the constraint $G=0$. The forces of constraint act on th...

I am not able to understand this lagrange's multiplier can anyone help me please.

On the one hand, some book says it doesn't depend upon generalized coordinates and on the other hand goldstein say it's a simplifying assumption. And in the answer given to to a duplicate, i am not able to understand it.

Slereah

Let's read that Carter paper on causality

user228700

07:05

@JohnR: Oh, you did watch the video!

Las Vegas Raiders

How long is it? @Slereah

user228700

@JohnR: Is it really not mentioned in the books?

Slereah

40 pages or so

John Rennie

@Kaumudi.H The first description of how horcruxes work is in The Half Blood Prince, when Slughorn and Riddle are talking. I can dig out the quote if you want, but there is no mention of the soul splitting in half.

Balarka Sen

i thought soul splitting was part of the creation of horcrux?

isn't that a standard in world fact

user228700

07:12

Hmm, yes, I've been reading about it here and this is what I have found:

user228700

> To create a Horcrux, a wizard first had to deliberately commit murder. This act, said to be one of supreme evil, would result in the murderer metaphysically damaging their own soul. A wizard who wished to create a Horcrux would then use that damage to their advantage by casting a spell which would rip the damaged portion of the soul and encase it in an object.

Las Vegas Raiders

Are "souls" divisible?

user228700

the damaged portion may not be a proper half of their soul. Hmm, but how long before there is nothing left? How does one know when to stop? Didn't Voldemort kill thousands of people?

Sid

My Mom today morning: "Go to the saloon. You look like you have come straight out of a forest.

Balarka Sen

That's fine though

it's a critique of postmodernist materialism

Sid

07:15

Me after coming from a saloon, My Mom: "Why did you shave everything? Now, you look like a kid". Me: facepalm

user228700

Haha :-)

Las Vegas Raiders

lol

user228700

I remember having read somewhere else that in addition to killing people, one has to eat human flesh as well, to create a horcrux but I doubt if this is mentioned in the books.

Balarka Sen

irrelevant, but: this is a fantastic song

John Rennie

@Kaumudi.H Presumably Riddle's plan was to split his soul in seven, so he knew in advanec that one seventh of his soul would go into each horcrux.

Though we are assuming that souls divide like material objects and that may not be a fair assumption.

Las Vegas Raiders

07:17

A "saloon" is for drinking. @Sid

Sid

duh. Salon and saloon are too similar to know the difference

user228700

@JohnRennie Right, right, but the problem is that he did kill thousands of other people and surely, there was a way to ensure that his soul didn't keep on getting damaged to the point where he died...Oh OHH, so he did die when he killed Lily but even after he came back in GOF, noseless and all, he must've killed others, no?

John Rennie

@Kaumudi.H: but I doubt JKR cares too much about the mathematical details of dividing souls. We're probably reading too much into this.

user228700

I'm sure that JKR once tweeted something about flesh eating and horcruxes because I remember having seen that in a video...

John Rennie

According to the book killing someone damages your soul. I guess that's just a moral comment on killing people. But that doesn't make a horcrux. Making a horcrux involves siphoning off damaged bits of your soul.

So you must kill to make a horcrux, but killing doesn't automatically create a horcrux.

Las Vegas Raiders

07:21

Dividing by zero must be avoided in literature also.

3

user228700

AHHH, right, right. And this siphoning off has to do with flesh eating I think.

Abcd

user228700

@Balarka: How long did it take u to read Notes from Underground the first time?

John Rennie

@Kaumudi.H I've never heard that you had to eat bits of your victim to make a horcrux.

Abcd

Please help me with the question.

Sid

07:23

@Abcd That looks like HCV's book

John Rennie

Perhaps making horcruxes is hard work and makes you peckish.

Abcd

@Sid That's exactly what I am solving

user228700

@JohnRennie I'm sure JKR tweeted something about this; it's definitely not in the books though.

Sid

@Abcd What have you done so far?

Abcd

@Sid You are done with JEE?

@Sid All the questions before this one (except a few which I couldn't solve)

Sid

07:24

Yep. I am glad that thing is over

Balarka Sen

@Kaumudi.H I read it in one go. Once I started it started to sort of pull my consciousness.

Abcd

@Sid I am 2019 aspirant :(

Sid

No, I mean what have you done with the said question

Abcd

@Sid Rank?

user228700

@BalarkaSen Right, but how long did it take you?

Sid

07:24

20k. Lol

Balarka Sen

approximately 3 hours?

Abcd

@Sid I tried various things but couldn't land anywhere around the answer.

Sid

So, what various things have you tried?

Balarka Sen

I am a madman though. For normal people I suppose the first bit (Underground Man's journal) would be disturbing and the second bit (Apropos of White Snow) just unbearable to read.

Abcd

@Sid It's okay. Any tips for me?

Sid

07:26

@Abcd Don't give JEE too much respect. They don't deserve it.

user228700

@BalarkaSen Wow, I see.

Abcd

@Sid Created the diagram of velocity vectors. Tried to find theta (angle of velocity vector) Found displacement in y direction

user228700

@BalarkaSen Right, right. I think I'll definitely start it sometime today.

Balarka Sen

It's a rather gripping story, so there's that.

Excellent! Let me know what you think of it

Abcd

@Sid Why?

user228700

07:27

I found this on the flesh eating thing.

Sid

@Abcd You will realise soon.

user228700

@BalarkaSen Certainly.

Sid

Anyway, let's look at that question

Mann

I am already 3rd year student in IITKGP xD

And i agree don't give too much respect.

Balarka Sen

"press F to pay respect"

5

Abcd

07:28

@Sid Ok. Just a question: Are you done with full HCV?

Mann

There are better places to go. If you are more academically oriented,

Sid

@Abcd Don't think I ever did it completely. I think I did a fair bit in Class 11. In 12th, everything went haywire.

@Abcd Could you show your work? Or the diagram?

Abcd

@Sid There's lot of untidy stuff on the page. You won't understand.

Mann

Try anyway

user228700

@JohnR: I must admit that I understood little to nothing of the cannibalism theory.

Avantgarde

07:32

@Balarka Bowie again! I thought you had come up with another artist

Las Vegas Raiders

He's a Bowie groupie now :P

Sid

@Abcd Still, show the work. I don't want to do your homework for you.

Balarka Sen

I am an avid Bowie fan, what can I do :P

Avantgarde

It's nice, but you need more artists

:D

Balarka Sen

@LasVegas Well, for one, I'm into Byrne and Eno now

Talking Heads and post-Talking Heads

Las Vegas Raiders

07:34

Try Floyd

Balarka Sen

No

Las Vegas Raiders

:(

Balarka Sen

2popular4me

Las Vegas Raiders

Led Zeppelin?

Balarka Sen

America is waiting - David Byrne & Brian Eno

►

Quality content

user228700

07:36

Did anybody read that theory?

Avantgarde

Never heard of it Kaumudi

user228700

@Avantgarde Of what?

Avantgarde

cannibalism theory

user228700

Dude, no, I mean this.

Avantgarde

Oh I don't know anything about harry potter either.

user228700

07:38

tl;dr - Voldemort eats people to connect his soul to theirs so their soul rips his out on their way to the afterlife

Abcd

@Sid See the scribbling:

John Rennie

@Kaumudi.H: I can't find any comment from JKR on this.

user228700

No, nothing from JKR; it's just a fan theory. What do u think tho?

Abcd

@Sid You won't understand anything from this scribbling therefore I wasn't uploading :( . Moreover, I never post questions without trying at least 5 times !

Avantgarde

@Balarka You might like Asia: youtu.be/E-ALlRGynTE

John Rennie

07:41

@Kaumudi.H Making up theories is an entertaining pastime and I don't want to wee on any bonfires ...

user228700

Hmm, alright.

Las Vegas Raiders

You're FLOODING the chat with page images @Abcd

Abcd

@LasVegasRaiders Sorry. He wanted to see what I had tried.

John Rennie

@LasVegasRaiders Abcd is asking about physics, which is after all the main point of this chat.

user228700

^

Balarka Sen

07:43

@Avantgarde Checking out.

Las Vegas Raiders

I never said it was wrong

Sid

@Abcd I probably understand what you have written there

Your first equation is correct.

Las Vegas Raiders

It's a matter of presentation :-)

Sid

For the second equation, look along Y-Axis. What is the displacement and what is the relative velocity?

Abcd

@Sid Which is my first equation?

user228700

07:44

@JohnR: Remember that theory about Fawkes being Dumbledore's horcrux though? This is what JKR said to that:

Sid

250=150Cos(theta)*t

Abcd

ok

user228700

Abcd

@Sid Displacement is 500 root 3/ 3

Sid

If you want to make theories, you should have done that before Season 4 of Sherlock. There were all kinds of theories.

@Abcd root 3 by 2.

Balarka Sen

07:47

I don't know a thing about Sherlock. Probably for good.

Las Vegas Raiders

Did you know Twitter is blocked in China? @Kaumudi.H

Sid

(And I am personally happy that I got at least one thing right in the whole season of Sherlock. That there was a Holmes Sister and not a brother)

Balarka Sen

@Avant Hey, that's not bad

I like it

Abcd

@Sid okay so 250 root 3

user228700

@LasVegasRaiders Wut? No!

Avantgarde

07:48

@BalarkaSen :) The members are from various prog rock bands. Glad you like it

John Rennie

@Kaumudi.H for Dumbledore to want to siphon off his guilt at (possibly - he doesn't know for sure) killing his sister would be an act of gross irresponsibility. Dumbledore lives with the consequences of his actions, as should we all.

Abcd

@Sid velocity along Y axis is 20 m/s + 150 sin theta

user228700

Yes, I completely agree.

Avantgarde

twitter, among many others, I think, are off in China

Las Vegas Raiders

Yup. That's what I heard.

John Rennie

07:49

@Kaumudi.H I'm not surprised that JKR wasn't best pleased at the suggestion.

Sid

@Abcd Correct! Now, equate that with displacement. Divide and remove the "t". You will get a probably ugly value of theta.

Abcd

Let me try

user228700

Put it that way, it's quite beautiful, how those fragmented pieces may be the causes of guilt among us, how...yeah, that's quite beautiful and I don't know how to put it into words.

Abcd

@Sid I can't remove the 't'

John Rennie

@BalarkaSen for those of us old enough to have listened to Yes albums when they were released, the band Asia is a sad pastiche of the glory that was once Yes.

Abcd

07:51

(20 + 150 sin theta) t = 250 root3

Balarka Sen

I dunno why but in-world theorization about many of these fantasy literature makes me cringe a bit

Sid

@Abcd Divide that by the first equation

Balarka Sen

@JohnRennie Huh. I have actually never listened to Yes.

Sid

t will be removed. Then, solve for theta

Las Vegas Raiders

Try it.

user228700

07:51

@BalarkaSen Most things make u cringe -_-

Las Vegas Raiders

lol

Balarka Sen

The good things don't

for an appropriate definition of good

John Rennie

@BalarkaSen if you're interested (Yes isn't to everyone's taste) try listening to the albums Fragile and Closer to the Edge first.

user228700

> for an appropriate definition of good

user228700

My point exactly.

Abcd

07:52

@Sid There are cos and sin together I can't solve for theta

Balarka Sen

@JohnRennie Thanks a lot, I'll put it up in my list

Abcd

@Sid I got this equation: ( 20+150sin theta)/ 150 cos theta = root 3

John Rennie

@BalarkaSen I'm sure they're on YouTube somewhere ...

Sid

That's the point

Abcd

@Sid ?

Sid

07:54

I think cos and sin together are solvable

user228700

My dad's urging me to participate in this TOI writing thing, BTW.

Abcd

@Sid sin theta = cos 90 - theta

Sid

@Kaumudi.H What do you have to write about?

user228700

You've gotta write a short story about pretty much anything, I assume.

Sid

Yep, put Cos= Root(1-Sin^2 (theta))

Balarka Sen

07:55

Are you given the topic or do you cook up a topic

user228700

This

user228700

@BalarkaSen The latter.

Balarka Sen

Ok, great. Use the cut-up technique.

user228700

I'll probably get caught for that one.

user228700

I suck at writing short stories though.

Balarka Sen

07:57

caught? what for?

Sid

how short?

Balarka Sen

it's a well recognized and well admired technique

i can write you a great pomo shit if you give me a day and money

Abcd

@Sid can't solve it

Sid

What's your final equation?

user228700

@BalarkaSen Oh, you were suggesting I write something first and then use the technique. Nice, but that's just double the work and I already suck.

Balarka Sen

07:58

you write something which sucks, and then cut it up to make it great

2

Abcd

@Sid 4 + 228 sin^2 theta + 60sin theta = 3 -3 sin^2 theta. Can we be quick? I gtg

Sid

Put Sin theta=t. Solve the quadratic equation

You know the formula of quadratic equation, right?

Abcd

@Sid Yes

user228700

@Sid Dunno. Haven't looked into it yet...

Sid

you will get theta. Then, find t.

Abcd

07:59

@Sid Is my equation corrrect?

user228700

@BalarkaSen Hmm, I'll think about it but...yeah, thanks, I'll try.

John Rennie

@Kaumudi.H write a story about a student who discovers an old manuscript that explains how you can guarantee a high JEE ranking by eating your grandmother.

Sid

@Abcd I am counting on it being correct. I haven't solved it myself

Balarka Sen

i was honestly joking. don't use the cut up technique if you want to write a coherent story

Las Vegas Raiders

Why not?

user228700

08:01

No, I really do want to try using it but I don't know how it'll turn out and I'm a little apprehensive about submitting a proper one, let alone something I experimented with.

Las Vegas Raiders

Just put coherence between the "cuts."

Balarka Sen

do you get money for publishing this?

user228700

Oh. OHHHHHHH.

user228700

Yeah, no, chuck everything I said so far.

user228700

> Write India is a collaborative writing initiative by Times Internet, which will have a set of 11 celebrity authors writing the preface of short stories. These prefaces will be later opened to public for completion. Every month we will select one winner from the entries received with the help of the 'Celebrity Author of the Month'.

Balarka Sen

08:03

meh

Sid

Boo..

Las Vegas Raiders

^

user228700

Why? That does sound interesting, seeing as Ruskin Bond is on the list...

user228700

Ruskin Bond being on the list is terrifying, not comforting but still.

Balarka Sen

Oh, I mean, go ahead.

It's just not my kind of thing.

Las Vegas Raiders

08:05

So they're giving you an outline to follow.

Balarka Sen

It'd be amusing to see a totally obscene and nonsense passage following after a Ruskin Bond outline though

Las Vegas Raiders

Less cutting.

Balarka Sen

probably would not be selected by times of india

Las Vegas Raiders

Sounds like good practice

Go for it!

user228700

08:08

I will. I'll go for Jeffery Archer (although, maybe not), Ruskin Bond, Twinkle Khanna and Sudha Murthy.

user228700

The trick with a short story is not to try too hard, which is why I suck.

Las Vegas Raiders

Just have fun with it, no pressure.

Sid

@BalarkaSen That would be so awkward..

user228700

@BalarkaSen I agree with both the things you said :-P

Balarka Sen

Let me find my favorite passage from Naked Lunch.

Here you go, but NSFW

John Rennie

08:14

@Kaumudi.H I knew Jeffrey Archer's wife (not in the biblical sense)

user228700

@BalarkaSen What's with all the digging?

user228700

@JohnRennie Wut, really?

John Rennie

@Kaumudi.H yes, she was a lecturer at the Chemistry Dept when I was doing my degree there.

user228700

Oh, wow, nice!

Balarka Sen

He's just talking. "You dig" means "you get it?"

John Rennie

08:16

She used to supervise students from my college in electrochemistry.

user228700

@BalarkaSen Isn't there supposed to be a comma or something there, then? OK, sorry, not the point :-P

user228700

@JohnRennie Ohh, nice, I see!

Balarka Sen

Comma? Comma? You're reading the first ever postmodern book!

John Rennie

@Kaumudi.H she is the quintessential sophisticated English lady - we were all in awe of her! :-)

user228700

@JohnRennie Haha, I see! :-) This Jeffery Archer guy is old huh?

user228700

08:19

@BalarkaSen :-/ Right.

Balarka Sen

The first line of the book starts with saying he wrote most of the book while high on cocaine and nuked out of the orbit on morphine

and doesn't even remember writing most of it

user228700

That explains things.

John Rennie

@Kaumudi.H Both of the Archers are 20 years older than me give or take a few years

Balarka Sen

It's friggin brilliant

user228700

@JohnRennie WHAT?! Oh, wow, that is old.

John Rennie

08:20

I'm going off you :-)

Sid

JR isn't old. He is wise. :P

Balarka Sen

Guest Performer: William S. Burroughs - Saturday Night Live

►

I think it's from Nova Express but another good paragraph

user228700

@JohnRennie Huh? (I'm finding it difficult to parse this sentence)

John Rennie

@Kaumudi.H English idiom. To go off something means you used to like that thing but are beginning to dislike it.

user228700

Oh. OH. Sorry :-P

John Rennie

08:24

Your English is so good I keep forgetting it's not your native language!

Better than my Hindi anyway.

user228700

@JohnRennie Haha, do u know any Hindi at all?

John Rennie

I know some Urdu swearwords that my father taught me, but that's all.

user228700

Ah, swear words. I bet they're the first words that anybody learns in a new non-native language.

John Rennie

:-)

user228700

@JohnRennie It'll give u a jump to hear me actually talk, then, in my accent and all.

Sid

08:26

Yeah, I was surprised when someone on Puzzling mentioned the Hindi word for Prostitute

Balarka Sen

Grr, I'm going down the deep end of William S Burroughs literature again. Brilliant stuff but I need to get work done

user228700

Toodles!

Sid

TIL that Osama was an engineer.

Slereah

obviously not a very good one

Not very lucky with buildings

Mann

08:52

Why is friction causing problem? Can't we just consider it in applied forces itself?

And get rid of the problem that net virtual work of constraint forces is non-zero.

Abcd

09:26

@DawoodibnKareem I didn't understand your method

user029857

09:46

How can i create physics problems to solve.

Can you give me some examples.

John Rennie

@user029857 Use one of the many physics problems books. Irodov is probably the most popular example.

user029857

Are the problems close to real life

John Rennie

@user029857: I prefer to keep discussions here in the main chat room.

user029857

ok

sorry for that

I like to create my own models for problems then test if my models are consistent with nature

I mean thats the point

Slereah

The problem with Carter's paper is that he uses that old timey notation for causality where $A \ll B$ means $I^+(A) = B$

John Rennie

09:50

@user029857 yes, the problems are pretty realistic. They are designed to train young physicists in the sort of problem solving techniques that they need if they plan to work in physics as a career.

Slereah

a bit annoying to translate

user029857

thanks

@JohnRennie

John Rennie

@Abcd: I think I have a really simple way to solve your problem if you're still interested ...

Abcd

@JohnRennie I am interested

user029857

@JohnRennie Also i know most of classical mechanics like % 90 percent

Abcd

09:52

@JohnRennie i have Irodov. Do you reccomend solving it?

user029857

which book do you prefer for qm

and gr and qft and qcd

John Rennie

@Abcd if you're studying for JEE then stick to the texts your teachers recommend. Irodov is excellent general training for physicists, but your aim will be to get a good rank in the JEE rather than general training.

Abcd

@JohnRennie Irodov is the most popular book among JEE aspirants. Any other tips for JEE? That'll be a big help

John Rennie

Irodov is the most popular book among JEE aspirants - ah, I didn't know that. Irodov is kind of a standard as in if you want a book of problems Irodov will be the first book mentioned.

If you can do all the problems in Irodov then you're pretty good at physics :-)

Abcd

Oh.

Slereah

09:57

Causality stuff has a lot of relation equality, which is fairly confusing

John Rennie

Plus there are lots of web sites with solutions for the Irodov problems. so if you do get stuck you can easily get help with solving the problem.

Slereah

Like $\ll \subset \leq$

Abcd

@JohnRennie Yes.

@JohnRennie I get stuck on HCV problems so I don't think I should think of solving Irodov.

@JohnRennie You were going to tell your interesting method to solve the problem?

user685272

finished the paper already? :^)

John Rennie

@Abcd give me a moment to sketch a diagram. I've just realised my method might might not be as useful as I thought.

@Abcd: OK, have a look at this diagram ...

On the left is the problem as stated. The line from A to B is 30º east of north. and the wind is due north. OK so far?

Abcd

10:08

@JohnRennie yes

John Rennie

OK. What I'm going to do is rotate everything 30º to the left. Then work out the required velocity, then at the end we just have to rotate everything 30º to the right again to get the final answer. Does that all make sense?

Abcd

@JohnRennie This is the first time I will deal with rotation while solving a problem

John Rennie

Shall I explain how it works, then you can see if you think it's simpler or not.

Abcd

@JohnRennie yes, but briefly

John Rennie

OK. If you take the diagram on the left and rotate everything 30º then you get the diagram on the right. Does that makes sense so far? The wind is now rotated so it is 30º west of north and the plane's velocity is now due north.

The red line shows the velocity we have to calculate to compensate for the wind.

Kenshin

10:14

@JohnRennie if I create a computer simulation that models particles using all three of Newton's laws, will the computer accurately model classical mechanics?

@JohnRennie, i.e. are the three laws sufficient or do I need to invoke other things like energy etc.?

John Rennie

@Kenshin yes, though this is harder than you probably think. You need a symplectic integrator to get accurate results.

Abcd

@JohnRennie didn't get this

Kenshin

@JohnRennie ok so John does this mean conservation of lienar momentum and consaervation of angular momentum are both consequences of newton's laws?

John Rennie

@Kenshin yes

Kenshin

@JohnRennie is it possible then to derive the conservation of angular momentum? I've never seen this done

John Rennie

10:16

@Kenshin that's what Newton's third law is ...

Oh hang on, angular momentum

Kenshin

yea

John Rennie

@Abcd suppose you are standing facing due north. Then the plane is heading off at 30º to your right and the wind is blowing straight ahead. Yes?

Abcd

@JohnRennie yes

John Rennie

@Abcd Now suppose you rotate yourself 30º to the right. Now you are looking directly at the plane and the wind is 30º to your left. Yes?

Slereah

John Rennie

10:22

@Abcd: maybe this is too difficult a method for now ...

user228700

Yo @Mithrandir24601/anybody else who has played Portal (1). Do u remember this door on level 19:

Abcd

@JohnRennie yes

user228700

Abcd

@JohnRennie yes probably

user228700

Isn't it supposed to open up? It doesn't for me! PANIC!

John Rennie

10:24

@Abcd OK, we'll abandon this for now. Ping me if you want to return to the idea later.

Kenshin

wat game is dat

John Rennie

@Kenshin Portal v1

@Kaumudi.H: there are Portal walkthroughs on YouTube. I watched one of them for a bit to get an idea of what was in the game.

If you jump to the end it will show you how to get through the door.

Kenshin

@JohnRennie so is angular momentum conservation in classical physics an emergent property of Newton's 3 laws?

John Rennie

I don't know offhand. Yes, it must be but I can't remember how to prove it.

Secret

Is torque also part of newton's laws or it is treated separately?

user029857

10:37

How can i calculate the time it takes for an object to fall to the surface of the from a height H

Assume that the acceleration increases by time

Kenshin

@Secret well if it's not then you must acknowledge that Newton's laws alone aren't enough to simulate particles

But surely Newton's laws are enough to model particles

I don't know of any particle-particle collision result that can't be explained using Newton's laws

Secret

right. In that case apply newton's 3rd law to a pair of torque similar to the linear case to get angular momentum conservation, I guess

Kenshin

@Secret yes is this possible? If so are you able to show me? I'm happy to pose the question on the main site

John Rennie

0

Q: Time to fall in a non-uniform gravitational field

In an uniform gravitational field the loss in potential energy (the work done) by gravity is given by $\text{-}mg\Delta h$ and the time to do so (assuming no friction and no initial velocity) is given my $\sqrt{\frac{2 \Delta h}{a}}$, where $a$ is the strength of the field as commonly referred to...

homework-and-exercises newtonian-mechanics newtonian-gravity orbital-motion

user029857

thankz

ACuriousMind

10:40

@Kenshin Yes

Kenshin

@ACuriousMind are you able to assist me in deriving conserviation of angular momentum from Newton's laws?

ACuriousMind

Able, but not really willing :P I consider the Newtonian formulation cumbersome and pretty uninteresting theoretically

Secret

$r \times F = L$

$r \times F - r \times F=\Delta L$

$0=\Delta L$

$0=\dot{\Delta L}$

this looks really problematic...

Kenshin

@ACuriousMind but you have done it before?

ACuriousMind

@Kenshin I'm pretty sure we did it in my Newtonian mechanics course, yeah

Then we went on to Lagrangian mechanics and Noether makes all these conversation laws so much better

@JohnRennie How has that one not been closed as homework-like? Am I missing something?

AccidentalFourierTransform

10:44

$\dot{\boldsymbol L}=\boldsymbol r\times\boldsymbol F$

Kenshin

0

Q: Deriving Conservation of Angular Momentum from Newton's Laws

Given that a collection of classical particles can be modelled using Newton's three laws, it must be the case that both the conservation of momentum and the conservation of angular momentum are emergent features of Newton's three laws. I can easily show the conservation of linear momentum from t...

classical-mechanics angular-momentum

^my question for you guys to show ur support

I know alot of ignorant people are going to downvote saying it is impossible

AccidentalFourierTransform

$\frac{\mathrm d}{\mathrm dt}\boldsymbol L=\frac{\mathrm d}{\mathrm dt}\left(\boldsymbol r\times \boldsymbol p\right)=\dot{\boldsymbol r}\times \boldsymbol p+\boldsymbol r\times \dot{\boldsymbol p}=\boldsymbol 0+\boldsymbol r\times\boldsymbol F$

ergo $\boldsymbol L=\text{const.}\quad\Leftrightarrow\quad\boldsymbol r\times\boldsymbol F=\boldsymbol 0$

ACuriousMind

@Kenshin Now what makes you say that? (Also, if you had searched the site a little more, you'd have found what I just commented there)

John Rennie

@ACuriousMind I wouldn't vote to close it. While it seems obvious to us to integrate $\ddot{x}$ it isn't obvious to new students. So the question has conceptual elements in that to solve it you need realise that integrating the equation of motion is the required approach.

ACuriousMind

@JohnRennie ...that would be an argument to consider all HW questions conceptual is obviously the required approach is not obvious to the person asking it, else they wouldn't be asking about it.

Kenshin

10:50

@ACuriousMind look at the first comment, he's using the "angular equivalent of newton's law" which is not one of newton's three laws

ACuriousMind

@Kenshin But it is derived from Newton's laws, i.e. equivalent.

Kenshin

yeah but the question wants to see that

and they guy seemed to think his comment was sufficient

ACuriousMind

So what? They didn't have time or willingness to write an answer, and left a comment pointing in the right direction. You weren't downvoted. What's your problem with that?

And I think CR Drost's answer (and knzhou's answer) I linked really already does answer your question, including the distinction between the "weak" and "strong" versions of the third law

Kenshin

@ACuriousMind because his point was rather obvious, so clearly my dilmena was in deriving newton's angular law

ACuriousMind

@Kenshin Many points I make (even entire answers I write) are obvious to me but not to the people asking; your question is brief enough that you cannot fault people for not immediately divining what exactly your state of knowledge is.

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