The h Bar

4:14 AM

@Abcd This is just a complicated way of asking when the angle of the particle's velocity vector is 30º to the horizontal i.e. when $v_y/v_x = \tan 30º$

@JohnRennie What if the inclined plane made an angle of 45 degree with horizontal

@Abcd The question asks at what time the particle is travelling parallel to the inclined plane. This is the same as asking at what time is the angle of the particle's velocity the same as the angle of the inclined plane.

So if the angle of the plane is 45º then the question is when is the angle of the particle's velocity 45º. The angle of the velocity is always given by $\tan\theta = v_y/v_x$.

Ok, TY

rob

@JohnRennie You're up early.

I'm usually up by 5:30 though I don't always come straight to the chat room :-)

My job involves checking (as early as possible) that no servers have died overnight.

On the good side, I've finished work by 9 a.m. :-)

4:30 AM

@JohnRennie How to solve further?

@Abcd well $v_x$ is constant so we only need consider changes in $v_y$. Yes?

@JohnRennie yes

And $v_y$ is given by $v_y = v_y(0) - gt$, where $v_y(0)$ is the initial value of $v_y$. Yes?

That's just the usual SUVAT equation $v = u + at$

@JohnRennie yes .... g= g cos alpha

@Abcd $\alpha$ being the angle of the plane?

4:35 AM

@JohnRennie Yes (though I want to know why g is g cos alpha)

It isn't $g\cos\alpha$.

Forget the plane is there. All the angle of the plane does is tell you what the angle of the velocity has to be.

In this question $g$ is just $g$ i.e. 10m/s^2

@JohnRennie It's given in my book that vertical component of g is g cos alpha :(

The component of $g$ perpendicular to the plane is $g\cos\alpha$. But we aren't interested in the component of $g$ perpendicular to the plane.

@JohnRennie Ok. What next?

I can't help feeling we've misunderstood each other somewhere along the way, because you have all the info you need and it's a really easy question.

4:41 AM

@AccidentalFourierTransform ROFL

@JohnRennie I don't think acceleration should be g. It should be g cos alpha for the vertical comp of velocity

The question asks at what point is the velocity parallel to the plane. The little arrows show the velocity at various points, and the velocity is parallel to the plane when the little arrow is at 30 to the horizontal i.e. the red arrow.

yes

@JohnRennie How do you draw these figures?

Now forget the plane exists:

4:48 AM

ok

Q: What does it mean for a law to be fundamental?

So the question is still at what point is the velocity 30º to the horizontal. It's still the red arrow and still in the same place.

@TheDarkSide Google Draw. It's not the greatest drawing app in the world but it's good enough and it's free :-)

Yashas

8

I was recently reading up about Coulomb's law and Gauss law and several sources seemed to state that the Gauss law was more "fundamental" than Coulomb's law even though one is deducible from the other, which got me thinking: what does it even mean for a law/theorem to be more fundamental?

opinion based?

@JohnRennie Have a look:

I don't see the relevance of that to the question

Hi, everybody.

4:59 AM

hi pal

pa?

hit send too soon :(

nobody I know uses "pa" or "pop" anymore

@DanielSank do you know swimming?

Yashas

@DanielSank there's a speaker below?

Yes.

5:13 AM

@JohnRennie Ahh. I see. Thanks.

@TheDarkSide I was a competitive swimmer many years ago. Why?

@Yashas I believe so.

So if Daniel knows swimming, how come Daniel sank?

:P

lol

user228700

5:34 AM

Hi, everyone :-)

user228700

@JohnRennie Write a book. I'd read it, I really would.

hi pal

Oh, :: incoming food talk ::

user228700

Hey :-)

:P

user228700

5:36 AM

@TheDarkSide Lol, we don't always talk about food.

@Kaumudi.H Kidding :)

user228700

:-)

But when you do I always get hungry :P

@Kaumudi.H John's guide to buying laptops

user228700

:-/ That would take you only 2 hours or so to write!

5:38 AM

Call it "The Laptop Dance"

user228700

(Unless you suffer from chronic perfectionism like me, in which case it would take you several months, if not years :-P)

user228700

Oh, this is great! :

user228700

5:56 AM

@TheDarkSide I assure you that you are not the first to think of that joke.

@Sid Forgot to add water? How did you cook it then anyway?

@Kaumudi.H Look what is half price at my local supermarket!!!

@avantgarde Didn't cook it. Brought out another packet and then, added water and cooked it "correctly"

@Sid You're amazing. This needs to be on your biography

Oh, and that episode helped me a lot in understanding the value of high specific heat Capacity of Water. :P

"value" as in importance

user228700

6:02 AM

@JohnRennie Oooh, go for it!! :-)

@Kaumudi.H I'm going to! :-)

user228700

Those don't look very much like doughnuts though. Presumably a side view?

But at the weekend - today is another healthy lunch.

@Kaumudi.H In the UK doughnuts come in three shapes: balls, rods and tori

Those are rod shaped doughnuts.

user228700

Tori being the most common, of course?

user228700

@JohnRennie Wow, I see.

6:04 AM

I like how you used the word tori

@TheDarkSide You were right. :P

Balls

Tori

Actually I suspect the ball shaped doughtnuts are the most common though I've never attempted a proper survey.

user228700

Haha OK :-)

The advantage of the ball and rod shapes is that you can fill them with jam, custard or whatever you want. That's harder with the rings.

user228700

Why is it harder? That it might not spread out evenly?

6:07 AM

Making the doughnuts into a ring makes the body of the doughnut quite thin.

You'd have to slice the doughnut in the plane of the ring to put in the filling.

I suppose that is done with bagels, but then they tend to be a lot bigger than ring doughnuts.

user228700

If I remember correctly, they just put a small hole on the side and fill it up...

With a ring doughnut? How would you get the jam to spread out round the ring?

user228700

Yes, even with a ring doughnut. Look:

@Sid cooking is not that hard though. You just need a little practice, like anything else

And I'm not talking about maggi

duh.. practice

SOunds like too much work

6:12 AM

If you gotta fill that tummy, you gotta work

No wonder I am thin... :P

@Kaumudi.H :: drums fingers ::

@JohnRennie delishably.com/desserts/Why-are-Doughnuts-Called-Doughnuts

Just make your lunch every day for a month and by the end of it all, you'll be a decent cook. At least for yourself

user228700

@JohnRennie Heh. I'm unable to find any videos of filling up ring doughnuts. I had been thinking of this one but they're ball-shaped.

6:15 AM

:: John grins smugly ::

@Avantgarde I am not going to forego Mom's tasty food to experiment with my poor cooking skills..

user228700

@JohnRennie :-)

Maybe, I should just try to make breakfast..

@Justwinbaby I think doughnuts are always (or at least almost always) ring shaped in the US.

@Sid I understand :P Maybe not right now, but you will have to do it sometime in the future ;)

6:17 AM

@Justwinbaby but in Europe doughnuts have long been ball shaped

That's interesting. I've never seen a ball donut

Gosh, look at all these doughnuts

Looking at pictures of doughnuts isn't a great idea when there are still hours to go before lunch

user228700

Stop! Stop talking about doughnuts!

:-)

user228700

Yesterday, I watched the book-->film adaptation of a book I'd been meaning to read (but eventually took off my list because it's YA and I think I'm finally done with YA).

user228700

6:23 AM

...and I couldn't even finish it because literally every character in that movie was an ***hole.

What film/book?

@Kaumudi.H ****hole in the UK :-)

user228700

Nick and Norah's Infinite Playlist.

Looks dangerously chickflicish to me

user228700

It wasn't. It was bad but I wouldn't call it a chickflick, whatever that really is.

user228700

I had watched another one right before I watched this and in that too, man, both films featured American teenagers and literally every last one of them was being an ***hole.

6:29 AM

Not ***hole. An ass is something Jesus sat on when entering Jerusalem.

user228700

...right, right :-P

user228700

This one was the other one.

*hole will suffice :P

@Kaumudi.H a romantic comedy drama?

No rayguns, spaceships or flesh eating aliens then?

user228700

@JohnRennie Nope. The trailer gives the wrong impression.

6:44 AM

@Sid :P

@DanielSank Haha :P

It was locally original, i.e. not consciously plagiarized.

And I still got you to respond :D

“We’re featherless two-legged linguistically conscious creatures born between urine and feces whose bodies will one day be the culinary delight of terrestrial worms.”

Wow..

Where is that from?

@Sid I find that sort of consideration quite comforting really. Every time I worry about how little I've achieved compared to the great names in physics I remind myself that we're all worm food in the end :-)

Food. Again :P

No physics allowed here!

6:56 AM

@Justwinbaby Said by Cornel West

Physics of cooking possibly, though that's really organic chemistry

Cornel West

Cornel Ronald West (born June 2, 1953) is an American philosopher, political activist, social critic, author, public intellectual, and prominent member of the Democratic Socialists of America. West is an outspoken voice in American leftist politics, and as such has been critical of many center-left figures, including President Barack Obama and former Secretary of State and 2016 Democratic Presidential nominee Hillary Clinton. He has held professorships at Harvard University, Princeton University, Yale University, Union Theological Seminary, and the University of Paris during his career. He is also...

Thanks

"he received a Ph.D in 1980, becoming the first African American to graduate from Princeton with a Ph.D. in philosophy.[23]"

When did the first African graduate from Cambridge with a Ph.D. @JohnRennie

Oh, @JohnRennie You are a Cambridge guy?

I thought you were an Oxford guy..

Oxbridge?

:-)

@Justwinbaby No idea.

@Sid I did my degree at Peterhouse and my PhD at Jesus.

7:10 AM

Ah.. Peterhouse. I think that is the oldest of the Cambridge colleges?

Yes

and the smallest, I think

Also true

I think it's one of the most fun colleges, or it was in my day. It was one of the last colleges to go mixed - it was still men only when I was there. Being old, small and single sex it tended to attract eccentrics, which made it ... erm ... lively :-)

Wow, 1965

Hello

7:20 AM

Hi

Why don't we have more GR people

I can basically only talk to @0celo7 for it and even then he mostly knows topology and analysis

7:43 AM

@Slereah Doesn't John Rennie fit the description?

GR gold badge.

unfortunately not!

Invite Ben Crowell to the hbar.

physics.stackexchange.com/tags/general-relativity/topusers

Save @ben-crowell, you're my only hope

I guess he's never been on the chat

Nope you gotta invite him.

optics polarization waves double-slit-experiment electromagnetism homework-and-exercises interference

7:47 AM

We also have:

Albert Einstein, Sikkim, India

24 4

Albert Einstein never really did topological GR stuff I think

here on P.SE

@TheDarkSide I know basic GR, but Sam is doing research in GR and his work is far, far beyond anything I understand.

@JohnRennie Oh. I see.

Surprisingly Einstein wasn't actually that great at GR.

general-relativity topology causality

7:48 AM

0

Q: Conformal infinities

What is the exact definition of the conformal infinities in a conformal compactification of a spacetime (not necessarily asymptotically flat)? I want to say that it's something of the type (for a time-oriented spacetime) : The future and past timelike infinity $i^+$ and $i^-$ corresponds to the...

@JohnRennie Einstein had the poor luck of having to understand what GR meant

Most of his papers were about that

Einstein had the genius to see what was required, but he needed help with the maths.

Most of Einstein's papers were about why GR makes sense

@Slereah Besides upvoting, all I can do is to assure you that I'll help you with it.

In some 20 years.

Comments about Einstein and the development of GR here

Einstein's paper on the twin paradox is the one that made me finally understand it

7:51 AM

@Slereah really?

It's a fine paper

Twin paradox from a GR perspetive

Isn't it obvious once you know that the proper time is the length of a world line?

The SR resolution isn't very satisfying

Because you can't switch to accelerated coordinates

Einstein wanted SR and GR to be both taught to high schoolers.

basic SR isn't terribly complex, mathematically

plus of course Einstein invented wormholes

Kinda

7:57 AM

Conceptually, he wanted GR to be taught right after SR.

So @Slereah, who are the world leaders in your field?

For GR causality stuff?

I guess the still active one is mostly like

Miguel Sanchez

No, Topological GR.

Same yeah

Miguel Sanchez is the guy I mostly see in modern papers

:: Googles ::

7:59 AM

If we count old and dead people that would be like

Geroch

Hawking

Penrose

Misner

Thorne

Wheeler

I'm not sure actually

I don't recall any big paper on the topic by wheeler

@Justwinbaby :P

Didn't he coin the term "black hole."

could be

He tried to rename GR into "geometrodynamics"

That didn't work out

8:03 AM

How about, try Physics Overflow?

Maybe

:: gets thrown out of the hbar ::

I don't go there too much, it's not terribly active

Nah, they will import it anyways. :)

I dunno, I ask a lot of weird questions and I don't get that many imports

8:08 AM

How is physicsoverflow? I've never been there. It seems relatively deserted

It's a bit quiet

seems like it

Ground level being higher may also be a factor.

I'm not even sure if the whole notion of conformal spacetimes as diamonds makes sense generally

You could compactify Minkowski space to a disk

???

8:12 AM

Although I'm not sure you could embed that into another spacetime

Thinking out loud?

Yes

Rubberducking?

are you a rubber duck

I feel that way.

A Dark Rubber Duck.

Anyways, 'cya hbar.

8:19 AM

See you around

No, you only see my Dark Side. Nothing around.

'cya

What about your backside

I'm impressed

I have 55% boy side, 45% girl side, a dark side, a light side, a gray side, a true side (which is mostly shroud in mystery), an analytic side, an artistic side, a spiritual side (developing), a cynical side, a tranquil side. The default side is tranquility

above that, there's also my subconscious, of which the girl side is known to be one silver of it, and the rest mostly revealed itself via the dreams, and the rest is part of the true side which self delusion means I cannot see into it without someone with opposite worldview of mine

I like this username Dark Side.

8:26 AM

(they may be more layers to my subconscious I don't know. That side is basically a mystery...)

There can be plenty of sequels.

What are you trying to say secret?

I am talking about what I know about my various personality facets

Any numerical analysis experts here?

depends on what problem

8:29 AM

Given position time data, at specified time intervals, how accurately can we know the velocity?

One part is average velocity.

Octopus at work!

Simple numerical differentiation.

Anything better we can do?

There are loads of algorithms for doing numerical differentiation

Numerical differentiation? There are various methods.

Do you know them?

Finite elements my man

8:33 AM

Well at the introductory level, yes.

@JohnRennie Can you point me to some starting references.

Same to @Slereah, @Avantgarde and the others.

Wikipedia would be a good place to start

Follow the links from that page for more details.

@UserAnonymous I suggest the computational physics book by Hasbun and DeVries. Here's the link: amazon.in/First-Course-Computational-Physics/dp/076377314X

I think there's a big standard book

Numerical analysis in C or something

www2.units.it/ipl/students_area/imm2/files/…

@UserAnonymous The book is pretty accessible, and as far as I recall, they introduce numerical differentiation early on. It's a fun read. You'll learn quickly.

Thanks a lot all three of you.

Automatic differentiation

8:37 AM

Is this better or worse?

In mathematics and computer algebra, automatic differentiation (AD), also called algorithmic differentiation or computational differentiation, is a set of techniques to numerically evaluate the derivative of a function specified by a computer program. AD exploits the fact that every computer program, no matter how complicated, executes a sequence of elementary arithmetic operations (addition, subtraction, multiplication, division, etc.) and elementary functions (exp, log, sin, cos, etc.). By applying the chain rule repeatedly to these operations, derivatives of arbitrary order can be comput...

I only know it is faster

@Slereah 947 pages!

It's the standard book

@UserAnonymous Welcome. Though which programming language do you intend to use?

@Avantgarde php

Fair enough

8:40 AM

@Avantgarde There are no stipulations. Anything goes. Even software packages. Just have to get the derivative. No matter how.

Just use C

OK. Thanks.

@UserAnonymous all the maths libraries will have functions for numerical differentiation. In my day we used NAG library though I'm not sure if that still exists and if so for which languages.

@JohnRennie What exactly do you mean by a maths library?

NAG Numerical Library

The NAG Numerical Library is a software product developed and sold by The Numerical Algorithms Group. It is a software library of numerical analysis routines, containing more than 1,700 mathematical and statistical algorithms. Areas covered by the library include linear algebra, optimization, quadrature, the solution of ordinary and partial differential equations, regression analysis, and time series analysis. Users of the NAG Library call its routines from within their applications in order to incorporate its mathematical or statistical functionality and to solve numerical problems - for example...

8:48 AM

That is a very useful resource to me. Thanks John Rennie.

a maths library is a library devoted to math literature only

It's just a package that you can add to your language (like C). It comes with predefined functions and such, so you don't have to write them down yourself.

To be fair

Writing a function to do a differentiation isn't terribly hard

Just do $(f(x + dx) - f(x)) / dx$

@Slereah I know that. But that's just the average velocity over an interval.

That is why you do it for a sample of points

with a small $dx$

8:52 AM

22 mins ago, by User Anonymous

Anything better we can do?

Short of doing a symbolic computation there aren't a lot of much better methods, I think

the subintervals dx controls the error of the derivative, which I remeber is bounded quadratically in terms of dx

The problem is when you are accelerating, the velocity isn't constant over the interval.

So what

Average velocity doesn't make sense.

8:54 AM

then you need higher order finite difference terms

If you only need the first derivative, that's pretty much the only formula

If you need second order derivatives it's a bit more complicated

@Secret On those lines, anything superior to Mr. Newton?

@UserAnonymous: when you use $(f(x + dx) - f(x)) / dx$ you are effectively fitting a straight line to two points then returning the gradient of that line. You could instead take three points and fit a quadratic, or four points and fit a cubic, and so on.

All give a better approximation to the derivative but at the expense of increased complexity.

If your position time data is not very bumpy, usually quadratic order fitting is sufficient in terms of minimising errors

@JohnRennie Yes. On these lines, which technique is this? Newton's? Are there superior methods?

8:59 AM

Did you read the Wikipedia article? That describes the various methods available.

@JohnRennie No. I got stuck in this chat. Maybe I should return after a while.

what do you mean by superior, in terms of smaller errors or faster computation?

smaller errors.

Emilio Pisanty

9:37 AM

-1

Q: Barry centres theory

Actually I am new to the concept which is known as ' Barry centre ' . I read it in Google but it was difficult for me to understand so can anyone please explain it to me with all its cases . I am very eager to know about it . Thank you

astrophysics astronomy

Fantastic

Is it worth editing? It won't stop it getting closed, and it's lovely that way

...for curating purpose

Who is Barry Centres

Barry, Vale of Glamorgan

Barry (Welsh: Y Barri pronounced [ə ˈbarɪ]) is a town in the Vale of Glamorgan, Wales, on the north coast of the Bristol Channel approximately 7 miles (11 km) south-southwest of Cardiff. Barry is a seaside resort, with attractions including several beaches and the resurrected Barry Island Pleasure Park. According to the 2011 census, the population of Barry was 51,502, making it the fifth largest town in Wales. Once a small village, Barry has absorbed its larger neighbouring villages of Cadoxton and Barry Island. It grew significantly from the 1880s with the development of Barry Docks, which in...

I was thinking more

Barry Manilow

Barry Manilow (born Barry Alan Pincus; June 17, 1943) is an American singer-songwriter, arranger, musician, and producer with a career that has spanned over 50 years. He is best known for a long string of hit recordings such as "Mandy", "Can't Smile Without You", and "Copacabana (At the Copa)". He has recorded and released 47 Top 40 singles, including 12 that hit number one and 27 of which appeared within the top ten, and has released many multi-platinum albums. He is ranked as the top Adult Contemporary chart artist of all time, according to R&R (Radio & Records) and Billboard magazines, and Rolling...

Barry White

Barry White (born Barry Eugene Carter; September 12, 1944 – July 4, 2003), was an American singer, songwriter and composer. A three-time Grammy Award–winner known for his distinctive bass-baritone voice and romantic image, White's greatest success came in the 1970s as a solo singer and with The Love Unlimited Orchestra, crafting many enduring soul, funk, and disco songs such as his two biggest hits, "You're the First, the Last, My Everything" and "Can't Get Enough of Your Love, Babe". During the course of his career in the music business, White achieved 106 gold albums worldwide, 41 of which also...

sciencedirect.com/science/article/pii/…

This smells like advertisment

9:45 AM

Barry On?

Bayon

The Bayon (Khmer: ប្រាសាទបាយ័ន, Prasat Bayon) is a well-known and richly decorated Khmer temple at Angkor in Cambodia. Built in the late 12th or early 13th century as the official state temple of the Mahayana Buddhist King Jayavarman VII (Khmer: ព្រះបាទជ័យវរ្ម័នទី ៧), the Bayon stands at the centre of Jayavarman's capital, Angkor Thom (Khmer: អង្គរធំ). Following Jayavarman's death, it was modified and augmented by later Hindu and Theravada Buddhist kings in accordance with their own religious preferences. The Bayon's most distinctive feature is the multitude of serene and smiling stone faces...

oops missplet...

Baryon

A baryon is a composite subatomic particle made up of three quarks (a triquark, as distinct from mesons, which are composed of one quark and one antiquark). Baryons and mesons belong to the hadron family of particles, which are the quark-based particles. The name "baryon" comes from the Greek word for "heavy" (βαρύς, barys), because, at the time of their naming, most known elementary particles had lower masses than the baryons. As quark-based particles, baryons participate in the strong interaction, whereas leptons, which are not quark-based, do not. The most familiar baryons are the protons and...

Barry Meter?