The h Bar

John

10:00 AM

Oh thank you, I'll go watch that now

John Rennie

@John angular frequency isn't constant ...

user246160

@John ω = 2πf is the angular frequency, not angular velocity

user246160

Simple harmonic motion

In mechanics and physics, simple harmonic motion is a type of periodic motion or oscillation motion where the restoring force is directly proportional to the displacement and acts in the direction opposite to that of displacement . Simple harmonic motion can serve as a mathematical model for a variety of motions, such as the oscillation of a spring. In addition, other phenomena can be approximated by simple harmonic motion, including the motion of a simple pendulum as well as molecular vibration. Simple harmonic motion is typified by the motion of a mass on a spring when it is subject to the linear...

user246160

See the mathematical derivation from Wikipedia after watching the video

user246160

I assume that you know a little bit of calculus.

John Rennie

10:06 AM

Ah, OK.

user246160

Rennie Sensei, I didn't mean you!

user246160

LOL XD

user246160

@JohnRennie That was the biggest joke of the year :P

John Rennie

Too many Johns!

user246160

@JohnRennie Change it to Jon :P

user246160

10:09 AM

or JonathanSensei XD

user246160

John Rennie (cricketer)

John Alexander Rennie (born July 29, 1970, Masvingo) is a former Zimbabwean cricketer who played in 4 Tests and 44 ODIs from 1993 to 2000. He used to wear prescription spectacles. As a fast-medium swing bowler, selected mainly for one-day matches, he always played with the utmost heart and enthusiasm, and often took early wickets for Zimbabwe. He was also a useful batsman and good fielder. He was never selected for any of the national age-group teams, and it took him some years to become a regular in his school first team, but he progressed steadily through club cricket to the national side. Born...

user246160

Are you are a part time cricketer ? :D

user246160

John Rennie (editor)

John Rennie (born 1959) was the seventh editor in chief of Scientific American magazine. == Biography == John Rennie was born in 1959. He received a Bachelor of Science in Biology from Yale University in 1981, after which he worked as a science writer and researcher at Harvard Medical School. He currently teaches science writing as adjunct professor at New York University's Arthur L. Carter Journalism Institute, and writes his The Gleaming Retort on the PLoS Blogs Network. === Scientific American === He joined the Board of Editors at Scientific American in 1989. Rennie has written and e...

user246160

John Rennie the Elder

John Rennie FRSE FRS (7 June 1761 – 4 October 1821) was a Scottish civil engineer who designed many bridges, canals, and docks. == Early years == Rennie, a farmer's younger son, was born at Phantassie, near East Linton, East Lothian, Scotland, and showed a taste for mechanics at a very early age, and was allowed to spend much time in the workshop of Andrew Meikle, millwright, the inventor of the threshing machine, who lived at Houston Mill on the Phantassie estate. After receiving a rudimentary education at the parish school of Prestonkirk Parish Church, he was sent to the burgh school at Dunbar...

user246160

And you even opened your own school XD johnrennie.lbpsb.qc.ca I'm freaking out at the awesomeness that John Rennie is ^ :D

John

10:14 AM

So for a system, angular frequency and angular velocity are not constant

user246160

@John yep

user228700

@JohnRennie: Halp! :-P

user246160

@Kaumudi John Rennie is playing cricket now. Don't disturb :P ............... XD

user228700

John Rennie

@TheStackExchange: Rennie is a Gaelic (Scottish/Irish) name and there are lots of Rennies in Scotland. A surpisingly large number of them seem to have been engineers.

user228700

10:17 AM

^ What can I do about this, computer guru? _/\ _

John Rennie

@Kaumudi the cover has come off your USB key?

user228700

Yeah :-/ Lost the cover too.

John Rennie

Just wrap it in insulating tape.

user228700

Insulating tape? Now where do I get that? :-|

John Rennie

@Kaumudi any hardware store

user246160

10:19 AM

@Kaumudi Amazon!

user228700

Wokay, thanks! :-)

John Rennie

This sort of thing

user246160

amazon.in/ETI-Masking-Carpenters-Painters-20Mtr/dp/…

John

So why does the equation w=root(k/m) show that w is constant

John Rennie

@TheStackExchange that's masking tape. Don't use that!

John

10:20 AM

I thought angular velocity and angular frequency would increase as Ek increased

user228700

_/\ _

user246160

amazon.in/Electrical-Insulating-Household-Adhesive-green/dp/…

user246160

Okay...masking tapes are way too sticky

John Rennie

There doesn't seem to be a lot of insulating tape on Amazon's Indian site

koolman

@TheStackExchange jee launch pad

user246160

10:22 AM

600 rupees seems too costly for this stuff amazon.in/Electrical-Insulating-Household-Adhesive-green/dp/…

user246160

I get it in local stores for 200 rupees at max

user246160

@Kaumudi You better try at local stores first

John Rennie

Just as cheap to buy a new pen drive

user246160

actually ^

John Rennie

@John the parameters $k$ and $m$ are both constant, so that means $\omega$ must also be constant.

John

10:24 AM

@JohnRennie wait so w is constant but angular velocity is not constant

John Rennie

@John it depends on what you mean by angular velocity. If you have a pendulum of length $l$ and the velocity of the bob is $v$ then the angular velocity of the bob around the pivot is $v/l$.

Obviously that changes because the speed of the bob changes as it swings from side to side.

But if we write $x = x_0 \sin (\omega t)$ then the parameter in that equation $\omega$ is also called an angular velocity and it is constant.

John

@JohnRennie so what is angular frequency, is that the omega in x0sin(wt)

John Rennie

It's a slightly unfortunate terminology. When I said the angular velocity isn't constant I meant the value of $v/l$ isn't constant - which is obvious.

Oh hang on, I'm managing to confuse myself.

$v/l$ is the angular velocity

$\omega$ is the angular frequency

Sorry...

John

@JohnRennie no problem! But what exactly is angular frequency

@JohnRennie I mean not in terms of the equation 2pif

John Rennie

@John Any form of simple harmonic motion can be related to circular motion.

Let me see if I can find a video to show this ...

Aha:

The angular frequency is the frequency of rotation in the related circle (multiplied by $2\pi$)

John

10:35 AM

@JohnRennie is 2pi/T also angular frequency

Swapnil Das

If physicists aren't geeks, then who are?arstechnica.com/science/2012/04/…

John Rennie

@John where $T$ is the period? If so yes because $f = 1/T$.

John

@JohnRennie but isn't 2pi/T= theta/T which my textbook says is angular velocity

@JohnRennie oh wait 2pi/T would be angular vel for circular motion

John Rennie

Angular frequency and angular velocity are both radians per second - they are the same thing. However we tend to use the term angular frequency only for circular motion.

John

@JohnRennie but for shm, 2pi is not necessarily covered in time T and so angular velocity =/= 2pi/T

John Rennie

10:40 AM

I wonder if we're getting mixed up here ...

Does that picture I posted above make sense i.e. explain how SHM is related to motion in a circle?

John

@JohnRennie It does yeah but I didn't think a system had to turn through 2pi in shm in a cycle

John Rennie

The $\omega$ in the equation is the angular frequency/velocity in the associated circle on the right

And obviously the motion in the circle is moving through $2\pi$

John

So a system will turn through 2pi in a time period T regardless of the initial displacement

user246160

@John T is defined as the time taken to cover 2pi angle.

John Rennie

@John the associated circular motion will go through $2\pi$ in a time period yes.

John

10:47 AM

But say for a pendulum, if the initial displacement = Pi/6 then the angle covered in one cycle = pi/6 * 4

Initial displacement - Eqm - other max disp - Eqm - initial displacement

John Rennie

@John Careful, that's a different angle. That's not the angle in the associated circle.

user246160

@John You are confusing the motion of a pendulum with the projected motion on a circle.

John

@JohnRennie so 2pi is not the angle covered, it is just the angle in the circle

John Rennie

There are two different angles here. One angle is the angle that the pendulum string makes to the vertical - call this $\phi$. The other is the angle in the associated circle - call this $\theta$.

The angle in the circle, $\theta$, goes from zero to $2\pi$ every cycle.

The angle of the string, $\phi$, starts at some value (e.g. $\pi/6$ as in your example) and goes to zero then $-\pi/6$, then zero again and back to $\pi/6$ every cycle.

John

@JohnRennie ok there's one confusion sorted

So angular frequency = 2pif = 2pi/T for any oscillating system

And this angular frequency = root(k/m) = constant

John Rennie

11:00 AM

Yes

John

x=x0sin(wt) where w is angular frequency not angular velocity

John Rennie

Yes

John

For a circle, angular velocity = dphi/dt = 2pi/T (assuming horizontal plane) as this is the angle covered over time

But for a pendulum, angular velocity =/= 2pi/T, angular velocity = dphi/dt at some point

John Rennie

Yes

John

Ahh ok it makes sense now

What equations do we use angular speed in?

John Rennie

11:06 AM

@John anything that isn't circular motion I guess ...

John

@JohnRennie ok well you've been a massive help, I can finally move on now!

John Rennie

@John Cool :-)

user228700

11:35 AM

@JohnRennie: The page is down (?) :-/

John Rennie

@Kaumudi it's working here:

> 10 December 2016 15:07 (Local time)
BANGALORE
10 DECEMBER 2016 15:07 Shipment Received At Tnt Location

user228700

Nope! I'm impatient :-P

user228700

Thanks! :-)

user228700

Wtf, it's still over there :-/ How does the company work?

user228700

Does it hand over the package to the post office or something like that, after it nearly reaches its destination?

John Rennie

11:38 AM

I would guess TNT handle the delivery right up to your door.

user228700

TNT= what?

John Rennie

TNT is the delivery company

tnt.com/express/en_gb/site/home.html

user228700

Oh, I see. Do they work on weekends?

Mew

probs not

John Rennie

The depot will work at the weekend so the package will get to your local TNT office over the weekend. Then the TNT delivery man will bring it on Monday.

Mew

11:41 AM

what if noone is home though?

John Rennie

However there will be import duty to pay and TNT will contact you to ask you to pay the duty before they will deliver.

Mew

wtf i thought it was free

user228700

Oh, okay. Then it will probably reach Chennai and just sit in the office till Monday. Sigh, what a waste.

John Rennie

You could fly to Bangalore and demand your package :-)

user228700

@JohnRennie Oh, okay. You absolutely have to mail me about paying for the shipment once I get it.

user228700

11:42 AM

@JohnRennie :-) If only I could.

Mew

@Kaumudi I'm sure the workers don't think getting the weekend off is a "waste" :p

user228700

Yes, true :-P

John Rennie

Shiftless layabouts. They should consider it a privilege to deliver K's laptop at the weekend :-)

user228700

Ey, the website says that I can collect it from the office itself. I might just convince my dad to let me do that if it gets to Chennai before Monday.

Mew

What if she can't afford the import duty?

John Rennie

11:45 AM

The import duty should be under £20. We've already checked that's fine.

user228700

^ JR has assured me that I definitely can. Fingers crossed.

Mew

awesome

John Rennie

I was young and impatient once. It was a long time ago ...

Mew

WOW

the import duty is 15% of the laptop's value

that's insane

John Rennie

That's fairly standard. It's effectively the sales tax.

user228700

11:47 AM

This is probably one of the most exciting things to have happened to me yet. You can interpret that in two ways: a. This is really really huge for me for personal reasons perhaps or b. I have a boring life :-P

John Rennie

If you buy a laptop in the UK you pay 20% sales tax (called VAT = Value Added Tax). If you import a laptop then obviously you aren't paying UK sales tax on it so that nice UK tax man charges you an import duty to make up for it.

Mew

I wonder how customs assesses the value

user246160

@Kaumudi Need help. You know which metal hydroxides dissolve in excess NH4OH ?

user246160

I can't find it any book :/

John Rennie

Well I declared the value as £50. It depends on whether they accept my declaration or not.

Mew

11:49 AM

oh k

user228700

@TheStackExchange: Nope, I'm not good enough at Inorganic yet.

user246160

ahhh...let me ask on Quora...lets see...on Chem Se they don't answer list type questions

user228700

@JohnRennie They're allowed to reject it?!

Mew

wow the stuff u guys have to know

@Kaumudi they can re-assess the value if they choose I believe

user228700

:-/ I see.

Mew

11:50 AM

and thus charge a higher duty if they think the laptop is worth more

John Rennie

@Kaumudi well suppose I import a gold bar and claim its value is one dollar.

The customs guys are unlikely to be impressed.

user228700

:-P Makes sense.

Mew

I think the laptop is worth more, but hopefully they will just take the stated value

John Rennie

@Mew Ssh, they might be listening :-)

Mew

lol

user228700

11:51 AM

:-)

Mew

@JohnRennie if she can't afford the duty, who keeps the goods?

Is it seized by the government?

or can it be returned to sender

or what?

user228700

@JohnRennie: Did u buy the chocolate pizza?

John Rennie

@Mew A good question, and I don't know the answer

@Kaumudi it isn't lunch time yet. In any case I won't be buying a chocolate pizza today - if ever! I'll probably wait for it to be a reduced price.

I haven't decided what to have for dessert today.

user228700

Okay. I'll try it someday.

user228700

I had chocolate ice cream!

John Rennie

11:55 AM

Chocolate ice cream is one of those things I'll happily eat if it's placed in front of me but I wouldn't seek it out. My favourite icecream is mint choc chip.

Mew

my favourite is fish food

John Rennie

I really, really love good quality mint choc chip icecream.

@Mew Ben and Gerry's?

Mew

@JohnRennie yeah :)

John Rennie

I find B&J ice cream a bit too sweet.

user228700

@JohnRennie Nevert tried it :-(

Mew

11:56 AM

Yeah I don't have it often

but when I do it's great

John Rennie

@Kaumudi then you have the finest (culinary) experience of your life ahead of you :-)

user228700

Haha :-)

Mew

Mango is good too

John Rennie

@Kaumudi I'd send you some, but after four days in an aircraft hold it wouldn't be great :-)

@Mew ooh yes. When I was a boy there was a Lebanese shop that sold home made mango icecream and it was the best icecream I have ever tasted.

user228700

:-D I bet I could it find it in some shop...but I also bet it wouldn't be very good if it doesn't cost like, 200 rupees, which is too much.

John Rennie

11:58 AM

I've never found anything as good since.

user228700

Bye, guys! :-)

Mew

why

user228700

:-P Gots to do some stuff. Will come back later.

Mew

BYe

user246160

@JohnRennie You should try Calcutta's Mango Kulfi once :)

user246160

12:02 PM

With spices

John Rennie

@TheStackExchange Hmm, I wonder if I can get that in the UK?

@TheStackExchange are you Bengali? I remember you asking if I had tried Bengali food.

user246160

@JohnRennie Probably not. You can make it at home though :)...do you plan to come to India sometime in the future ? :)

user246160

@JohnRennie I am actually Bangladeshi who lives in India

user246160

My ancestors are from Bangladesh

user246160

But I always lived in India

Mew

12:04 PM

what's better bangladesh or india?

user246160

Its something like this ^

user246160

@JohnRennie :-D

Mew

that looks like a mango

John Rennie

My knowledge of Indian geography isn't great, but aren't Bengal and Bangladesh next to each other?

user246160

12:06 PM

@JohnRennie A few 1000 kms apart...but yeah...close :)

John Rennie

@TheStackExchange Ooh that looks good :-)

@TheStackExchange I mean they share a border ...

user246160

Oh, yes...they do share a border!

user246160

But we live quite far from the border

user246160

:-D

user246160

I went to the border several times

user246160

12:08 PM

@Mew Both are nice places. Bangladesh isn't much developed though

user246160

@Mew Its a mango, which is scooped out and then mixed with cream/milk and spices and then put back

Mew

yummy

user246160

in the freezer

user246160

The spices make it amazing

user246160

I mean I can eat 50 of them in one day

John Rennie

12:10 PM

Ooh look what I have found

user246160

(and end up in the hospital)

user246160

:-P

Mew

@JohnRennie what countries have u been to?

@JohnRennie internal severe error

user246160

@Mew yeah...

John Rennie

Sudan, Egypt, Turkey, Greece, Italy, Austria, Germany, Holland, Belgium, France, US, India.

Mew

12:12 PM

I guess that site can handle a traffic volume of 1

John Rennie

I think that's it.

Oh, and Eire.

Mew

@JohnRennie that icecream looks good value

That's alot of countries

user246160

@Mew If you ever visit India bring along a dozen ear plugs :P...no one follows traffic rules here

Mew

what was the best one?

are you interested in Japan?

John Rennie

@Mew Not for Europeans. I suspect most Europeans have been to lots of European countries.

Mew

12:13 PM

true

cos ur so close

here in Australia we're far from everywher

John Rennie

@Mew Italy. I just loved Italy.

Mew

yeah I wanna visit there

and taste some napoli pizza

user246160

@JohnRennie Is Sudan's condition as bad as South Sudan or is it better ?

user246160

@JohnRennie Thats a lot of countries :)

user246160

@Mew You are in an island

user246160

12:15 PM

:)

John Rennie

@TheStackExchange I was last there in 1969 and it was Ok then. These days it's in a pretty bad state.

Mew

@TheStackExchange the biggest island in the wordl

user246160

@Mew yep XD hehe

John

Is mgL(1-costheta) + 1/2mv^2 = 1/2mVmax^2 correct for a pendulum

user246160

only country which is a continent

John

12:17 PM

I.e it's gpe + Ek = max Ek

Mew

@John this is a bar mate, not a school

John Rennie

@John yes, that's correct.

I assume $\theta$ is the angle of the string to the vertical.

John

@JohnRennie ^yes, I followed the equation through and ended up with 1/2theta^2 = 1-costheta

@JohnRennie I'll write what I did:

@JohnRennie mgL(1-costheta) + 1/2mv^2 = 1/2m(Aw)^2 = 1/2mA^2*g/L

John Rennie

Wrong I'm afraid

You've mixed up those two angles again

Remember we used $\phi$ for the angle of the string and $\theta$ for the angle in the associated circle.

John

@JohnRennie isn't w^2=g/L

John Rennie

12:24 PM

Yes, but that isn't related to the velocity of the pendulum bob.

John

@JohnRennie and vmax = Aw

Secret

What Happens at the Event Horizon? | Space Time | PBS Digital Studios

►

Now I kinda start to get the penrose diagram

Are these rindler coordinates?

John

@JohnRennie where w is angular frequency

John Rennie

$A$ is what?

John

@JohnRennie amplitude

@JohnRennie because x=Acos(wt) v=-Awsin(wt) so vmax= Aw

John Rennie

12:30 PM

Ah, OK, yes $v_{max} = A\omega$

John

A/w?

Or is that Aw I can't tell with the code

John Rennie

\omega is the MathJax expression for the greek letter omega.

1627

Q: MathJax basic tutorial and quick reference

To see how any formula was written in any question or answer, including this one, right-click on the expression it and choose "Show Math As > TeX Commands". (When you do this, the '$' will not display. Make sure you add these. See the next point.) For inline formulas, enclose the formula in $......

John

Ah ok

John Rennie

So yes it's A times omega

John

So anyway, gL(1-costheta) + 1/2v^2 = 1/2A^2*g/L

Should I write this down and upload it to imgur

John Rennie

12:35 PM

No, carry on here ...

John

Alright, gL(1-costheta) + 1/2(Awsin(wt))^2 = 1/2A^2*g/L

John Rennie

I think your middle term is wrong

Let me work it out to check ...

Oh no, ignore me it's fine

John

gL(1-costheta) + 1/2A^2*g/L*sin^2(wt) = 1/2A^2*g/L

Factoring, 1/2A^2(1-sin^2(wt)) = L^2(1-costheta)

are they ok?

John Rennie

Yes, that looks fine

John

1/2A^2cos^2(wt) = L^2(1-costheta)

Hey-men-whatsup

12:42 PM

helo..how are you?

John

x=Acos(wt) so 1/2x^2 = L^2(1-costheta)

John Rennie

@John yes

John

x=Ltheta so 1/2(Ltheta)^2 = L^2(1-costheta)

So 1/2theta^2 = 1-costheta

I've just put a value of theta in and it works

John Rennie

Yes. You're about to discover the Taylor series for $\cos\theta$ :-)

John

Well it's a good approximation

Oh cool

Oh yeah I know the Taylor series

Ah that's really cool

John Rennie

12:48 PM

@John The reason it's an approximation is that you said $x = L \theta$ and it's really $x = L \sin\theta$.

But $\theta$ is approximately the same as $\sin\theta$ if $\theta$ is small.

I have to go. It's time for lunch. See everyone later ...

John

oh ok I see, thank you again!

user246160

3:15 PM

@Kaumudi.H yo....why does the cat in the dp have an extra tail ? O_o (BTW you finally contracted my habit of changing dps and display names :P)

user228700

:-P That's the NAND cat.

Bernard Meurer

@JohnRennie I need your help

How do I get rid of this hangover

user246160

@Kaumudi.H what the hell is that ?

user228700

No, no, I had the Pizza John for far too long and the NAND cat is hilarious so :-) I haven't contracted your illness :-P

user246160

@BernardMeurer How many bottles did you drink ? :P

user228700

3:18 PM

@TheStackExchange Use Google bhai.

Bernard Meurer

@TheStackExchange If I remembered I wouldn't be hungover

user246160

firse bhai bola ? hff pff...ok @Kaumudi.H

Bernard Meurer

@JohnDuffield Hello darkness my old friend

user246160

camo.githubusercontent.com/…

user246160

NAND Cat [original]

►

user246160

3:20 PM

Ugh....this is so so anti-hilarious ^ !

user228700

@TheStackExchange Haan bhai, bola maine :-P

user228700

@TheStackExchange To each his own :-) I find that very very hilarious.

user246160

>:[

Bernard Meurer

@Kaumudi.H Is right, it's really creative and it made me exhale at a rate faster than usual

user246160

@BernardMeurer It seems right because you're officially drunk :P

user246160

3:26 PM

Now I am sure that you are drunk!

Bernard Meurer

Huh?

Hugo Trombert

Did you hear about the physicist who froze himself to absolute zero?

Bernard Meurer

I'm having 1 (one) beer

Hugo Trombert

He's 0K now

Bernard Meurer

Sigh

Hugo Trombert

3:29 PM

(I confess that i did not invent this joke)

Bernard Meurer

That's barely a joke

user246160

That's a PJ.

user246160

I read something really funny today qph.ec.quoracdn.net/…

user246160

quora.com/…

heather

hello

Ramanujan

3:40 PM

@heather hello!

heather

@Kaumudi.H, you have confused me with the name/avatar change ::shakes head in dismay::

if i != "target" | (i == "target" & mem1 in globals()):

gives the error mem1 is not a global variable

which makes sense, but it shouldn't be a problem because it is or...and target is not used in the input.

github.com/physicsnerd/Quantum-Computer-Simulator/blob/TwoQubit/…

Bernard Meurer

What's that | and the & ?

If those mean "or" and "and" please just use or and and

heather

oh, really, you can do that? Okay, switching that out...

Bernard Meurer

:)

heather

done

alarge

3:49 PM

| and & usually mean bitwise operations. Not sure about python defaults, though.

and || && are the logical operators

Bernard Meurer

You should use classes, then you can reference that as self.mem1 or whatever

@alarge You're absolutely right about that, I don't even know what magic her interpreter was pulling

And wither way you should always just use or, and and so on

heather

huh, that seemed to be the error; now gives an error on a different line.

Bernard Meurer

Because the real error was you misusing operators

heather

yep

i should be more careful about that stuff =)

Bernard Meurer

You should really get a linter on VIM or just use atom, it will help you a lot

heather

3:54 PM

what's atom?

Bernard Meurer

The editor I use :)

atom.io

Developed by GitHub

heather

sudo apt-get install atom?

Bernard Meurer

Go on the website

heather

it looks nice

Bernard Meurer

download the deb file

and run

heather

3:55 PM

okay

Bernard Meurer

sudo dpkg -i [FILE]

OR

Do this tipsonubuntu.com/2016/08/05/…

This is what my atom looks like

user246160

4:12 PM

@BernardMeurer The background color is cool :)

ACuriousMind

It's the standard atom look

Bernard Meurer

@ACuriousMind Yep :)

user246160

I never used atom. But it seems really light on the eyes and comfortable too.

heather

hmm, it is only showing a "download for mac" button

Bernard Meurer

Then use the other method I linked

heather

4:14 PM

okay

vzn

4:31 PM

@GPhys what kind of homework? ps are your honors theses online? any more thought/ reaction on a spkr session?

Bernard Meurer

4:46 PM

@heather Did it work?

DanielSank

5:08 PM

Hi, everybody.

koolman

5:19 PM

Hii

Transcript for