The h Bar

If you had some application where you needed a well-controlled, feeble force, you could imagine using a hydraulic system "in reverse" as a force reducer.

ffahim

Where is the conservation law of energy here?

rob

@ffahim Ooh, fun question.

You should try to work it out yourself, but I'll guide you.

How much work is done by a force?

ffahim

5:11 AM

I just get that pressure is constant

Fs

rob

That's right, the pressure is the same throughout the system.

Fs means?

DanielSank

@ffahim If you remember how work is defined, you can surely answer your own question.

ffahim

F .s.costheta

DanielSank

@ffahim What is s?

Define your symbols, please.

s is distance?

ffahim

S means distance

rob

5:12 AM

(I'm a big fan of using plain language rather than mathematical symbols. I think I know what you mean, but you should be clear.

)

ffahim

Yeah

DanielSank

Ok, @ffahim, so I apply force F1 over the area A1. How much energy did I use?

rob

Right. So work is force times the (parallel part of the) distance over which the force acts.

DanielSank

I'll shut up, @rob. You take it away.

rob

@DanielSank I was about to say the same thing :-). I'll continue.

ffahim

5:14 AM

Sorry I am still not been able to understand pls.. @rob

rob

@ffahim So I push on end 1 of the piston with force F1, and the piston with area A1 is displaced by some distance s1. How much work gets done?

ffahim

Fs?@rob

F1s1

rob

@ffahim Fine. At the other end, the hydraulic force F2 moves the piston with area A2 by some distance s2. How much work gets done there?

ffahim

F2S2

@rob

rob

Yes, but: sign matters. One piston goes in, the other comes out.

ffahim

5:18 AM

But work is scalar

@rob

rob

6 mins ago, by ffahim

F .s.costheta

@ffahim (You don't have to ping me every time, and if it's been less than two minutes you can actually edit the ping into your existing message)

ffahim

Hmmm.. what sign u r talking about?

rob

Scalars can have positive or negative sign.

If the force is parallel to the displacement, it puts energy into the system: positive work

ffahim

Ohh I remember

rob

If the force is antiparallel to the displacement, it takes energy out of the system: negative work

ffahim

5:21 AM

Go ahead.. :-)

user116211

@DanielSank \o

rob

So you've got work in and one end, W1 = F1 * s1, and work out at the other end, W2 = F2 * s2, with opposite signs so we're not creating energy, just moving it around.

Now, you've said that Pascal's Law relates the pressures at the two ends of this hydraulic system. Can you find W2, at the output end, only using things we know from the input end?

(I always need scratch paper for this part, I can't do it just by typing.)

Sir Cumference

@MAFIA36790 What is that even?

ffahim

Ummm no

rob

@ffahim You said F1/A1 = F2/A2, yes? Solve for F2, eliminate that from W2, see where we are.

user116211

5:26 AM

@SirCumference This is Nazi salute.

Sir Cumference

@MAFIA36790 ...

user116211

Really, I waved my hands to address DS.

rob

@MAFIA36790 Hmmm. I had decided it was a low-fi friendly wave.

ffahim

W2=F1*A2*S2/A1 @rob

rob

@ffahim Just so, very nice.

Have a look at the numerator, which contains (A2*s2). Can you think of a physical way to interpret that product?

It's an area multiplied by a length.

user116211

5:29 AM

@rob yes, it is indeed.

Sir Cumference

@MAFIA36790 Daniel Sank?

user116211

yes.

Sir Cumference

@MAFIA36790 By the way, I've been meaning to ask

ffahim

@rob .. nah.. pls help me.. its Volume

user116211

Ask what?

rob

5:31 AM

@ffahim Yes. Do you see which volume?

Sir Cumference

Oct 10 at 2:34, by Sir Cumference

@bl00 But you really don't need to worry about having a childish name here. @MAFIA36790 Literally has "mafia" in his name. @AlfredCentauri mixed his name with a star.

Since then, I've wondered why you have Mafia in your name

user116211

My internet connectivity sucks.

Sir Cumference

@MAFIA36790 What

ffahim

How much fluid was replaced? @rob

Is it so

rob

@ffahim That's it. (A2*s2) is how much the volume changes at the second end of the tube.

Likewise (A1*s1) at the first end of the tube.

If the hydraulic fluid is incompressible, what's the relationship between these volumes?

ffahim

5:34 AM

Not yet learned . Sorry

user116211

@SirCumference The connectivity is really weak; don't know why suddenly. It is weak lately for few days.

ffahim

What's the relationship

Sir Cumference

@MAFIA36790 That's...not what I asked

rob

@ffahim You're not sure what I mean by "incompressible"?

user116211

@SirCumference That's my dog's name and I love mafias.

ffahim

5:35 AM

No no.. I don't know the relationship

Sir Cumference

You named your dog "mafia" o.0

rob

@ffahim By incompressible I mean that the density of the fluid doesn't change with the pressure.

This is one way to distinguish a liquid from a gas.

user116211

@SirCumference Well, I call him maf.

ffahim

Ohh l see.. .. then the volume chance is same?

rob

Bingo. Same volume change at both ends.

ffahim

5:37 AM

*change

rob

So now, finish relating the work done at end 1 to the work done at end 2.

user116211

BTW, you were supposed to be in bed by now, @sirC, isn't it? Or is it that you are still feeling not sleepy?

Sir Cumference

@MAFIA36790 I am in bed

I have a laptop

user116211

You should sleep.

rob

@SirCumference This is known as "poor sleep hygiene"

Sir Cumference

5:38 AM

I wish I could

@rob It's not a choice

I know how important sleep is, it just doesn't come

rob

@SirCumference Insomnia isn't a choice, but the laptop is.

Sir Cumference

@rob The only difference is using a laptop vs. lying in bed, frustrated that I can't sleep

rob

The internet is specifically designed to grab your attention and keep you awake

ffahim

Omg... awesome.. @rob

user116211

Take pills.

rob

5:40 AM

It even makes little noises, @SirCumference

Sir Cumference

@MAFIA36790 No pills are good

ffahim

Thanks a lot... buddy

Sir Cumference

@rob So do the trucks outside

rob

Read a book. Sing a song. Use your own brain.

Sir Cumference

@rob I am reading

A paper, Relativistic Cosmology

rob

5:40 AM

@ffahim Hey, glad to help. You can get the same result from a lever.

@SirCumference You are talking to me.

user116211

@rob, SirC is tortured by being forced to listen to the SU anthem.

Sir Cumference

@rob I am multitasking

ffahim

Really very thanks...

Sir Cumference

@MAFIA36790 That's in the morning

rob

@SirCumference Multitasking is poor sleep hygiene.

user116211

5:42 AM

^^

Sir Cumference

@rob Sleep hygiene isn't something I really have control over atm

Right now my best bet is to learn physics here

And prepare myself for tomorrow

Ah, I have an idea

I'll write a book

user116211

@SirCumference Maths book?

user116211

Don't say Asexual book T__T

Sir Cumference

@MAFIA36790 Nah, a book on the Universe's evolution

user116211

ohh.

Sir Cumference

5:43 AM

@MAFIA36790 It's not a sexual book

>_>

user116211

yeh....

rob

@SirCumference When my daughter was about four, she came out of her room after bedtime, weeping about she couldn't sleep. I told her "no one has ever fallen asleep standing up in the hallway and being loud."

Writing is good.

Multitasking is bad.

user116211

@rob, Do researchers write book? Does it help them?

Sir Cumference

@rob Looks like high school english classes are gonna pay off

rob

@MAFIA36790 Writing is very educational. Nothing concentrates the mind on your hidden areas of ignorance like trying to explain them.

Sir Cumference

5:45 AM

First of all can someone help me define what mathematics is

user116211

ohh; Milnor is a great example.

user116211

@SirCumference Mathematics is Bourbaki.

Sir Cumference

@MAFIA36790 Very clever

But seriously, I need a definition

...no one?

rob

Goodnight, all.

user116211

@rob o/

Sir Cumference

5:47 AM

No wait!

I need a definition ;-;

rob

@SirCumference You don't need my definition.

You need your definition

user116211

You should study metamathematics then @SirC.

Sir Cumference

@rob It would help to get a professor's opinion

rob

Which you find by listening to yourself for a while.

user116211

But I don't think you require to know metamathematics to do maths.

Sir Cumference

5:48 AM

oh good thing I guessed correctly that you're a professor

@JohnRennie Please help

user116211

Morning @JohnRennie.

Sir Cumference

I need a definition for mathematics

user116211

@SirCumference You will get it in metamathematics.

Sir Cumference

@MAFIA36790 I'll get to that after metaphysics :P

user116211

Metamathematics is logic theory.

John Rennie

5:50 AM

Morning all. @SirCumference what do you want help with?

Sir Cumference

@JohnRennie Could you define mathematics for me?

John Rennie

@SirCumference No. I have no idea how to define mathematics and I'm not even sure if it can be defined.

Sir Cumference

:\

user116211

Start doing first-order logic.

Sir Cumference

Mathematics is the only thing I can think of that legitimately has no definition...

user116211

5:53 AM

@SirCumference Do you really need it?

Sir Cumference

@MAFIA36790 It'd make for a killer opening I have in mind

John Rennie

Mathematics has always been whatever mathematicians agree amongst themselves should be included.

Sir Cumference

@JohnRennie I'd ask, why would they agree on certain things being included and others not?

John Rennie

When Cantor started doing his work on set theory other marthematicians denied it was maths, but it's now mainstream.

Who knows what new areas today will be part of mainstream maths in the future?

Sir Cumference

At first glance I'd define math as a way for us to understand the Universe, but even still math goes beyond the physical realm

John Rennie

5:55 AM

I'd guess that pretty much anything to lends itself to the axiomatic approach could be part of maths.

Sir Cumference

Darn it math. Why does it have to be such an abstract thing?

It isn't constricted by the Universe yet it can describe the Universe

rob

@SirCumference I read a great essay a few years ago that concluded "mathematics is ... well, whatever mathematicians do, that's mathematics."

Sir Cumference

@rob Where'd you read that?

rob

@SirCumference Couldn't remember in five minutes.

Sir Cumference

Sigh...it just seems a bit irksome that we always use something, yet we have no idea what it intrinsically is

rob

6:01 AM

@SirCumference Possibly "Lockhart's Lament," which you should read.

Sir Cumference

It's beyond our understanding, but it somehow works

I don't know, maybe I'm the only one bugged by it

user116211

You should read texts of Hilbert on metamathematics @SirC.

user116211

I dare you to read Principia Mathematica.

Sir Cumference

@MAFIA36790 Newton's?

user116211

It's lethal on its notations.

Sir Cumference

6:04 AM

Or that one that tries to prove mathematics without axioms?

user116211

@SirCumference Russell's.

Sir Cumference

Oh, thought ya meant this at first

Philosophiæ Naturalis Principia Mathematica

Philosophiæ Naturalis Principia Mathematica (Latin for Mathematical Principles of Natural Philosophy), often referred to as simply the Principia, is a work in three books by Isaac Newton, in Latin, first published 5 July 1687. After annotating and correcting his personal copy of the first edition, Newton also published two further editions, in 1713 and 1726. The Principia states Newton's laws of motion, forming the foundation of classical mechanics; Newton's law of universal gravitation; and a derivation of Kepler's laws of planetary motion (which Kepler first obtained empirically). The Principia...

Huge work of Newton, but you probably know that

user116211

It tried to make everything rigorous.

Sir Cumference

@MAFIA36790 Russell's?

user116211

yes.

Sir Cumference

6:06 AM

It was kind of an interesting idea

Proving math without axioms

Shame it isn't really possible

user116211

Did he succeed? No.

user116211

Thanks to $\mathrm{G\ddot{o}del} \,.$

Sir Cumference

@MAFIA36790 Darn Gödel with his theorems

user116211

I like that ;))

Sir Cumference

It just makes math all the more mysterious to me

user116211

6:08 AM

@SirCumference You don't need that to do maths, I re-iterate.

Sir Cumference

It's something that is entirely based on axioms, yet it beautifully works and can't be defined

It just seems bizarre

It isn't even bound by physical laws, like everything else in the Universe

It basically sounds impossible...

user116211

Maths is abstract; you can't expect some physical motivation/intuition for that.

Sir Cumference

@MAFIA36790 It's aggravatingly abstract

Its basis is so darn confusing

user116211

Unless, of course, it is applied maths.

Sir Cumference

@MAFIA36790 Yeah, but the existence of pure mathematics is what befuddles me

The fact that math is based entirely on axioms, isn't bound by the universe and can't be defined, yet describes the universe and more

It's seems like baloney

rob

6:26 AM

@SirCumference You should definitely read Lockhart. His "Mathematician's Lament" is an essay on the difference between mathematics and what gets taught in math classes, and his 2012 book "Measurement" expands on the idea.

One thing to realize is that mathematical cognition is a biological phenomenon, honed because people who were good at it tended not to die.

And yet, a logically consistent application of those rules has this shocking predictive power in contexts that are apparently totally unrelated to each other.

And furthermore, thinking about it is fun (another biological phenomenon).

user116211

6:44 AM

0

Q: Condensed Matter Physics and Quantum Mechanics

How to analyze the quantum mechanical spin state of magnetic nanocomplex systems ?

quantum-spin

John Rennie

@MAFIA36790 Unclear

user116211

No efforts at all?

user116211

unclear?

user116211

oh, yes.

user116211

I hate my internet ;/

Sir Cumference

7:12 AM

@rob Mathematical cognition may be a biological phenomenon, but math itself must be something more, right?

user116211

GO TO SLEEP @sirC ;(

Sir Cumference

@MAFIA36790 I was lying in bed, but suddenly I was wondering if anyone replied

user116211

Hmm; it's already 3:15 at your place, right?

user116211

Well, I go to sleep at 3 or max 3:30; so legit....

Sir Cumference

Huh, what time is it for you?

You live in Venezuela right?

Oh, same here

user116211

7:19 AM

@SirCumference Mine is 12:49pm.

Sir Cumference

Huh?

user116211

India.

user116211

Wow! It's raining heavily!

Sir Cumference

$\small \text{...oh...}$

nvm

@MAFIA36790 You're lucky you never had to take our crappy standardized tests

like the ACT

user116211

@SirCumference What test?

Sir Cumference

7:21 AM

@MAFIA36790 tests to get into colleges

user116211

I have a Linear Algebra test tomorrow.

user116211

@SirCumference ohh.

user116211

You never heard of JEE?

Sir Cumference

No, what's that

user116211

hmm.

Sir Cumference

7:22 AM

Oh, just looked at wikipedia

user116211

Well, that's one of the major entrance tests in India.

Sir Cumference

...

> It is regarded internationally as one of the most challenging engineering admission tests.

user116211

Huh~~

Sir Cumference

Guess I can't complain then...

lemme try to take one

user116211

@SirCumference Not that difficult; it's just too numerical...

Sir Cumference

7:24 AM

is there a difference between jee and iit jee?

user116211

It's the old name of JEE.

Sir Cumference

Okay, this is pretty tough...

I have to recall a lot of physics

Most of which I had to teach myself for the SAT II test

(another college entrance test)

user116211

@SirCumference I know that.

user116211

It is conducted in India too.

Sir Cumference

Wait, really?

How about the ACT?

user116211

7:28 AM

yes.

user116211

@SirCumference Don't know about that.

Sir Cumference

But...you know what ACT stands for, right?

American College Test

@MAFIA36790 It's an alternative to the SAT

user116211

ohh; no idea.

Sir Cumference

I assumed both were American because they were alternatives, and one is called the American College Test

user116211

I know about SAT and Oxbridge exam only.

Sir Cumference

7:30 AM

So did you take the SAT II in physics?

user116211

No.

Sir Cumference

did you take an SAT II?

user116211

I hardly recall whether any of my classmates took SAT.

Sir Cumference

Oh

user116211

@yuggib o/

Mew

7:59 AM

Gday peeps

yuggib

@MAFIA36790 \o

Mew

upvoting on comments has now been enabled

notifactions have now been implemented

user228700

8:22 AM

@MAFIA36790 There too?! :-D

user116211

You know about the new room @mew, right?

user228700

Hi everyone :-)

user116211

chat.stackexchange.com/rooms/info/47874/physics-problem-solving

user116211

Write the updates there too @mew.

user116211

@Kaumudi hello

Mew

8:25 AM

@MAFIA36790, thanks for that i didn't see that site

bbl dinner

user116211

@Mew You are the room owner there.

John Rennie

9:17 AM

Magnetic monopoles - spoiler, not really ...

arxiv.org/abs/1607.07460

Slereah

9:32 AM

Gotta have a catchy title, @JohnRennie

Title : IS TIME TRAVEL REAL???

Abstract : No

John Rennie

:-) Arxiv clickbait!

Slereah

"I will start with a short argument in support of ER=EPR. By now it is clear that the existence of an ERB between two black holes implies that they are entangled"

Is it actually clear?

Secret

how will ERB ensures the two black holes are unable to be described as a product state (criteria for entanglment)?

John Rennie

@Slereah Susskind paper?

Secret

Especially (if I recall), black holes with ERB are not actually topologically connected like that of a transversible wormhole, i.e. not a tube

Secret

9:53 AM

-2

Q: Quantum time travel?

this is a theory i made up but is it possible, could quantum tunneling go thru time? In quantum mechanics everything exist every where, so can it exist every when? let me know if this is possible

quantum-mechanics quantum-tunneling

It would have been a meaningful question if time is an observable in quantum mechanics

Otherwise we cannot really say anything along the lines of "the wavefunction is delocalised through time"

Actually on second thought, the reason why this question is not meaningful is that there isn't a notion of a "energy barrier in time", that is, we cannot expect the wavefunction to suddenly become exponentially decaying and then return to oscillatory as if passing through a barrier

->actually, won't that basically means the wavefunction is evolved by some time dependent potential that rotates its imaginary phases so it can become real momentarily...? Might investigate this later...

user228700

@JohnRennie: Morning! :-)

John Rennie

Morning

user228700

10:31 AM

@0celo7:

user228700

in The Periodic Table, 22 mins ago, by M.A.R.

The only thing worse than a homework question is a homework doubt.

John Rennie

???

user116211

@Secret a piece of pop-science myth and lack of knowledge of QM.

user228700

The classic "u don't mean doubt, you mean QUESTION" @JohnRennie

user228700

in The Periodic Table, 24 mins ago, by M.A.R.

No. It is my doubt. I know Birch reduction (Metal Reduction of Alkynes) and how that happens. However I saw this question in a paper. So I am wondering how reduction will happen. Anyway, thanks for reply. I will try to figure out if you don't tell. — Rohit 5 hours ago

John Rennie

10:33 AM

Ah. I'm used to Indian students saying doubt when the mean question.

Secret

@MAFIA36790 Well technically, due to how dynamics are defined, the wavefunction does exists every when (because dynamics means the quantify is well defined for every point in time). The problem is that time is not an observable thus one cannot do a time measurement

John Rennie

It's just a manner of speech, and all social groups have their own argot.

user228700

:-) 0celo7 and I had a looong discussion about this a few weeks ago

user228700

Set me straight :-P

John Rennie

Well, you know, Ryan is such an easy going and relaxed person :-)

user228700

10:35 AM

Ryan?

John Rennie

0celo7's secret identity, well not really very secret, is Ryan Unger.

DHMO

11

Q: I have a doubt about "doubt" in doubt titles

From the depth of the stack exchange network I bring to you the newest season (7) of our community effort against buzzwords in question titles. The word of the season this time is doubt. In Indian English the word doubt is used as a synonym for question (see ELU), which makes it as terrible as q...

discussion editing question-titles

John Rennie

It's on his blog.

einsteinandtheevidence.wordpress.com/about

That's why it's not very secret :-)

Secret

google.com.au/…

user228700

Huh. @JohnRennie: How did u find this?

John Rennie

10:41 AM

I think 0celo7 posted here when he created the blog.

user228700

Oh, OK...

user228700

And Sam Lereah is Slereah?

John Rennie

Yes

user228700

Cool. Ryan Unger is the CEO of some company. Unrelated, I assume :-P

John Rennie

I'm sure there are many Ryan Ungers in the world, just as there are many John Rennies :-)

user228700

10:43 AM

Yes :-)

user228700

Ryan. It's a nice name to have if u listen to the podcast "Dear Hank and John". They have this running joke about it, how John Green prefers to answer questions from people named Ryan and how Ryan is the best name in the world, etc. :-P

user116211

@Kaumudi It's in his profile and he told us about this when he started to write the blog.

Secret

user116211

Only Voldefield knows the significance of the name of his blog.

John Rennie

@Secret Ah, I recognise that picture at the bottom right :-)

user116211

10:45 AM

@JohnRennie That insulted you in a video.

Secret

yup, that guy

John Rennie

I'm a bit offended I only rank 20th.

user228700

Oh, yeah, that guy.

John Rennie

Still, I suppose 20th isn't bad given how many John Rennies there must be out there.

Secret

and some of these are historical, it seems

John Rennie

10:47 AM

A lot of John Rennies were Scottish engineers.

One of London's bridges was built by a John Rennie

Rennie is a Scottish name, or to be precise a Gaelic name.

Not that it's of any great significance, but I'm currently reading an Indian SF book ...

Secret

what is the scifi about?

John Rennie

amazon.co.uk/Cripple-His-Talismans-Anosh-Irani/dp/1565124561

user116211

0

Q: Is it possible that big bang are still happening?

I was reading on this site about big bangs, and it got me wondering, about two things. There’s no exact spot that the Big Bang happened. In fact, the Big Bang happened everywhere in the Universe. 1) if there were multiple big bangs, then is it possible that one galaxy from another explosion...

universe big-bang

user116211

11:13 AM

$\Large{+1}$ from me. ;-) — Babak S. Apr 28 '13 at 2:20

user116211

I'm shocked.

user116211

flagged..

user116211

0

Q: Does bonfire night in the uk increase temperature of the country / local area?

I am not sure if this is the right forum for the question but failing to have any better location to ask it, i have come here. In the UK we have a tradition of lighting bonfires on the 5th of November (or the weekend closest to it). With numerous amounts of bonfires of a large size being create...

homework-and-exercises thermodynamics temperature estimation

user116211

Do you participate in this ritual @JohnRennie?

John Rennie

@MAFIA36790 Not these days. Bonfire night is mainly an occasion for children these days. I used to love going whan I was a child - fireworks and a huge bonfire, what more could a child ask :-)

user116211

11:25 AM

ahh.

Sanya

11:58 AM

@JohnRennie come on, we went as students and there were lots of adults too ...

John Rennie

@Sanya at my age students are little more than children :-)

Transcript for