The h Bar

0celo7

05:03

@JohnRennie you should read a book

user228700

@JohnRennie :-) Morning sir.

John Rennie

@0celo7 I read lots of books

user228700

@0celo7 Why?

John Rennie

@Kaumudi Morning

0celo7

@Kaumudi because

it's the only logical possibility

user228700

05:04

@0celo7 As opposed to him just being asleep?

0celo7

hmm

no I don't think that makes sense

user228700

@0celo7 Right, 'cause he doesn't need sleep like the rest of us..?

0celo7

no because he wakes up at 5AM

so when he's not here at 5AM, I conclude something horrible has happened

user228700

@0celo7 Riight. What you need is hope; it's the thing with feathers! :-)

John Rennie

I was a bit late today because Amazon asked me personnally to write a review of a mattress I bought a few weeks ago. These important tasks have to be done first thing :-)

user116211

05:08

@JohnRennie Mattress?

user116211

Not a book?

user116211

hmm.

user228700

@MAFIA36790 Lol :-P

0celo7

feathers:

John Rennie

I recommend not sleeping on books or reading mattresses

0celo7

05:08

user116211

@JohnRennie I sometimes make books as my pillow.

user116211

They are really awesome.

John Rennie

Doesn't that make for a rather hard pillow?

user116211

Till your neck pains.

user228700

0celo7

05:10

@JohnRennie no you commie

user116211

^

user228700

^Feathers :-)

John Rennie

I'm a communist because I like soft pillows?

0celo7

no

you're a communist because you hate america

explain that, commie

John Rennie

When did I give the impression I hated America? I've been to several places in the US and I loved them all.

0celo7

05:12

you hate eagles

Since eagle $\cong$ America...

John Rennie

It's not as good as Great Britain of course :-)

0celo7

we won the war

John Rennie

When did I give the impression I hate eagles? Everyone likes raptors don't they? Raptors are such wonderful birds.

→ 1 message moved to Trash

0celo7

Just ban me

If you hate America there's no reason for me to be here

Sniff

John Rennie

I don't hate America. I'm at a bit of a loss as to how I've given that impression.

0celo7

05:15

You said eagle feathers wouldn't be soft

user228700

@0celo7 Are u high? :-P

0celo7

I was

user228700

@JohnRennie: Quick question...sort of.

user228700

@0celo7 (Y)

John Rennie

@Kaumudi ask away ...

user228700

05:17

@JohnRennie Will u please watch a 1 minute-long video?

user228700

When Is A Bungee Jumper's Acceleration Max?

►

John Rennie

Groan. Oh OK then ...

user228700

@JohnRennie Eek. Sorry :-( It would be much quicker than me explaining, sir, that's why...

John Rennie

Ok, watched it. Have you studied simple harmonic motion?

Ramanujan

@Kaumudi d?

user228700

05:20

@JohnRennie Yep. Where's the fastest point sir? (I'll try to figure it out on my own...)

user228700

@Ramanujan d? :/

0celo7

Ok I'm going to sleep.

Some of us aren't vampires

user228700

@0celo7 Good decision (Y)

Ramanujan

@0celo7 it's morning in India

@Kaumudi this might help you m.youtube.com/watch?v=vKsof8ax_Ns

John Rennie

@Kaumudi so you know that in SHM $$\frac{d^2x}{dt^2} = -kx$$ so the acceleration is greatest when $x$ is greatest. Yes?

user228700

05:22

@Ramanujan I know, I watched it; my question is different from the one being asked in the video...sort of.

user228700

@JohnRennie Yes.

John Rennie

And that's going to happen at the lowest point in the jump.

user228700

@JohnRennie Yessir, I understand that. My question is, where exactly is the point at which the velocity is max? ie. point c?

user116211

Hey @MikeMiller.

Mike Miller

Are these the sort of comments that get upvoted on your site?...

user116211

05:26

Welcome at Physics.

John Rennie

@Kaumudi The velocity increases when the force due to gravity is greater than the force due to the bungee cord, and it decelerates when the force due to the bungee cord is greater than the force due to gravity. Yes?

user116211

@MikeMiller What do you mean? the comment that recommended Math.SE?

user228700

@JohnRennie Yes.

Mike Miller

Both of them. The first is wrong, because bad questions don't belong on any site. The second is just nonsense.

John Rennie

So the point where the velocity stops increasing and starts decreasing, i.e. the maximum of $v$, is when the force due to the bungee cable equals the force due to gravity.

user228700

05:28

@JohnRennie Ah! But hang on, if the tension in the rope equals gravity at that point, won't the rope become taut?

user116211

While I agree with the second one, first one is just an advice; not a rigid order.

user116211

But yeh, 3 upvotes is exaggerating.

Mike Miller

So it is.

John Rennie

@Kaumudi No, the rope becomes taut when the distance the jumper has fallen is equal to the natural (unstretched) length of the rope. At this moment the tension in the rope is zero because the extension of the rope is zero.

user228700

@JohnRennie Oh, and then the rope begins to stretch, yeah..?

John Rennie

05:32

@Kaumudi Yes.

The motion has two different parts. At first it's just parabolic motion under gravity and the rope plays no part. Then once the rope becomes taut the motion is a simple harmonic oscillation.

user228700

@JohnRennie So even after he reaches the point at which the force on him due to gravity is equal in magnitude to the tension in the rope, he keeps going down...and the tension keeps increasing until the velocity becomes zero at the extreme point and then he starts to rebound?

John Rennie

Yes

The point where gravity and the tension are equal is the maximum velocity.

Below that point the jumper is being decelerated, but it takes time to decelerate that maximum velocity to zero so the rebound can start.

user228700

Okay, two questions: under the action of which force does he keep going down past the poin at which his velocity is maximum..? And secondly, how is there a difference between the point at which the rebound starts and the point at which velocity is zero?

John Rennie

Remember that force is proportional to the rate of change of velocity.

Ramanujan

@Kaumudi (1) under gravity ?

John Rennie

05:39

So given that the initial velocity is non-zero at the maximum $v$ point the jumper keeps going below that point while their velocity is decreasing.

user228700

@JohnRennie It's a lot similar to S.H.M, right..?

John Rennie

The net force on the jumper is $$F_\text{net} = mg - kx$$ where $k$ is the elastic ocnstant of the rope and $x$ is the displacement from the point where the forces balance.

Once the rope is taut the motion is SHM.

But above that point te motion is just free fall under gravity because the rope can't exert a force on the jumper when it's slack.

user228700

Maybe I should replace the word "rope" with spring in my mind...

John Rennie

To analyse the motion we need to split it into the two parts and analyse each part separately.

user228700

@JohnRennie Yes, riight...

John Rennie

05:42

@Kaumudi actually that's not a good analogy because unlike a spring the rope can't be compressed.

user228700

@JohnRennie Oh snap, correct...

user228700

Okay, I'm gonna try to analyze the two parts now, with all the points that u told me about...

John Rennie

The analysis is straightforward but a bit messy.

I'd be inclined to start at the lowest point of the jump and work upwards, which by symmetry is the same as starting at the highest point and working downwards.

user228700

@JohnRennie Hm. It didn't even occur to me to split the whole motion into two different parts. Sigh.

John Rennie

You can do it by solving the equations of motion, but you can also calculate the velocity by conservation of energy.

user228700

05:47

@JohnRennie I'm trying to do it just qualitatively and make sure I believe everything...

John Rennie

Suppose we take the downward direction to be positive, so the gravitational force is positive and the rope tension negative, and take the origin to be the unstrtetched length of the rope.

user228700

@JohnRennie No, no, no! I'll do it on my own! :-P

John Rennie

OK :-) But I recommend using the conventions I've described above.

That way the motion splits neatly into $x > 0$ and $x < 0$ parts.

user228700

@JohnRennie OK, on it :-) Thanks sir!

DanielSank

06:10

@JohnRennie you've been "sir"'d.

John Rennie

@DanielSank a lot of Hindustani students use the honorific sir. I suspect it's just a standard way to approach any teacher rather than the way it's used in UK or US English.

DanielSank

@JohnRennie I prefer to imagine that @Kaumudi has implicitly knighted you.

user228700

06:39

@JohnRennie Yes, u are correct! :-) How is it used in the US/UK?

user116211

07:04

@Kaumudi knighthood

user116211

Don't know about US though.

John Rennie

07:15

@Kaumudi In the UK the honorific sir is used when schoolchildren are addressing a teacher in school, but outside of school it's only used in exceptional circumstances. People who have been knighted are entitled to be addressed as sir or maam, but few of them insist in it.

It feels very odd to have anyone address me as sir.

Swapnil Das

Sir in India is used to address an elder citizen/ teacher/A Learned person

John Rennie

Oh well, I meet two of those three criteria :-)

Swapnil Das

Of course, Sir :)

I'm feeling drowsy and tired after spending like hours pondering over an optics concept, bleh..

If I won't get any image of my answer, I'll put my doubt here. I hope a kind somebody would help me. TEs are hard 😛

user228700

07:35

@JohnRennie Oh, I see!

user228700

I can't help it :-P I hope that's OK.

John Rennie

@Kaumudi it's not a problem, it just feels odd :-)

user228700

@JohnRennie OK :-)

user116211

@yuggib o/

yuggib

07:56

@MAFIA36790 \o

user228700

08:14

@JohnRennie: Are u familiar w/the valencies of atoms and exceptions to the octet rule..?

John Rennie

Yes, sort of.

user228700

Okay. May I ask u a very small question?

user228700

..?

John Rennie

Yes, go ahead

user228700

Okay. Boron. It has only 6 valence electrons at most, even when bonded to other atoms.

user228700

08:24

This is an explanation that I found:

user228700

*"The principle of full octet is that most reactivity patterns are described well if there is a driving force / energetic gain for full octets in main-group atoms."*

*"However, the only facet of an atom that dictates anything about the number of valence electrons is its nuclear charge. Boron has a charge of 5. This is balanced by 5 electrons. Two of them are core electrons and the remaining 3 are valence electrons. The valence electrons may participate in bonding through sharing with other atoms, to make three bonds. Three bonds = six electrons"*

John Rennie

ok

user228700

No, actually, never mind. The periodic Table usually takes too long to respond but Rubisco has responded there :-) Thank you anyway!

John Rennie

08:40

@Kaumudi I had a quick look in the periodic table, and I would have answered the way that Rubisco did. The octet rule is ony a guideline and breaks down even for such common elements as transition metals. That's why those elements have many different oxidation states.

Slereah

@yuggib !

yuggib

@Slereah yes?

Slereah

If I have a distribution defined by the series of functions $(f_n)_{n\in \Bbb N}$

yuggib

?

Slereah

Do you think a good embedding in hyperreal would be the function $(f_0(x), f_1(x), f_2(x), ...)$

yuggib

08:43

what do you mean by "a distribution defined by a series of functions"?

Slereah

Well distributions can be defined as equivalence classes of sequences of functions

yuggib

mmmmmh, that does not sound as a good definition

Slereah

I don't think it would be the same values for the intervals where the function is standard

But I think it might be at least infinitesimally close to it?

yuggib

at most distributions can be defined by minimal Cauchy sequences of compactly supported functions (with respect to the weak topology).

Slereah

Compactly supported?

Isn't the heaviside function a distribution?

yuggib

08:49

@Slereah do you have the faintest idea on how distributions are defined? ;-P

Slereah

The dual space of test functions with respect to the inner product?

yuggib

the topological dual space of test functions

endowed with their usual locally convex topology

since the compactly supported functions are dense in the distributions (endowed with the dual topology), any distribution can be seen as a Cauchy sequence of compactly supported functions

Slereah

Well I suppose you could make a heaviside function from a sequence of compact functions

enlarging it as $n$ grows

Tho I'm not sure how that prevents the embedding I described

yuggib

I don't see why you want to see distributions as sequences of functions

user116211

0

Q: Idealizations of objects

Why is it that an object can be idealized as a particle or 'particle like' to solve problems? What are the limits of such a tool?

classical-mechanics

user116211

08:55

Does it want the analytical mechanics point of view?

Slereah

For the aforementioned definition in the hyperreals

yuggib

the distributions form a (topological) vector space, why don't you enlarge it as you would enlarge any other vector space?

user228700

@JohnRennie Yes, OK. Thanks :-)

Slereah

@yuggib Well the functions of hyperreals already form an algebra

While distributions do not

And by transfert their extension would also not be an algebra

yuggib

that is the way it is

Slereah

08:59

I know

But the goal is to have an algebra

yuggib

the extension you propose is meaningless, it defines a function $f:\mathbb{R}\to ^*\mathbb{R}$

Slereah

Well yes

But I don't think algebras of distributions ever map to $\Bbb R$

Colombeau algebras certainly don't

yuggib

the question is always the same...then what?

Slereah

$\delta^2$ is not integrable in $\Bbb R$

Well then that is where the renormalization comes in

yuggib

I really don't see anything interesting in this whole stuff, sorry

user116211

09:02

It should be noted that while in vectorial mechanics, a point is isolated from the whole system, in analytical mechanics, the whole system is considered without decomposing it into into individual points. Then we can transform the body concerned by a one-to-one correspondence from the regular coordinates to the abstract realms of more general Configuration space where the body though composed of infinitely many points is considered as a single point known as Configuration point. All the regular coordinates can be expressed as the function of generalised coordinates. — MAFIA36790 12 secs ago

Slereah

Oh well

Physics Meta

1

Q: Put on hold and rephrasing the question

OK - so I rephrase the question to highlight the physics content as suggested and... zilch. It still stays on hold. So the only option appears to be to re-post the revised question under a different heading. Is that how this is supposed to work or should the person who put it on hold actually c...

discussion close-reasons specific-question

yuggib

$\delta(x)^2:\mathbb{R}\to ^*\mathbb{R}$ would probably be not in the range of your embedding of distributions into functions

Slereah

Well no

yuggib

so you cannot interpret it as a distribution

Slereah

09:04

The embedding would not be a bijection

yuggib

I know

I am saying that then you lose predictive power completely

Slereah

Why would I, though

$\hat H$ doesn't converge by itself

yuggib

because you can't transfer back to usual stuff

Slereah

The point would be to have all observables properly renormalized to be $\in F$

Finite numbers

As it stands we already do products of distributions

yuggib

but that's just using the word shit instead of the word poopoo

no new information, only the plain old problem with the plain old solution

given in a (slightly) different language

sorry, I don't want to be destructive, but really I fail to see the interest in simply changing the language

if it does not lead to new insights/results

Slereah

09:11

Well as it stands the product of distribution is done without being well defined

I don't think it will lead to particularly new insights

It just sounds like a decent idea

yuggib

@Slereah it is rigorously done only when it is well defined, i.e. when you have compatible supports and the alike

I gave you some reference at the rigorous level.

Slereah

Still I'd like to try to make something

Might look good on a resume

Don't want to lose my physics edge :p

Slereah

09:27

If someone can write a paper about GR with quaternions I can write a bloody paper on QFT with hyperreals

Slereah

10:56

Bane Star (Big Guy For You) ft. CIA

►

Sir Cumference

12:13

@0celo7 I thought you were raised in DC?

user116211

@SirCumference He then came to US.

Sir Cumference

Huh.

user116211

Recently DS quizzed 0celo; you can check the transcripts @SirC.

Sir Cumference

DS?

Oh Daniel

user116211

@SirCumference yes.

Sir Cumference

12:19

@MAFIA36790 Uhh...what should I be searching for?

I don't know any of the words they used

user116211

@SirCumference hmm... grew up in Germany?

user116211

I'm not sure though what the exact words were.

GPhys

large physics books with poorly filled indexes = Q_Q

Slereah

Get a GR book

They are full of indexes

rimshot

0celo7

12:45

DO WE HAVE NEW MODS YET

user116211

@0celo7 Why? Election still has not ended.

0celo7

that's what I'm asking about

@SirCumference just ask @ACuriousMind whenever he decides to come back

but once he loses the election I doubt he'll show up again :/

Sir Cumference

@0celo7 I trust you

You think he'll lose?

user116211

@0celo7 I betcha he will not.

0celo7

idk how he expects to win

Sir Cumference

12:48

I'm betting he'll win

0celo7

he just disappeared

I'm sure his constituents retracted their votes

Sir Cumference

What

No

Idk

user116211

8 hours ago, by 0celo7

I did not vote for ACM

Sir Cumference

Oh shoot

user116211

It's ACM!

Sir Cumference

12:49

That timing tho

ACuriousMind

@0celo7 I'm not "disappeared", I'm just a bit more busy than usual because I'm involved in the welcoming of the freshmen

0celo7

Dw, he's not a mod

nothing to be afraid of

ACuriousMind

@Slereah With what?

0celo7

@ACuriousMind I see, what have you been doing?

Jim

@0celo7 Incorrect. You should be afraid. Very afraid

Slereah

12:51

@ACuriousMind What's the justification of using distributions for QFT rather than functions

Is it just the CCR?

Sir Cumference

@0celo7 Because if he becomes a mod

ACuriousMind

@0celo7 Yesterday: 11-13 Teaching about sets and relations 14-19 Roleplaying workshop 20-03 Bar tour

Sir Cumference

You should be afraid

0celo7

BAR TOUR

WHAT KIND OF SHIT IS THAT

Jim

Fun

ACuriousMind

12:52

It's...where you go to bars and drink.

I'm not sure what's unclear about that :D

0celo7

That's illegal for Freshmen

ACuriousMind

Not here

Sir Cumference

@0celo7 Not everywhere

Jim

@0celo7 Maybe for your freshmen

0celo7

Jim, for yours too

Jim

12:52

nope

user116211

@0celo7 US is weird.

0celo7

I was 17 as a Freshman

Slereah

Those prudish US people

0celo7

So I was illegal everywhere

Slereah

nah

17 can legally drink here

Like not liquor, I think

ACuriousMind

12:53

@0celo7 Nope, beer & wine are legal from age 16

Slereah

But beer is fine

ACuriousMind

And it's not like most bars want to see ID in the first place :P

0celo7

@ACuriousMind I don't understand the German system

you're teaching them PhD set theory as Freshmen

but not about Banach spaces?

when the latter is actually useful?

Sir Cumference

Crap, misread you

Jim

19 here, but most frosh are 19

Slereah

12:54

The German system :

Sir Cumference

"Frosh"?

ACuriousMind

@Slereah Yeah, I think it's mostly the CCR

Slereah

aight

ACuriousMind

@0celo7 You can't define what a map is without talking about sets and relations :P

Slereah

I can work with that I think

0celo7

12:55

I don't want to define maps

they just are

Sir Cumference

Seriously tho, most freshmen are 17

Some 18, others 19

Jim

@SirCumference freshman in canadian

0celo7

ok @ACuriousMind I have to go

Sir Cumference

@0celo7 Woah

0celo7

I will revote for you

ACuriousMind

12:56

There are more 17 year olds now that we've shortened school by one year and removed the draft, but until a few years go the freshmen age was more like 19-21

Slereah

Were you drafted

In the Wehrmacht

Jim

yeah, with no grade 13, we have more at 18 now, but it's still pretty common to have students at 19 when they come in

ACuriousMind

@Slereah Nope - they called me in for the medical exam, but the draft was removed before they could get me

0celo7

Unpatriotic

Slereah

You draft dodger

ACuriousMind

12:58

I would've objected anyway

Slereah

what will the Kaiser think

0celo7

Why?

Loong

0celo7

Are you rooting for the enemy

ACuriousMind

@Slereah I think I'm obliged to point out that that's the Bavarian system. The rest of us doesn't dress like that.

Slereah

13:02

much better engineering

Jiminion

@SirCumference I think the default pic is AWESOME!

Sir Cumference

@Jiminion Huh?

No you don't

Stop

You have 5k rep on SO and you still haven't changed it?

user116211

@Loong Hitler must be proud.

Jim

@SirCumference I use it because why not? Before you question me, you should question QMechanic

strictly speaking though, my pic is not the default

Sir Cumference

I question all of you

It's generic

Shing

13:10

um.... is Jiminion = Jim?

Jim

it is a screenshot of what my original default was. The default for me now is different

Sir Cumference

No.

Jim

@Shing No. There is an inequality there

Sir Cumference

No...

Shing

lol

Sir Cumference

13:10

@Shing Well they both use the default pic...

user116211

@Shing Jim is the Jimperor.

Jim

I am the one true Jim. Thou shalt have no other Jims before me

user116211

^

Sir Cumference

Oh boy...

@Qmechanic You're included here too

ACuriousMind

I don't get why you care at all that people use their gravatar

Sir Cumference

13:14

It just bugs me. You know?

Like people who capatalize every first letter of their words

Or people who put "aspostrope's" before every "s"

ACuriousMind

The latter two are errors. The picture is a personal choice.

Sir Cumference

Idk, it just seems like something a newbie would do

Yet these people have thousands of rep

Dunno what to expect

user116211

What is the problem of retaining the old gravatar @SirC? I never thought it's anon-trivial stuff.

Sir Cumference

I dunno, maybe I'm just being a picky idiot

Ya ever been on Youtube, and seen someone with lots of subs but the default pic?

I haven't at least

Jim

13:30

I use it to keep anonymity. It allows people to have a picture they can identify me with without revealing much about my personal likes/dislikes

0celo7

@ACuriousMind huh?

Then change yours back to a default one

ACuriousMind

huh?

0celo7

Tough talk but can you walk the walk?

ACuriousMind

I have no idea what you're trying to say: I obviously want a picture different from my gravatar but I don'T see why I should care what picture others use

user116211

ACuriousMind

101 1

0celo7

13:42

@ACuriousMind I just missed you, sorry :(

Jim

@0celo7 I feel so loved right now. But if people don't like me, they don't have to leave. I just ask that they not foment insurrection against me. Unless they've already fomented insurrection. I'm not a monster; I'm willing to grandfather in pre-fomented insurrection

0celo7

What are you referring to?

Jim

click the arrrow

0celo7

Don't know how

I've never understood the arrow system

Also how do I click on mobile?

user116211

@0celo7 Click on the arrow.

Jim

13:45

Step 1) turn on computer. Step 2) find adjoining mouse/pointer system. Step 3) RTFM!

0celo7

RTFM?

Jim

acronym. Not user friendly

user116211

@0celo7 ^^

ACuriousMind

@Jim Read the funny manpage?

0celo7

13:46

@MAFIA36790 IM TRYING

Jim

@ACuriousMind Exactly

user116211

@0celo7 :(

Jim

9 hours ago, by 0celo7

Jim has a PhD in likability

and succeeding message

0celo7

Ok thank you

user116211

It works in my damn Lumia; yours is IPhone; so it will work.

0celo7

13:48

I am mentally deficient

I can't click

user116211

;/

Jim

@MAFIA36790 But in the event that it is too difficult. It's easier for us to just link the message

user116211

@Jim sure.

0celo7

My fingers are too fat

Jim

I'm not touching that one

0celo7

13:53

@Jim why

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