The h Bar

@JohnRennie: Morning :-) I have a very quick follow-up question regarding yesterday's discussion about boiling. We agreed that the external pressure on a sealed steel vessel will be $P_atm - F_{Walls}/A$. For the vessel to not be crushed, we must have $$F_{Steel} /A + P_{Vap} = P_{atm}$$, correct? But $P_{Vap}$ just keeps on increasing. Does this mean that $F_{steel}$ is a self-adjusting force (since $P_{atm}$ is a constant)

dmckee

@DanielSank The blames falls squarely on the white button mushroom that were the only things on supermarket shelves throughout my childhood. Those remain nasty, tasteless, and disgustingly like limply damp sponges in texture.

But I am—slowly—getting over the psychic damage they did me and learning to accept dirtberries in my life.

heather

@DanielSank, @Skyler, sorry, had to do some chores =)

Skyler

@heather dont worry about it

DanielSank

@dmckee Yup. I thought I hated mushrooms because of button mushrooms and people putting them on pizza.

heather

1:29 AM

@DanielSank I don't think so...

@dmckee ick, yes, those are gross

some mushrooms are good, but those aren't

I don't mind the gray ones

I don't know what those are called

DanielSank

@heather Correct!

So... what $f$ does solve it?

Can you explain why $f(t) = t^3$ doesn't work?

heather

well, the derivative of $t^3$ is $3t^2 \neq t$

f(t) = t....the derivative of t is 1, so that doesn't work

DanielSank

@heather Right.

@heather Right, $f(t) = t$ doesn't solve our problem.

heather

f(t) = $t^2$...the derivative there is 2t, which doesn't work

DanielSank

@heather No but it's sooooooo close....

Can you make the $2$ go away...?

heather

1:41 AM

f(t)=$t^2/2$, or like times 1/2, so it's like $1/2t^2$, so 2*1/2t, so 1t, so t

right, does that work?

DanielSank

Yes.

You solved a differential equation.

heather

yay!

DanielSank

Everyone congratulate @heather for solving her first ODE.

heather

\o/

=D

DanielSank

Oh, by the way, $f(t) = t^2/2 + c$ works for any value of $c$.

Can you see why?

heather

1:43 AM

well, with a derivative, the constants just kind've go away, because parallel lines have the same slope.

so it doesn't matter what c is

it'll just go away.

DanielSank

right

heather

and then because of that with integrals we have to put back in the c

DanielSank

@heather You have to put it in because it's a solution!

heather

yeah

DanielSank

No matter what $c$ you pick, $f(t) = t^2/2 + c$ is a solution.

You don't really have to think about integrals. You can if you want.

HDE 226868

1:45 AM

@heather Congratulations!

heather

@HDE226868, thank you =)

@DanielSank, okay.

Is there a general "rule" to solving ODEs, or no not really?

is it guess and check like that?

DanielSank

There is no general rule.

There are rules for certain cases.

There are a lot of differential equations for which you can write the solution as an integral.

...and in those cases, there are simple procedures to get to those integrals.

In many cases, you can actually do the integrals too.

heather

i think i'm starting to see why my dad says differential equations are a bit of a pain =) could i try a harder one?

DanielSank

@heather Sure, do you know the derivative of sine and cosine?

heather

derivative of sine is cosine and derivative of cosine is -sine, then derivative of that is -cosine, and then derivative of that is sine again.

if i remember right, anyway.

DanielSank

1:57 AM

Yep.

Ok, so here's a super-duper important differential equation:

$$D^2 f = - C f$$

Here $D$ means "derivative".

IMHO the $(df/dt)$ notation is kind of misleading.

You can use it if you want though!

heather

i'm not sure what that equation means.

the derivative squared of f equals -Cf (what's C?)

DanielSank

$C$ is a constant.

heather

okay.

negative some constant f.

DanielSank

yep

heather

and derivative squared means the second derivative?

DanielSank

2:00 AM

Yes.

If you want, start off with $C=1$.

heather

okay. so =-f.

hmm.

DanielSank

Yeah, what function, when you take it's derivative twice, gives you the same thing but with a minus sign?

heather

okay, let me think about it

Ramanujan

Is damping force is the only force which is directly proportional to velocity?

DanielSank

Well, that's the definition of a damping force. At least, in the most usual case.

koolman

2:06 AM

Hii @heather

Ramanujan

So there are some other forces which can be directly proportional to velocity?

DanielSank

@Ramanujan If a force is proportional to velocity and with the opposite sign, it is called a "damping force".

Ramanujan

If a force is proportional to velocity and with the positive sign, it is called a "?? force".

DanielSank

@Ramanujan I dunno.

ACuriousMind

@Ramanujan Stokes friction is, for example.

Ramanujan

2:19 AM

@ACuriousMind hmm

heather

@koolman, hi

koolman

@heather the same problem

heather

@koolman ?

koolman

On that site

heather

ah

I'll confer with @Mew.

koolman

2:27 AM

Please @heather do it as soon as possible

heather

@koolman, i will talk to him as soon as I can, but really, it would be much faster if you added as much effort as you could to both your open questions. you still have an open question that you haven't added any effort too.

koolman

@heather I have shown all my efforts to them . seriously I have no more work to show.

Sorry ,if it feel rude

heather

physics.qandaexchange.com/?qa=515/nuclear-physics

^this one you have not edited since it was reopened

physics.qandaexchange.com/?qa=524/electric-field-density

^this one you have not added your thoughts on option C. even if you just add your thoughts that will make it better. I'm sorry, I just don't see what the trouble is here. why are you having trouble with C? just explain that.

koolman

@heather I have doubt in the first step , then how can I show much work

heather

why do you have a doubt?

explain that!

koolman

2:32 AM

@heather in that I have written the electric field equation which is related to all the options

heather

sorry if i'm coming across wrong, but people on this site are volunteering their time for you, the least you can do is put in a little effort.

and i don't think you're doing that.

koolman

@heather I have already explained that

heather

you have not

you aren't even making an attempt at the small things

like putting it in in mathjax not a photo

DanielSank

^

2016 December 14: Heather starts to become curmudgeonly.

heather

lol

sorry

DanielSank

2:34 AM

Don't be.

koolman

@heather its not small thing . It would be for you not for me . The answer given is also not that what I have asked . You can see

heather

"curmudgeonly: crusty, ill-tempered" @DanielSank =P

DanielSank

@heather oh, wrong word...

Ramanujan

@heather Google search! Also archaic

DanielSank

2016 December 14: Heather starts to show symptoms of "read-to-many-bad-questions-itis".

4

better?

heather

2:36 AM

@koolman how about this. How about I give you a couple of links to format your posts with mathjax. Then I'll come through and try to help you out with the grammar a bit. then we'll see where we stand.

@DanielSank, now that is definitely true =P

@koolman, do you agree that would be an appropriate step, or...?

koolman

@heather I have asked mew about this , he said its fine to post image of question

@heather but It would take monthsss

heather

@koolman, perhaps mew misunderstood the situation. I'll talk to him about it. But especially with your nuclear question, the image is hard to read. Please trust me when I say that everyone would like you more if you used mathjax.

koolman

I don't have enough time

heather

@koolman It would not take months, please don't be ridiculous.

ACuriousMind

@DanielSank Alas, that's something you can never be cured of once you've caught it :P

heather

2:39 AM

You can format the posts in mathjax very quickly, trust me. Considering you are a little newer, it'll take maybe an hour for both, and that's a high estimate @koolman

and I'll be doing the grammar.

koolman

@heather yeah I can understand for nuclear . But not for others , they are clear and easy to read

heather

that's not much work at all for you.

@koolman, it shows people you're putting a little effort in. it also standardizes the formatting of math equations so people don't get mixed up on which variable you mean.

ACuriousMind

@koolman If you don't have enough time to type the text in a picture why do you presume people should make time to answer your questions?

6

heather

@ACuriousMind, thank you.

koolman

But I mean whats the problem in it

Ramanujan

2:41 AM

@heather you are moderator of that site?

heather

@Ramanujan, of physics problems q&a, yes.

koolman

@ACuriousMind I have no problem if they also post picture

heather

::bangs head against desk::

@koolman, that's not the point

if you aren't willing to put in a small amount of effort, why should you expect people to even type an answer?

koolman

I have done all things I can do

heather

You have not.

ACuriousMind

2:44 AM

@koolman 1. Pictures are not searchable. This is a big one. One can't find your posts if the relevant keywords are hidden from the search engine in a picture. 2. I have almost never seen a question where there wasn't irrelevant stuff in the picture that could've been trimmed out. Posting a picture means you didn't really try to extract the parts relevant to your question.

4

koolman

Now what more you want . :(

heather

1. Rewriting the pictures as mathjax.

2. Fixing the grammar up a bit for readability (I can do this, I understand English isn't your first language and don't blame you one bit for this).

Then we'll see where we stand.

koolman

And

Okay we can try them

heather

Okay, let me give you the mathjax links.