Problem Solving Strategies

Koolman

7:00 AM

$V\sin k \cos k - V\cos k \sin k$

$=0$

This is the same case @blue

Anonymous

@Koolman In this case you don't know the direction of net velocity of the cylinder....

Anonymous

It is not along the incline....

Anonymous

So there can be a component of net velocity perpendicular to the left wedge

Koolman

According to solution the net velocity is inside the left wedge

Anonymous

@Koolman What?

Anonymous

7:05 AM

Inside?

Koolman

direction of velocity

Anonymous

The diagram given in the solution you posted seems to be wrong...

Anonymous

Anonymous

This is the correct diagram ^

Anonymous

Just solve for $v_x$ and $v_y$ and you are done

Koolman

7:06 AM

No that is correct

It is in frame of left wedge

@blue This one is also correct wrt ground

I think I got it

Anonymous

Oh right

Anonymous

I was thinking in ground frame all this time

Koolman

Thanks a lot @blue

Anonymous

Welcome...it was a nice question....I need to revise mechanics :-P

Anonymous

I getting weaker in mechanics day by day...I will do it after I complete the rest of the syllabus

Koolman

7:09 AM

I will do inorganic at last

Anonymous

Do inorganic from now...you will literally die of heart attack if you do it at the end!

Anonymous

The qualitative analysis is deadly

Koolman

oh god

Anonymous

(Also inorganic is very very scoring)

Koolman

I hate that

Anonymous

7:10 AM

Chemistry is the only dependable subject in the exam

Koolman

yes

Anonymous

Maths and Physics can screw you anytime

Anonymous

:_P

Koolman

@blue but they are interesting

Anonymous

If you get atleast 70 percent questions correct in chem...then you have a good chance of getting top 5000 ranks

Koolman

7:11 AM

I think top 3000

Anonymous

Maths and Physics...I will target something like 50 percent and up

Anonymous

I am really scared now :-P

Anonymous

Need to study!

Anonymous

Bye!

Anonymous

Have a good day!

Koolman

7:12 AM

Bye

Madhuchhanda Mandal

Oh shit!!!! We were fucking around uselessly

Power need not be conserved in Interference

Hence what I thought is absolutely correct

You need to ignore diffraction

For the rest, the problem is absolutely perfect

@skillpatrol @skullpetrol Are you somewhat related :P ??

skill patrol

identical

I can only get away with it because my accounts do not interact :-)

Anonymous

@MadhuchhandaMandal Total power is always conserved. However, at a point it may be increased or decreased due to interference.

Anonymous

Otherwise conservation of energy will be violated.

Anonymous

@MadhuchhandaMandal That is right.

Madhuchhanda Mandal

7:21 AM

"At a point it may be increased or decreased" @blue absolutely.. This exactly happens in the question.

Anonymous

@MadhuchhandaMandal Yep.

Madhuchhanda Mandal

Hence the problem is solved

Although I am not sure about the notion of total power

Or does the notion of total power exists in wave motion?

Anonymous

It does....

Anonymous

Total power is just sum of energy passing through the slits in unit time...

Madhuchhanda Mandal

Although I am not sure about how to define it

Anonymous

7:29 AM

I don't understand what your question is.

Madhuchhanda Mandal

I am not able to understand the notion of "total energy" which you think must be conserved.

Anonymous

@MadhuchhandaMandal Total energy. Is the sum of energy incident on the slits.

Anonymous

Something like $P$ Joules per second

Madhuchhanda Mandal

And conservation ?

Anonymous

$P_A+P_B$

Madhuchhanda Mandal

7:31 AM

How do you define it?

Anonymous

Define "define"

Anonymous

I don't understand what you are asking...

Anonymous

I already defined it

Madhuchhanda Mandal

I mean how do you think you can define "conservation of total energy"?

Anonymous

"Is the sum of energy incident on either of the slits"

Anonymous

7:33 AM

3

Q: Conservation of energy in interference of light

In interference of light, I know that energy is conserved globally but how the energy disappeared at minima appears at maxima? Is there any path by which energy flowed or is it just energy couldn't reach at minima and all energy is at maxima. Is it right to say that energy can be destroyed if sam...

Anonymous

20

Q: Energy conservation and interference

I have a problem with energy conservation in case of interfering waves. Imagine two harmonic waves with amplitudes $A$. They both carry energy that is proportional to $A^2$, so the total energy is proportional to $2A^2$. When they interfere, the amplitude raises to $2A$, so energy is now proport...

waves energy-conservation interference

Madhuchhanda Mandal

That's different thing

Anonymous

van.physics.illinois.edu/qa/listing.php?id=1918

Madhuchhanda Mandal

I don't know how to convey my thoughts... Wait a minute

Okay lets put it like this : Pa + Pb not equal to Pi because we don't know about their sign

We know only their magnitude

Anonymous

Since when does power have sign....

Madhuchhanda Mandal

7:37 AM

It does

I mean definition of intensity does ignore sign

Anonymous

wtf...show me one example where light power or energy has sign

Anonymous

it is always positive

Anonymous

what does negative power even mean

Anonymous

no idea what you are talking about

Madhuchhanda Mandal

Consider a body moving with v m/sec forward. It is being acted by two forces f N backwards and f N forwards. If you define intensity analogous quantity , then the body will say I am receiving 0 Energy. Yet energy provided to the body yes (from intensity point of view) 2kf

Intensity does ignore sign

Anonymous

7:43 AM

@MadhuchhandaMandal wtf

Madhuchhanda Mandal

(The powers provided were 180 degree out of phase. In the given question they are 60 degree out of phase.)

Anonymous

"Yet energy provided to the body yes (from intensity point of view) 2kf"

Madhuchhanda Mandal

@blue wwtf?

Anonymous

This is hilarious.

Anonymous

^

Anonymous

7:43 AM

Give me a link or source

Madhuchhanda Mandal

I don't have the habbit

Anonymous

Give some standard link or source which says power has sign.

Madhuchhanda Mandal

The link is : intuition

Anonymous

Otherwise your interpretation of light power having positive or negative sign makes no sense

Anonymous

@MadhuchhandaMandal Ah, so I am talking to a troll.

Anonymous

7:45 AM

Come back when you have some link or source to fall back upon.

Madhuchhanda Mandal

Yeah.. You are doing so

Ok a question :

Ok by the way do you know formulae for intensity?

(Connecting amplitude, frequency...)

Anonymous

yes.

Madhuchhanda Mandal

If you know it, I mean if you can derive it, you will see that it's just a time averaged quantity. There exactly the abstraction of sign takes place. Instantaneous intensity does have sign and its a time dependent quantity. When two instantaneous intensity (which are out of phase) superimposes then the net instantaneous intensity is given by vector addition which is then time averaged to give the net averaged intensity or the more popular term intensity (which is what asked in the question).

Thus Pa+Pb not equal to Pc. But it does not violate conservation of energy as well. Because at every instant energy is conserved.

Anonymous

Intensities are never out of phase. The waves are out of phase.

Anonymous

Intensities are out of phase. makes no sense.

Madhuchhanda Mandal

7:58 AM

Very good

Keep on

Bye

Anonymous

k.

Madhuchhanda Mandal

Had you known or ever tried to derive intensity you would know what I mean

Anonymous

LOL

Anonymous

I derived it over 10 times.

Anonymous

Here is it

Anonymous

7:59 AM

:

Pritt Balagopal

Hello

Madhuchhanda Mandal

Then it's your bad not to notice it

Anonymous

@MadhuchhandaMandal Or....you are totally cofused

Anonymous

LOL

Anonymous

Bye

Madhuchhanda Mandal

8:00 AM

Or I am speaking something which is totally tangential to your heaf

*head

Anonymous

Just go man.

Anonymous

I am not interested.

Anonymous

You are speaking nonsense.

Anonymous

And not even wanting to hear why you are wrong.

Madhuchhanda Mandal

@blue LOL you have not read what I said !!! Instantaneous Intensity and Intensity are totally different quantities !!!

Anonymous

8:03 AM

Not interested. Bye.

Madhuchhanda Mandal

But I am very much interested to pluck the idiocy out of you

Bye

Anonymous

There are many other idiots in the world. Go and find one.

Anonymous

When one starts using personal attacks to protect their point of view, it is evident that their ground is shaky. :-)

Madhuchhanda Mandal

No.. They are total idiots. It will take a lot of hard work to take idiocy out of them. But you are not a total idiot. Hence it will be very much less work to pluck idiocy out of you.

Anonymous

And since you started using personal attacks, I refuse to talk to you any further.

Anonymous

8:06 AM

Putting you on ignore.

Madhuchhanda Mandal

I have never personally atacked you. Idiocy is not an attack. Everyone has it. Even me. But you should at least listen to someone who tries to take it out of you.

Putting you on ignore : that's a evasive approach my friend

Anonymous

First learn how to talk decently. Maybe, then we can talk.

Anonymous

Bye.

Anonymous

I've work to do.

Madhuchhanda Mandal

Hmm... I don't know how to talk decently.. I only know how to talk non-hypocritically. Can you claim that you have never thought me an idiot during the conversation ??

If you have thought , then why haven't you said that?? That's hypocrisy. Call md

*call me an idiot and point out my idiocy

I will never mind

Anonymous

8:13 AM

On the contrary I thought you were quite intelligent.

Madhuchhanda Mandal

As I can clearly point out the idiocy , I stand by my opinion

Anonymous

Anyway, I am not interested anymore. Find someone else.

Anonymous

You could say I am trying to escape. So be it.

Madhuchhanda Mandal

Ok cool... Calm down atleast... For the sake of physics

You can't deny Physics. Let me say my viewpoint.

If you are not interested... Let it be.

Anonymous

Yeah, let it be.

Madhuchhanda Mandal

8:16 AM

Good bye.... You don't love JEE not Physics :( :(

*you love JEE not physics

Anonymous

Yeah, you could say so. I am the most money-minded, greedy, capitalist guy you will ever see who is preparing for JEE. LOL

Madhuchhanda Mandal

Oh.... Had you said it at the beginning this argument would have never aroused.

Anonymous

@MadhuchhandaMandal That's true.

Madhuchhanda Mandal

I throught you loved it.

Anonymous

@MadhuchhandaMandal Right, right. My bad.

Anonymous

8:20 AM

Go and find some other guy now.

Anonymous

Leave me alone.

Anonymous

Cya.

Madhuchhanda Mandal

Your bad.. That's true

Lemme find

Yashas

9:56 AM

You cannot use conservation of energy with interference.

@MadhuchhandaMandal @blue

It won't work.

If you have two beams of light which are out of phase which intersect at point X, the power at point X is given by the power formula which was used in that video.

If you keep a piece of ice at point X, the ice will melt according to the power obtained by that formula.

What happens to the rest of the energy? It appears on the other side of the ice.

provided the ice is much thinner than the wavelength of light

if the ice is thick, then the light which destructively interfered at the surface of the ice will melt the layer of ice half a wavelength ahead of the surface of the ice

Measure the Speed of Light with a Chocolate Bar

►

@MadhuchhandaMandal what? whatever direction the light comes from, it is going to carry energy E. Whatever gets affected by it does not care where the light came from.

@MadhuchhandaMandal You take average of the amplitude "squared"!!!!!!!!!!!!!!!!!!

You take the average of $\sin^2 (\omega t))$. Intensity is always positive!

And so is power

becaz power is also proportional to A^2

where A is amplitude not area

Moreover, if you send two beams of light in opposite directions, the intensity of both the light will be positive but there may or may not be a phase difference.

Intensity is also the square of the electric field

even that way, you get intensity to be positive

always

Madhuchhanda Mandal

"Intensity" quantity itself abstracts the sign.

Yashas

what do you mean?

even power of light does not have sign

Madhuchhanda Mandal

Letme think

Yashas

power is proportional to squared of amplitude

hyperphysics.phy-astr.gsu.edu/hbase/Waves/emwv.html

Poynting vector is in JEE syllabus I think.

idk

Madhuchhanda Mandal

Indeed intensity is squared amplitude. I don't argue with that

Ok. So why I1+I2 follows vector addition ? Because Intensity alone cannot convey the energy crossing the area at a instant of time. It only averages the energy crossing over a period of time.

*convey the information

Yashas

10:12 AM

No

Madhuchhanda Mandal

about

Yashas

Intensity does not follow vector addition

Amplitude does

Intensity is proportional to amplitude squared

for constant frequency, veolcity of wave, etc. , the intensity = constant * amplitude^2

the sign goes off

Madhuchhanda Mandal

Sorry... Vector addition was not the perfect term. I mean why Intensity does not follow scalar addition?

Yashas

I need to think about that

but that does not make intensity a vector

Madhuchhanda Mandal

Because at a point (instant) of time intensity cannot convey information about energy traversing

Yashas

10:15 AM

there are third rank tensors which do not follow vector addition

nor do they follow sclar addition

Madhuchhanda Mandal

(I never claim intensity to be vector though)

Yashas

omg

intensity follows scalar addition if the waves are in phase

the intensity formula deals with light beams which are out of phase

if they are in phase, intensity adds up numerically

Madhuchhanda Mandal

Because intensity are in phase for two identical waves

(Don't confuse it with phase of vectors)

Its something like :

Yashas

you needn't have identical waves

hmm maybe you need :p

if they are off by multiples of 2pi then they look identical

if the frequency is different, then you WILL see interference

the interference pattern fluctuates though

our eye might see it as plane light

becaz it wud be too fast for our eyes to notice

@MadhuchhandaMandal something like?

Madhuchhanda Mandal

A moving body being acted upon by two forces in opposite directions. If you ask force 1 what was the average energy that crossed the body due to you , you will get reply like something : k f. If you ask the same question to force 2 it will also say kf. But in reality the body is not receiving any energy because the forces are doing opposite work

Yashas

10:25 AM

The power is zero if the forces are not doing any work.

zero + zero = zero

but you can have negative power in mechanics

power can be negative just like work

but that analogy does not work with light

You can ask in h bar. Few hours ago, I had some doubt with conservation of energy with interfering light and I got that cleared up.

You get better answers and they are from far more experienced and credible people.

Madhuchhanda Mandal

But here arises another doubt

On rigorously trying to derive what I was saying it arosed

Yashas

can you show your derivation?

Madhuchhanda Mandal

Yes I am saying

(I am similar like you @32kbps :P)

Lets suppose a String.

Two waves can traverse independently through a String. Right?

Yashas

yes

Madhuchhanda Mandal

10:40 AM

Wait.... I think there is a problem with the statement of superposition principle.

Or it's interpretation

It cann

*It cannot be independent

Yashas

in The h Bar, 49 secs ago, by Slereah

@Yashas the intensity of light is $W/m^2$

Madhuchhanda Mandal

Otherwise I=I1+I2 will hold true always

Yashas

in The h Bar, 51 secs ago, by Slereah

Unless you have negative energy light

in The h Bar, 53 secs ago, by Slereah

Probably not

in The h Bar, 58 secs ago, by John Rennie

@Yashas no.

the question was

in The h Bar, 1 min ago, by Yashas

@JohnRennie can intensity of light ever be negative?

Madhuchhanda Mandal

Oh no... It can't be negative. Because ... Wait I am turning mad

I think Superposition principle is saying about Mathematical aspect. Not physical significance

Because we know Integration(f(x)+g(x)) = Integration (f(x))+Integration (g(x)) so I=I1+I2

Yashas

how did u end up with that equation?

with the integrals?

Anonymous

11:12 AM

@Yashas Wait, what?

Yashas

Anonymous

Conservation of energy is valid in interference....

Yashas

nope

Anonymous

Okay, reading

Yashas

I mean the conservation of energy is valid

but not the way you expect

the light which destructively interferes goes through without affecting its locality

Anonymous

11:13 AM

How do you know what I expect ? :-P

Anonymous

tf.uni-kiel.de/matwis/amat/admat_en/kap_5/advanced/t5_1_3.html

Anonymous

This website explains it well

Anonymous

20

Q: Energy conservation and interference

I have a problem with energy conservation in case of interfering waves. Imagine two harmonic waves with amplitudes $A$. They both carry energy that is proportional to $A^2$, so the total energy is proportional to $2A^2$. When they interfere, the amplitude raises to $2A$, so energy is now proport...

waves energy-conservation interference

Anonymous

Motl's answer is nice

Anonymous

Interference and Law of Conservation of Energy

►

Anonymous

11:18 AM

Anyway, Madhuchanda's claim that light intensity can have negative sign is bullshit.

Yashas

I think we shudn't be rude towards each other.

skull petrol

^

Yashas

Anyway, my brain is done for today after watching that video.

that is not fair

adjusting the video so that they can explain

skull petrol

Like I said negative signs are just the opposite of positive signs.

Anonymous

@Yashas Yup, sorry.

Anonymous

11:22 AM

@skullpetrol Eh?

Yashas

@skullpetrol u told that in har bar, not here; these people are not aware of it

and I don't agree with you

intensity does not have direction

skull petrol

Note: 0 is neither positive nor negative.

Anonymous

My goodness, energy/intensity is not a vector.

Anonymous

If you mean Poynting Vector that's a different story altogether.

skull petrol

"Direction" itself is a matter of definition.

Anonymous

11:25 AM

To add to that "definition" itself is a matter of definition.

Yashas

aow where are we going?

Anonymous

And secondly, I don't think any standard book uses Intensity as a vector

Anonymous

And thirdly, I am too bored about that topic :-P

skull petrol

All I'm saying is negative numbers are just as "real" as positive numbers :-)

Anonymous

Urgh, so how does it matter in this situation?

Anonymous

11:27 AM

Obviously they are real numbers

skull petrol

We could define negative intensity.

Or negative anything.

Yashas

negative moment of inertia?

Anonymous

@skullpetrol You "could". But that's not how it is defined by standard sources.

skull petrol

As the opposite of the positive.

Anonymous

@Yashas lol

Anonymous

11:31 AM

@skullpetrol Can you give any source/link which uses negative light energy or defines it?

skull petrol

All we need is the property of opposites: a + (-a) = 0

Anonymous

@Yashas I don't know if he is trolling. Is he? :-P

Yashas

I am not going to join you with insults

skull petrol

I'm just trying to convince you of the reality of negative numbers :-)

Yashas

@skullpetrol there are quantities which can never be negative

let me define a quantity

Anonymous

11:33 AM

I never insulted anyone.

Yashas

a = c^2 where c belongs to the set of real numbers

a is always positive

Anonymous

This is just hilarious.

Yashas

that is the case with intensity

intensity is a function of the square of amplitude, angular frequency, speed of light and few more quantities which are positive

you can also write intensity as a function of electric field squared

you always get a positive quantity

@blue idk what you meant by that statement, it sounded like an indirect insult or some sort of mocking

Anonymous

@Yashas LOL...don't try to be a saint now :-D chat.stackexchange.com/transcript/message/36382251#36382251

Anonymous

in The h Bar, Mar 30 at 8:45, by Yashas Samaga

How stupid can people be? -____-

Anonymous

11:36 AM

in The h Bar, Mar 28 at 17:36, by Yashas Samaga

I am trying to get an idea of how stupid people can be

Yashas

they aren't here :P

Anonymous

in The h Bar, yesterday, by Yashas

and I am talking to a headless human

Yashas

and those people are really insane

Anonymous

in The h Bar, yesterday, by Yashas

A person who writes shit like this: http://prntscr.com/eso5pm

Yashas

the flat earth people

Anonymous

11:37 AM

@Yashas I get the same feeling here, sometimes :-P I just say openly, while you say it behind their backs....LOL

Anonymous

same pinch ;)

Yashas

LOL I am arguing with someone who says the sun is 1km up in the sky

and it is few cms wide

proof? he can measure with his finger

Anonymous

o_o

Anonymous

0_o

Anonymous

o_O

Anonymous

11:41 AM

mwahahahahahahahahhahahhahhahahahhahahahhahahahhahhhhhhhhhhhhhhhhhhhhhhaaaaaaaaa‌aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaahhhhhhhhhhhhhhhhhhhhhhhaaaaaaaaaaaaaaaaaaaaaa‌aaaaaaaaaaa

Anonymous

They were professional trollers and they made a fool out of you :-P

Yashas

theflatearthsociety.org/forum/index.php?board=10.0

I don't think so

skull petrol

All I'm saying is don't get hung up on the sign of the number. Remember what is important.

"The scientist does not study nature because it is useful to do so. He studies it because he takes pleasure in it, and he takes pleasure in it because it is beautiful. If nature were not beautiful it would not be worth knowing, and life would not be worth living. I am not speaking, of course, of the beauty which strikes the senses, of the beauty of qualities and appearances. I am far from despising this, but it has nothing to do with science. What I mean is that more intimate beauty which comes from …

Anonymous

@Yashas You want me to read through that? I saw the first link and knew that it is bullshit. :-P Don't want to fail in advanced!!!!

Anonymous

lol

Yashas

11:45 AM

lol

Transcript for