12:40 AM
it's a shame all the hats this year require active participation on the site
which sounds absurd to say but lots of people have finals around now

1:29 AM
@JohnRennie I know little or close to nothing about Physics. Can you choose a suitable basic law and explain how it is tested? :)

8 hours later…
9:23 AM
@yh05 Try gravity

10:05 AM
@yh05 yes, gravity is a good example. But I don't think this will work well as an asynchronous discussion.

It is even more doable these days
The typical classroom experiment for gravity involving a camera
No need for little slides to make metal balls go down

10:19 AM
@AaronStevens All users should bring gifts to the king of physics

10:38 AM
...who might that be?

If we go by points I guess @JohnRennie

You're not worthy

But I bring gold, myrrh and frankenstein

are kings considered the most reputable in their kingdoms, sir?

@skullpatrol if we were to take lessons from history we'd conclude kings are generally not shining examples of the human race.

10:50 AM
were there any exceptions?

I can't think of any

Me
The king of all I survey

11:23 AM
taking about men with power, I found a funny coincidence
in 2020 United States presidential election, 51 mins ago, by skullpatrol
December 10th, 2009:

President Barack Obama accepts Nobel Peace Prize

December 10th, 2019:

House of Representatives introduces Articles of Impeachment against Donald Trump

@Slereah Who on earth would want Frankenstein?
Frankenstein; or, The Modern Prometheus is a novel written by English author Mary Shelley (1797–1851) that tells the story of Victor Frankenstein, a young scientist who creates a hideous sapient creature in an unorthodox scientific experiment. It is one of the great works of modern literature. Shelley started writing the story when she was 18, and the first edition was published anonymously in London on 1 January 1818, when she was 20. Her name first appeared on the second edition, published in 1823. Shelley travelled through Europe in 1815 along the river Rhine in Germany stopping in Gernsheim...
Frankincense (also known as olibanum, Hebrew: לבונה‎ [levona], Arabic: اللبان‎ al-libān or Arabic: البخور‎ al-bakhūr) is an aromatic resin used in incense and perfumes, obtained from trees of the genus Boswellia in the family Burseraceae, particularly Boswellia sacra (syn. B. bhaw-dajiana), B. carterii, B. frereana, B. serrata (B. thurifera, Indian frankincense), and B. papyrifera. The word is from Old French franc encens ("high-quality incense").There are five main species of Boswellia that produce true frankincense. Resin from each of the five is available in various grades, which depend on the...
=)

11:39 AM
While it's possible Sam doesn't know the difference between Frankenstein and frankincense it seems more likely he was making a joke.

@user21820 He is a doctor
Sounds like a good man to have around

@user21820 her?
:-)

@JohnRennie I couldn't tell.
@Slereah Lol.
@skullpatrol Nah. She's not on earth. =)

Courtesy of New Scientist:
3

Good one.

11:50 AM
@skullpatrol There is a Hammer Horror film called Dr. Jekyll and Sister Hyde, the premise being that Jekyll's potion turned him into a woman as well as a psychopath. As I recall it is a truly dreadful film.

12:09 PM
Merriam-Webster announced Tuesday that it has chosen “they” as the 2019 word of the year. The singular “they” is a pronoun used to refer to a person whose gender identity is nonbinary, a word that itself was added to the Merriam-Webster.com dictionary in September of this year.

> “When I use a word,” Humpty Dumpty said, in rather a scornful tone, “it means just what I choose it to mean—neither more nor less.” “The question is,” said Alice, “whether you can make words mean so many different things.”

12:38 PM
2

1 hour later…
1:46 PM
@JMac Is this better?

@AaronStevens Yes, you look much more like Bane now.

@JMac Nailed it

Hi @AaronStevens

@Knight Hello

Can talk to you regarding my academics
@AaronStevens Can I talk to you regarding some academic issues?
@JMac Are you around?

2:00 PM

@JMac Why failure sucks?

@Knight ... I don't think that's directly related to anything academic. That's just how most humans are wired... failure is not pleasant.

@Knight Yeah. Not fun to fail. But you can view it in a positive light as a chance to improve

2:06 PM
@JMac Is it possible to do something in the area of science without a degree from a reputable University? Well, all those NatGeo Series of Albert Einstein and other scientists documentary show something which is quite unreal. I want to know the reality

@Knight There are plenty of science jobs out there. Will you get to be in charge of a large, well-funded research lab at a top university? Probably not.
But there are many other jobs out there

@Knight I'm sure it's possible to do things "in the area of science" without a degree from a reputable university, but that also depends on what things you're talking about. If you're talking about contributing to the cutting edge of scientific understanding, it's often difficult to get people to even listen unless you have some sort of academic standing, or can very clearly express your thoughts in an academic way.

@JMac That's clearly what I'm talking about.

@Knight I don't think that was very clear at all
You just said "do something in the area of science"

@AaronStevens All I want is to earn my living through the wisdom os science

2:11 PM
@Knight Again, that is quite a vague statement.

@AaronStevens Why you're always so rude to me?
@JMac You too

@Knight I don't see how it is rude to try to have clear conversations where both sides know what each other are talking about...

@JMac I want to do something like Mr. A.L. Cauchy , he was a normal instructor at a Polytechnique College but he did something which is alive even today

I am not intending to be rude. Both JMac and I are unclear about what you mean specifically about your goals. Therefore it is valid to let you know we are unsure. If we did not, then we could not have a conversation
@Knight I suppose if you made a huge ground breaking discovery then your background wouldn't be as important.
But sometimes you need the background to get to that point
And a bit of luck

@AaronStevens Well ground-breaking is not something I want. I want something which really gonna my own idea, just like you and @JMac writes an answer with the understandign that is purely your own.
But See those Princeton University Phd guys, or Indian IITs' students they onyl seek for recognition and money.

2:18 PM
The foundation of my answers are from my education (from school and on my own). I do not claim to have created, established, etc. any of the physics I discuss in my answers on this site.

@AaronStevens A.L. Cauchy didn't discover anything new. He just used the ideas to describe some concepts, Lord Kelvin did the same. Your answers contain your understanding which stems from logic and with some axioms. I want something like that.
Where are you @JMac?

@Knight Well your degree doesn't give you knowledge. It is the other way around. If you want to do those things you don't need a degree

@AaronStevens Well then how I'm gonna live? I mean for how long my parents gonna feed me? If I would become close to 30 years then they will clearly tell me to do something regardless what I want to do

@Knight So you can get a job and still work towards your goal
It is an unreasonable expectation to put on yourself that you have to jump right into your dream job and you can't do anything else

@Knight I don't know how many of my "own ideas" I really have when it comes to physics. That's kinda the beauty of science and collective knowledge. I don't need to have my own ideas to apply the concepts of physics. People have already determined a lot of stuff, and I just need to look at what ideas they already figured out and apply them correctly.

2:25 PM
I want to teach eventually, but I am looking for post-doc research positions currently. I am working towards a goal
I am not staying put and wishing I could obtain my goals

@AaronStevens @JMac Here, in my society everyone is just running after those Engineering degrees, they can solve Physics very well in exams, and therefore the society strives to make me believe that I'm nothing.

@Knight Who cares what other people are doing
?
Sounds odd to let other people determine your worth

@AaronStevens Did no one ever compared you and made you believe that you are lower than him?

@Knight No, because I don't put my self-worth into what other people think of me
Heck, I don't even put my self worth in my career goals
If I couldn't do physics anymore, I would just find something else

@AaronStevens What if your society is an integral part of your life and you can't just igonre it?

2:30 PM
@Knight I am aware that there exist societies such as yours as you have described it. Unfortunately, the best you can do is to try to get into a university somewhere else.

@Knight Can you expand on that more? I guess I don't fully understand what you mean by society being an integral part of your life.
I mean I care about the people around me and respect their views. I just don't let those views define me

@AaronStevens There are some societies where unless you get out you face a huge amount of social pressure and bullying.

@user21820 That's quite a real-life advice.

@user21820 Oh I know. I am not saying there are not

@AaronStevens You always understand me but keep on asking for clarification. You always bully me , lol.

2:33 PM
@AaronStevens In some places, it is common for one's own family to behave in the way Knight described.
I don't presume it's the case here, but in such societies it can be harder to 'just ignore' it.

@user21820 I don't think I ever said to ignore it

@Knight don't feel "bullied," pal

@Knight I am not bullying you. You are asking questions and I am answering them. I have not said anything against you

@Knight You need to stop assuming everything is a personal attack, bullying, or rude. Much of what you say can be interpreted in many different ways. It's not bullying to make sure people are talking about the same thing before they start going into detail.

:-)

2:35 PM
@AaronStevens You wrote "Who cares what other people are doing?". I know none of you are bullying Knight, but what you wrote can come off as very insensitive.
Of course, ideally, we should not let others determine our self-worth. But that's ideal.

@user21820 Thank you for understanding me.

In reality, our emotional and psychological state is something that not many people have full control over.
Can try, but...

I am sorry, I did not mean to be insensitive. I thought I was being asked for my opinion and views

Sure, and I too agree with your ideals. That's why I suggest that one try to get out of oppressive societies into a society where the ideals are easier to achieve. =)

@AaronStevens Yeah, I just skimmed through it again... you are clearly just explaining the way you approach it.

2:38 PM
@user21820 Right. Support is always a great thing to have

@Knight we all are just trying to communicate in this "imperfect" chat room environment

It is probably my hat that makes me seem so rude

@skillpatrol Exactly. It's nice that we have now largely succeeded. =)
@AaronStevens I wanted that hat, because it suits me. But alas no hat for me yet.

@user21820 @AaronStevens Are there any University where we can get admission and study without any pressure(pressure regarding your self worth)? I know one of them i.e. South African Institute of Mathematics (AIIMS)

@Knight I think it is less about the University and more about the people you associate with at the university
I don't think you can really assume all people from a university are going to behave a certain way
Especially with regards to pressure, bullying, etc.

2:41 PM

@AaronStevens What's the difference? People will live in University and

@user21820 He did say "get admission and study".

they will judge

@JMac Oh well you're right he included that.

@JMac What you interpreted by that?

2:42 PM
@Knight Not everyone. I tried out some labs before landing on one. I picked the lab with people who were the nicest, most supportive, etc.

I agree with Aaron that it's more important whom you associate with, whether in the university or outside. But it's still important to choose a university that has a fair admission criterion.
And where to try depends a lot on where you are right now, so you might want to ask a trusted adult among your family or friends for advice on that.

@AaronStevens WON'T THEY GONNA TELL YOU THAT THE SENIOR MATHS PROFESSOR HAS PUBLISHED NUMEROUS PAPERS (BUT IN REALITY THEY DON'T EVEN DESERVE REVIEWING) HOW WOULD YOU FEEL?

@Knight Please don't use caps. It is considered shouting in online settings.

Oh sorry

np :-)

2:45 PM
@Knight I don't think I fully understand the scenario. Can you expand on what you are talking about?

I felt very little pressure when getting my bachelors TBH. It really depends on environment. There was competition between some people, sure; but the people who didn't care to compete for the absolute best grades still got by fine. I made plenty of friends with people like me who were more than satisfied to just make sure we were doing okay and learning; without worrying about being the top of anything.

@JMac You're getting me here.

@Knight how competitive are you?

@JMac But doens't your psychology gonna change eventually and you will start racing for top position?

@Knight That is possible. For me, that is why I have people in my life to keep me grounded :)

2:51 PM
@AaronStevens And/or hobbies to relax.
=)

@user21820 @AaronStevens How to tell yourself that your goals are better than those Gold Medals? How to argue with your brain?

@Knight Sometimes, you don't argue. You acknowledge that you want it, but accept that you may not get it. And still you work for it, and be satisfied with your own efforts.

@Knight Like I was mentioning earlier, I don't put my self worth into my career

@user21820 All it means is that we need to be strong, Ha?

@Knight Exactly :)

2:55 PM
try your best, that's all you can do

@Knight My psychology hasn’t changed in the 5 years since I graduated in that sense really. Being the top was nice the few times it happened, but it was never a goal of mine. I wasn’t driven in that way.

@Knight That's not all to it. There's a quote I like, and change slightly to fit my preference: God, give me the courage to change what I can, the serenity (peace) to accept what I cannot change, and the wisdom to know the difference.
If you don't believe in God, you can alter it further to your liking.

@user21820 WOW

Lord beer me strength.
2

beer?

2:58 PM
The Serenity Prayer is the common name for a prayer written by the American theologian Reinhold Niebuhr (1892–1971). The best-known form is: Niebuhr used various versions of the prayer widely in sermons as early as 1934. The prayer spread rapidly, often without attribution to Niebuhr, through church groups in the 1930s and 1940s and was adopted and popularized by Alcoholics Anonymous and other twelve-step programs. The Serenity Prayer appeared in a sermon of Niebuhr's as part of the 1944 A Book of Prayers and Services for the Armed Forces, while Niebuhr himself first published it in 1951 in a...

@skillpatrol Thank you.

> popularized by Alcoholics Anonymous and other twelve-step programs.

4 mins ago, by skillpatrol
try your best, that's all you can do

@skillpatrol Interesting, I didn't know that.
12 steps? So many? Haha..

3:00 PM
Why @JMac wrote Lord beer me strength

@Knight You wouldn't understand. It's a secret

beer = alcohol

@AaronStevens @skillpatrol So, beer is a noun, what he meant?

@Knight He was quoting a TV show :) Don't worry about it

Okay

3:02 PM
a play on words :D
see, nobody is being "rude"?
24 mins ago, by skillpatrol
@Knight we all are just trying to communicate in this "imperfect" chat room environment

@skillpatrol Yeah, but they always asked for clarification and I couldn't clarify myself anymore, @JMac and @AaronStevens always group together, hahahhah

yeah, communication is tough

yes @skillpatrol

I was quoting that same show. So yes, we were working together on that one

@AaronStevens and @JMac Your duo is very lethal to others, hahhah

3:17 PM
1 hour ago, by Aaron Stevens
@JMac Nailed it

@JMac Or "Boom, roasted."

pwnd?
In other news:
Merriam-Webster announced Tuesday that it has chosen “they” as the 2019 word of the year. The singular “they” is a pronoun used to refer to a person whose gender identity is nonbinary, a word that itself was added to the Merriam-Webster.com dictionary in September of this year.

3:55 PM

2 hours later…
5:26 PM
What is unreality that is not any of these: imaginary, fictional, surreal, virtual, dream, nonexistent, changing, contingent, dependent, immaterial, negative, unknown, philosophical zombie things? what is the essence of unreality?

@Secret The answer to that question.

6:02 PM
@JohnRennie Is it the explanation in the video?

At 09:18 of the video, section "How do we justify the Law of Gravity?".

> Failed to commit transaction
> Invalid or corrupted package:

> Oddly enough, there is no mention of which package it is.
that is actually a pretty terrible thing :(

@dmckee btw, my friend got shortlisted and is interviewing on Monday
any tips I can pass on?

don't mess up

@JohnRennie If so, then a natural question to ask is what is the criteria that a finite number of observations become sufficient for induction? And I'm sure the measurements in the observations will not accurately reflect the law. So how to choose the range of acceptable error?

And induction is quite problematic. For instance, I can say that "all swans are white" if I just happen to not find a black swan.

@KyleKanos I'll pass that along =)
I'm sure it won't add to the pressure

6:10 PM
for salary purposes, something akin to "i'd expect typical market value for the position given my history"
maybe add a "but we can discuss that when the offer is made", but that might not be completely ideal

@KyleKanos I think right now the focus is on getting the job, with salary on a background plane for now

@EmilioPisanty fair enough, that was just something dmckee & you had mentioned in the link

@KyleKanos sure =). That was mostly a pointer to set the right reference.

actually, messing up is okay, just need to recover adeptly
like i had a person i interviewed who asserted that this stochastic process was martingale and proceeded to prove it

@KyleKanos ... but no pressure? =P

6:13 PM
thing was, it wasn't martingale. and each of the three times he got to the point he should have concluded it was not martingale, he assumed he made a mistake and started over again
that dinged him a bit in my review
had he realized that his assumption of martingale was wrong when he did the math, i'd have been fine with it
but i'm also speaking of an indstry position, not faculty
so YMMV

@KyleKanos yeah, this is for a tenure-track faculty position

maybe just a little different of an environment

6:53 PM
is rep increase notification supposed to be changed to blue? or does that signify a hat?

7:31 PM
Hi people!
I have a question about the conservation of energy.
Assume that an elevator is hoisted by its cables at constant speed.
$\vec{F}_{net}=\vec{0}$
So, $\vec{V}=\vec{cnst}$
$\int{y_0}^{y}(\vec{F}_{net}.dx)=\Delta{K}=0$
But elevator's $K_{f}=K_{i}$ and It's not equal to zero.
I tried to write conservation of energy in this problem.
$E=K+U(x)$
$\Delta{E}=\Delta{K}+\Delta{U}=0$
So, $\Delta{K}=-\Delta{U}$
$\Delta{K}=0$ but $\Delta{U}$ is not equal to $0$.
So the equation is not available here.
Is that mean in this problem energy is not conserved and total energy is changed?

When we write the work-energy theorem, $\int{y_0}^{y}(\vec{F}_{net}.dx)=\Delta{K}$, the $\vec{F} in this equation includes the net force on the object. When the lift is moving with uniform velocity, there are two equal and opposite forces acting on it (the force due to the cable and gravitational force) which cancel to give a zero net force. ^Replace the word "includes" with "is" Yep, I'm listening to you 7:46 PM the energy density of an iphone is equal to the trajacent mass of the propulsory extratentant bragadon from the prostorus exticalus. (that's how I feel every time someone writes something here) mind blowing @NovaliumCompany well then do something about it & study something no one else here knows. like MOND In your example, the force due to the cable is doing positive work on the lift. When there's an external force (other than the conservative force) doing work on the object, conservation of energy doesn't hold. It's important to keep in mind that conservation of energy comes from the work-energy theorem. In your example, it is$W_{\text{done by the force applied by the cable}} + W_{\text{done by the gravitational force}} = \Delta KE$. And in your example,$W_{\text{done by the force applied by the cable}} + W_{\text{done by the gravitational force}}=0$since the gravitational force is doing negative work on the object whose magnitude exactly matches the work done by the force applied by the cable. @KyleKanos lol I don't care dude :D I mean, a lot of people are using complex language and words to show how smart they are and to feel good about themselves. (some have just read a lot of books and that type of language has been slowly integrated into their brains from the authors) But honestly, It's much harder to express something complicated in a simple and understandable way. Complex terminology are born out of necessity. 7:52 PM true, unfortunately. @AjayMohan In my example energy is not conversing, is it correct? I mean, talking with complex terminology and linguistic twists is fine as long as your goal is not for the average person to understand you. I really agree with you @NovaliumCompany Tasteless persons 😁 I often feel, when I'm reading a formal definition of some sort, like the goal of the person is to simply confuse everyone. I mean... you've seen those famous people saying some very wise things in such a way that it seems like it's purpose is confusion. Yes,$E = KE + PE$is not conserved in your example. 7:58 PM What is the reason of it? There is no non-conservative force I think. But, the force on cable is an agent force and it is not conservative force, is that correct? @NovaliumCompany Sometimes that goal directly counters efficient communication. That's why we come up with all these terms. If you want to communicate effectively, you need to come up with words, instead of repeating their definitions every time you want to use them. That's why learning the terminology is pretty much always a part of studying a field. @JMac I agree. I'm not talking specifically about terms of some subject. I'm just seeing a lot of people twisting their sentences like their goal is confusion for the sake of boosting their social status. @NovaliumCompany How do you know they are saying wise things? Sometimes people abuse the confusing nature of terminology to make it seem like they are saying something substantial, to people who don't know the meaning of the words; but then the people that do know the meaning see it as nonsense. It's unfortunate; but that's a problem with accepting anyone who sounds like they know what they are saying if you have no way to verify it. @JMac Exactly, it seems like they are using linguistic twists to cover up for their nonsense @NovaliumCompany I write with different word choices all the time depending on the context. In chat I (usually) use less formal language than I would in answers; and if the person I'm talking to is using a wider vocabulary I usually wind up doing the same. I think it's partially habit. 8:02 PM Okay. First, let's try to understand where the potential energy comes from. Let's say only a conservative force$\mathbf{F}_c$is acting on a particle. The work done by this conservative force depends only on the initial and final position of the path traced by the particle and hence, we can write it as$\int_1^2 \mathbf{F}_c \cdot d \mathbf{r} = U(\mathbf{r}_2)-U(\mathbf{r}_2)$, where$U(\mathbf{r})$is the potential energy corresponding to the conservative force$\mathbf{F}_c$. Anyways, I think that it all depends to whom you are talking to. @NovaliumCompany One big example I've seen mentioned is Deepak Chopra. He uses a lot of scientific terms; but the actual substance of what he says often actually is nowhere near what the science suggests. He abuses the pop-science explainations of quantum mechanics to mislead people about new age ideas on a regular basis AFAIK. (contd.) So,$\Delta PE$is just another way of writing the work done by the conservative force in the problem. But, here comes the twist in most textbooks : They define their potential energy as the negative of my potential energy. Therefore, if we choose that convention,$-\Delta PE$is another way to write the work done by the conservative force. Alright, I get it. I'm listening to you @AjayMohan Oh, wait @JMac I don't know. I'm just getting a bit frustrated when I see some dude talking like his goal is to confuse everyone and make himself look smart, when in fact he is saying something simple just complicating his language. 8:07 PM (contd.) Now, we know that the work-energy theorem says that the work done by the net force acting on the object is equal to the change in kinetic energy of the object. In my case, it is only the conservative force acting on the object. Therefore, this implies :$W_{\text{by conservative force}} = -\Delta PE = \Delta KE \Rightarrow \Delta PE + \Delta KE = 0$. @MuhammedÇ.TUFAN I'll wait. Oh, I think I get a point that I always confused. @NovaliumCompany I think it's worth considering that often, they aren't necessarily doing it to confuse everyone and make themselves look smart; at the very least not consciously. Sometimes I just find myself in a wordy mood, and I'll read back on something I wrote and think "why the hell did I word it like that". @JMac It happens to me as well, but I try to speak in simple terms so I can keep the main reason why language exists, understanding. @NovaliumCompany perhaps the assumption that their "goal is to confuse everyone and make himself look smart" is false? My book includes a equation:$\Delta{U}=U(r_{B})-U(r_{A})=W(A\to B)=\int_{A}^{B}\vec{F}.d\vec{r}$Is this equation is only usable for conservative forces? Or true for conservative forces? 8:12 PM why would anyone upvote the answer there: physics.stackexchange.com/questions/519059/… ?? Considering the additional avails from excluding the inappropriately-behaved individuals from the publicly-established bar will result in the potential spark of action as to bring peace to the community. = Kicking out the assholes from the bar makes everyone happy. Ah, they haven't placed a negative sign. The$\mathbf{F}$in this equation is the external applied force that is equal and opposite to the conservative force acting on the object. @NovaliumCompany Language is about communication. Yes, it requires understanding; but you can also communicate a lot more efficiently if both people know the vocabulary, and complicated concepts can be described by fewer words, and subtle meanings can be included by specific word choice. Simple terms don't always make communication better. @JMac simple terms don't typically help for more advanced topics I would suggest you take this as the definition of potential energy:$U(r_{A})-U(r_{B})=W(A\to B)=\int_{A}^{B}\vec{F}_{\text{conservative force}}.d\vec{r}$. Note that A and B are switched in the LHS 8:16 PM @JMac I understand and I agree, but when the speaker knows that the audience doesn't have the proper term-set and consciously continues to speak in the complicated manner mean that either the person is not paying attention to his language or his goal is to confuse and boost his status. @KyleKanos Yeah, I mentioned that above too. For many things it obviously becomes too cumbersome to explain things by their definitions instead of just coming up with a word that encompasses the concept. @AjayMohan Oh, I get the idea. Is this equation available to use when we are calculating the conservative force? @NovaliumCompany I'd honestly be wary of anyone who considers talking past people to be "boosting their status". Being a poor communicator isn't something most people would brag about. I mean when we consider the work done by conservative force. Anyways, I think that complicated language and simply language both have their own place and time. I hope some day in the future we will communicate thoughts and not words. 8:17 PM @NovaliumCompany or that he has no other way to say anything except via the "complicated" manner Yes, the negative of the change in potential energy is nothing but the work done by the conservative force. I gave an example above that says the same thing in complicated and simple terms. I personally prefer the latter. also, with shows like The Big Bang Theory that exist to make fun of intelligent sounding people, i'd be really wary of even suggesting that people would choose to only speak in "complicated" manner to "boost" their status the typical result of "sounding smart" is actually mockery, not revelry wait, not revelry. that's not right that's like a party or something esteem, i guess? I don't know, I'm probably wrong on my views, I'm not familiar with the topic we're discussing, but I'm just seeing those disgusting formal definitions in my textbooks and I'm like "Whyy....??!?" when the same thing can be explained much much simpler. meh @AjayMohan I take an example of that we ascend a mass$M$from zero potential energy point to$y=h$point. In this example work done by the gravity:$W(\text{done by the gravity})=\int_{0}^{h}\vec{F}_{G}dy=(-Mg)\int_{0}^{h}dy=(-Mg)(h-0)=-Mgh
Object's potential energy at the zero potential energy point is $U(0)=0$ and the object's potential energy at the height $y=h$ is equal to $U=Mgh$
Now I found the $-\Delta{U}=U(0)-U(h)=0-(Mgh)=-Mgh$ and this equal the work done by the gravity (conservative force)
Is the main idea this?

8:30 PM
Yes, you've got it right.

Thanks @AjayMohan. I'm grateful to you so much because that help!
I have some more questions as well.

No problem. It's great to see that my sleeplessness has come of some use.

Is that Equation $\W(\{done by a conservative force})=-\Delta{U}$ is valid for any kind of conservative force? I especially think about the spring force. Is this valid for spring force as well?
I will take another example for myself for spring force as well after I understand the relationship of $-\Delta{U}$ with the whole mechanical energy of the system.

Yes, it's valid for any conservative force, including the spring force.

Alright, thanks @AjayMohan.

8:36 PM
Sure, play with the idea. That's how it starts to "stick" in your mind.

I will try an example for that relationship, If I get any error I will back to you happily 😁

thumbs up emoji

Wow!
That $-\Delta{U}=\Delta{K}$ thing is really awesome!
I have tried it in both energy conserved and energy changed systems.
I see that it doesn't work in systems that their energy don't conserve.
But I have forget it right now 😁
Everything is OK I think.
I'm really grateful to you @AjayMohan!

8:51 PM
It's no problem. Best of luck.

Thanks @AjayMohan! Have a good night, hope you will have a best sleep of your life.

9:03 PM
@AjayMohan I remember my question.

I'm here. Go on.

I saw an equation $W(\text{done by agent force})=\int_{A}^{B}\vec{F}_{ag}.d\vec{r}=\Delta{U}=U(B)-U(A)$
Is that equation correct?
I have seen this in my book, let me show you the whole text.

If the agent is applying force in such a way that the initial and final KE is the same (or $\Delta KE=0$), then this equation is valid. You can once again see this from the work energy theorem : $W_{agent} + W_{conservative force} = \Delta KE = 0 \Rightarrow W_{agent} = - W_{conservative force} = \Delta PE$.

Hmm...
I think that is equal to text in my book.
**Potential Energy** *We have mentioned (page 141) that only
differences of potential energy are meaningful. Our definition
of potential energy indicates that the difference in potential
energy at points B and A is the work that we must do in moving
the system without acceleration from A to B, so that;*
$W(\text{done by agent force})=\int_{A}^{B}\vec{F}_{ag}.d\vec{r}=\Delta{U}=U(B)-U(A)$
That's OK. I get this one as well.
Thanks for your help once again 😊
Have a good night!

You too.

2 hours later…
11:04 PM
@JohnRennie I've got a question about what is called "black body radiation". What exactly And precisely is black body radiation. For example, plasma. Plasma emits radiation called black body radiation. But yet no source really says what black body radiation is. Is it comprised of several wavelengths? Or what is it?

11:50 PM
@ScientistSmithYT Are you familiar with what a black body is?

heya guys

what is a formal argument why the angular momentum operator is given by $l=+i\hbar r\times\nabla$ when dealing with a hole instead of an electron?
I mean, it kind of makes sense intuitively
but I would kind of like to know the mathematical reason
I mean, we model a hole as a positively charged electron right, but I don't see how that would yield the opposite sign for the momentum operator
hm, it does kind of make sense, because we want the eigenvalues to have opposite sign
but that's backwards engineering from my part