How much energy is needed for a spaceship to go from 0 to Mach 30 in under 10 seconds?
The Spaceship has a circumference of $20 ft$.
A height of $6 ft$.
It weighs $7,000 lbs$
We, humans, stole the design and created our own, but our copy-cat crumbles up under the intense mass increase due to th...
@Semiclassical Homework-like calculation questions are explicitly off-topic. And a vehicle that can accelerate at 105 g without turning its passengers into paste is clearly using non-mainstream physics. ;)
Does anyone know if there is a notion of "neighborhood transitivity" in graph theory - i.e. it is vertex transitive, but also for any permutation of the neighbors of a vertex v, there is an automorphism of the graph that exhibits that permutation?
Not sure why this was closed:
How do you show that the Lorentz metric is preserved for $\zeta^{3,1}?$
I think it was closed because I bit off more than I could chew so to speak and should have just asked a more simple question.
That being said, what can I do to possibly reopen it? I've edited ...
Why is my screwdriver attracted to my screw? My guess would be that the screw is made from iron, which is a ferromagnetic material and gets attracted to magnets but why is the screwdriver a magnet? As far as I know, it's made out of steel, and it's not a magnet?
@Slereah I am reading about Einstein's equation. In the slide of an instructor, he said $G_N$ is Newton constant but in a textbook it says $\kappa$ is gravitational constant. I guess the two are the same, but they use different units so that the coefficients in Einstein's equation are different, so I am not quite sure.
@JohnRennie Got it, thanks :-) Now I'm curious to know how they magnetize only the tip :P How do they magnetize magnets (which are made out of steel) in general?
@FakeMod If you think a question is unsuited for HNQ, please flag it for moderator attention.
user434058
@ACuriousMind I mean there's nothing egregiously wrong with the question being HNQed, however it isn't a kind of question which would make visitors love this site, or in simple words, it isn't good enough. However I didn't know I could use mod attention flags for that. Thanks!
user434058
@ACuriousMind But flagging it for mod attention sounds really weird. There is no real fact using which I can claim that it shouldn't be a HNQ. It's just my notion of what should ideally be HNQed and what shouldn't...
@FakeMod I knew which question you were talking about before I clicked the link. :) I agree it's not a great HNQ question, but Aaron's answer is quite good. Note that the question itself hasn't scored any points from the HNQ effect.
I want to suggest a feature i.e. verifying some trusted users and giving them a badge.This will help the new users to trust the good answers given by them.
Consider the above drawn tanks (Tank 1 left Tank 2 Right)
What Would the hydrostatic pressure from the water at the points where the arrows are drawn.
In Case 1 for example
Tank 1) Hydrostatic pressure from atmosphere + hydrostatic pressure from the part sticking out
Is this the same for Tank 2)...
Can someone check out my question ? the person who answered only answered one part and not the other and its kind of unclear if he is desicive about his opinon. it should be an easy question
On another note, why is this wierd rule? A person who is willing to go on the intenret and post his question is clearly interested in learning, not helping him out because its a homework question is not a good argument in my opinion.
thanks for the comments Aaron i will check your links out
@AaronStevens In severe cases, such users may be warned via a mod message and suspended if they ignore that warning. Feel free to flag answers by repeat offenders with custom mod flags.
And, sure, many people ask these types of questions because they are struggling, in general, with how to approach them. But others simply want the answer so they can hand it in, we cannot distinguish the two, and so the policy does not really care about their hidden motivation
Well i dont know and i am not intersted in entering this argumentation. I am just saying if someone goes online regardless of what ist he motviation. its a good enough reason. In real life. if you keep taking shortcuts, you fail at end. So it doesnt matter to help them or not if they dont do good on their own.
Sorry i did not mean to i just wanted to present example.
I occasionally come here and ask John renne for example to help me with my homework. I work in real life and i study mathematics and physics and my professors sometimes put very wierd homeworks. Sometimes i cant do them on my own. But does it mean i need to fail physics because one professor give me wierd homework? ..
@ACuriousMind After days of programming it with book content preprocessed on phone, it turns out that not all fonts are monospaced, so... I'll be doing all the text processing on the go with no pages and I'll keep track of last character positions. After giving it some thought and chatting to a specialist, this is the better way to do it. I would be calculating the width and height in pixels of each word.
And looking forward, if I need unicode symbols from Chinese, Bulgarian... text needs to be processed on the go and I'll just leap with fseek.
You were right, as always. Thanks for helping me :-)
@ACuriousMind One thing that's unclear though is, how would I go to previous pages? If I've fseeked to some character and processed that page, how would I go to the previous page not knowing from which character to fseek?
@JingleBells Just make a rough guess. The user won't mind if there's a little bit of overlap between the content of the pages when they scroll back a page, but they will be annoyed if the program misses some of the text. You know how many chars are on the page you just rendered, so you can assume the previous page has roughly the same number of chars. If the current page had N chars, scroll back, say 0.9*N, or 0.95*N. You could even make that ratio a setting that the user can adjust.
Consider the Huygens–Steiner theorem. What i understand we can calculate the moment of inertia of a rigid object (that has infinite points of mass) in relation to an axis. that is not going thru the center of mass. Do we apply this concept as well to lets say a system of 3 balls of mass lets say they are connected to each other by massless sticks in a shape of a triangle and lets say we want to calculate the moment of inertia of those 3 balls in accordance to an axis of rotation, that is not going throught the center of mass. Do we still use the Huygens–Steiner theorem?
@JingleBells Not if you want it to be fast. I don't know the details of how you're rendering text, but you could render to an invisible page that's a bit longer than the visible page, then copy from the invisible page to the display. With a bit of juggling, that will let you reduce the overlap to a single line (or a fraction of a line).
@PM2Ring Maybe there's a way to process the page backwards where you start reading characters from the file backwards, so you start from the first character of the page you're on and you go backwards. That's probably a stupid way but worth mentioning. (also, how would I print the text backwards on the display...)
@JingleBells Yes, that'd work. But it basically doubles the size of the section of your program that renders text. And makes it a bit more confusing to work on, because you have to keep in mind if a chunk of code is for forward rendering or backwards rendering. And any features you add need to be handled correctly by the forward code & the backwards code.
But if you have the time & energy, it's certainly worth testing out.
That's another benefit of Git. You can do side experiments like that without your source code turning into a tangled maze. ;)
@PM2Ring I have my Github here and I do regular backups. So yeah, it's actually pretty easy and I'm happy I learned my lesson.
@PM2Ring Which one would you do? The "go to previous page" it's not a really crucial feature because most people read the book from start to finish but dunno. The doing it backwards thingy is not a bad idea because I'm getting it perfect every time.
I must confess my own Git skills are very rusty. I've rarely needed it since I learned it. But some good Git tutorials have been mentioned in the Python room. There's even a game designed to help you learn Git properly. I'll see if I can find a link...
@JingleBells I wouldn't worry too much about trying to go backwards perfectly. Most people won't mind if it's approximate, they'll be glad that they can go backwards at all. :)
"Geometric quantization is a marvelous tool for understanding the relation between classical physics and quantum physics. However, it's a bit like a power tool — you have to be an expert to operate it without running the risk of seriously injuring your brain."
Time for me to use it willy nilly
user434058
@AaronStevens It (the answer to the question) just feels a bit obvious to me, but it isn't a big problem, it being HNQ is okay but to be honest, I think there are many other better candidates :)
user434058
@AaronStevens I know :) It was just a kind of symbolism for showing that it was people who made this a famous and highly active question and thus it got into HNQ.
user434058
In other words, that question lacks basic prior research.
Hey guys. In what setting do we actually use infinitesimal rotations in physics? (infinitesimal rotations formally being the generators of the lie algebra of SO(3))
the thing is
I referred to them as "infinitesimal rotations that physicists always talk about"
and then I was asked rightfully: in what context?
and now I'm actually a bit confused... and I can't find a clear answer online
what I did find was that orbital angular momentum satisfies the same commutation relations as the lie algebra of so(3)
but as the lie algebra of so(3) and su(2) is the same, I feel like I would need some more to explain what infinitesimal rotations would have to do with orbital angular momentum
I do realise that a quantum system that is spherically symmetric gives us a representation of SO(3)
but hm.. ye, I feel like I still can't provide an answer to said question
Given a wave function $\psi(\mathbf{r})$, you want to compute how it changes under an infinitesimal rotation of $\mathbf{r}$, i.e. $\delta \mathbf{r} = \delta \vec{\phi} \times \mathbf{r}$, you plug $\mathbf{r}' = \mathbf{r} + \delta \mathbf{r}$ into $\psi$ and this gives the angular momentum operators showing they generate infinitesimal rotations of a wave function,
You then want to seek eigenfunctions of the angular momentum operators and you're forced to seek irreducible representations of $\mathrm{so}(3)$
The context of $\delta \mathbf{r} = \delta \vec{\phi} \times \mathbf{r}$ being an infinitesimal rotation of a vector $\mathbf{r}$ is different to the context of analyzing an infinitesimal rotation of a wave function $\psi(\mathbf{r})$ (even if both cases can be expressed in terms of $\mathrm{SO}(3)$ representations)
They are the matrices of the vector representation of the $\mathrm{so}(3)$ Lie algebra generators, if you look at $\delta \mathbf{r} = \delta \vec{\phi} \times \mathbf{r}$ and remember the cross product can be represented in terms of matrices you could figure out what those matrices are, you could also find those matrices by defining a 3x3 rotation matrix on general grounds (in terms of sines and cosines) and then infinitesimally expand,
or you could find them from the representation theory of $\mathrm{so}(3)$
Not sure why this was closed:
How do you show that the Lorentz metric is preserved for $\zeta^{3,1}?$
I think it was closed because I bit off more than I could chew so to speak and should have just asked a more simple question.
That being said, what can I do to possibly reopen it? I've edited ...
I want to suggest a feature i.e. verifying some trusted users and giving them a badge.This will help the new users to trust the good answers given by them.
