What is the significance of momentum?
What is momentum really?
How can I add the word "Chap" before the chapters numbers in the Toc in class report? At here  the word Chapter added in \documentclass{book}. Now I want to use them in \documentclass{report}. How can I do that? Tag scores update is supposed to happen at 03:00 UTC every day. Tag scores are updated by a scheduled job that is time consuming and often times out  (see for example the answers for  and ).  In the past month or two, the issue with not updating the scores has gotten much worse. Tag scores are now regularly not updated for 2-3 days in a row. The job fails more often than it runs which obviously lowers usability and appeal of tag badges (which are not awarded when the script fails) and scores to some extent.  I can also guess that if a job fails on consecutive days, it only makes it harder to make it work as it has more and more tags and answers to recount.  This issue happened in the past but not to the extent it's happening now.  Please do not close it as duplicate. I'm creating a separate post as I've been asked to do it by Adam Lear in a comment for the answer for the first of the linked questions. Other posts reporting this issue have accepted answers which makes them difficult to track for the SE team.  UPDATE  I've been writing down whether the scores were updated or not every day in April.  The results are:  Day      1   2   3   4   5   6   7   8   9   10  11  12  13  14  15   Updated  -   -   -   ✓   -   ✓  -   -   -   -   -   ✓   -   -   -   Day      16  17  18  19  20  21  22  23  24  25  26  27  28  29  30 Updated  ✓   -   ✓   -   ✓   -   -   ✓  ✓   -   ✓  -   ✓   -   -   The update succeeded on 10 days, failed on 20. The situation got somewhat better in the second half of the month. Has something been done about it? I've read that it is possible to add a method to an existing object (i.e., not in the class definition) in Python.   I understand that it's not always good to do so. But how might one do this? I'm trying to understand why the find command is not deleting files in one particular case. I've got the following command:  find C:\\path\\to\\Pictures\\pmcctv -name cap_*.jpg -o -name cap_*.ogg -o -name cap_*.flv -o -name cap_*.mp4 -o -name cap_*.webm -mtime +7   Which returns the files I would expect:  C:\path\to/Pictures/pmcctv/cap_20160915T193251_620067800.jpg C:\path\to/Pictures/pmcctv/cap_20160915T193322_742708800.jpg C:\path\to/Pictures/pmcctv/cap_20160916T081046_394767500.jpg C:\path\to/Pictures/pmcctv/cap_20160916T081125_615129600.jpg C:\path\to/Pictures/pmcctv/cap_20160916T081129_503678200.jpg C:\path\to/Pictures/pmcctv/cap_20160916T081255_842394600.jpg C:\path\to/Pictures/pmcctv/cap_20160916T161008_693586800.jpg C:\path\to/Pictures/pmcctv/cap_20160916T161012_749396800.jpg C:\path\to/Pictures/pmcctv/cap_20160916T161043_774132200.jpg C:\path\to/Pictures/pmcctv/cap_20160916T161150_497251900.jpg C:\path\to/Pictures/pmcctv/cap_20160916T161154_437379600.jpg C:\path\to/Pictures/pmcctv/cap_20160916T161337_350955300.jpg C:\path\to/Pictures/pmcctv/cap_20160916T161341_452596000.jpg C:\path\to/Pictures/pmcctv/cap_20160916T161412_870774000.jpg C:\path\to/Pictures/pmcctv/cap_20160916T161443_969064400.jpg C:\path\to/Pictures/pmcctv/cap_20160916T161554_468109900.jpg C:\path\to/Pictures/pmcctv/cap_20160916T161558_378086000.jpg C:\path\to/Pictures/pmcctv/cap_20160916T161916_668433100.jpg   Now if I try to run the same command with -delete, the files are not deleted:  find C:\\path\\to\\Pictures\\pmcctv -name cap_*.jpg -o -name cap_*.ogg -o -name cap_*.flv -o -name cap_*.mp4 -o -name cap_*.webm -mtime +7 -delete   I've also tried with -exec rm, but they are also not deleted:  find C:\\path\\to\\Pictures\\pmcctv -name cap_*.jpg -o -name cap_*.ogg -o -name cap_*.flv -o -name cap_*.mp4 -o -name cap_*.webm -mtime +7 -exec rm {} \;   find returns no error and the files are owned by me with permissions 644.  Any idea what could be causing this?  (note: I've cut the commands to make it more readable but normally there's no line break) Is there an easy way to convert a number to a word in PHP? No reputation for editing questions? If $A$ and $B$ are nonempty sets, prove that $A \times B = B \times A$ if and only if $A = B$.   Proving the first direction of this is easy. That is, if $A = B$ then it is obvious that $A^2 = A^2$. I am wondering how to prove the other direction now. Maybe proving it by contradiction will be easier than directly. Presentation $\langle x,y,z\mid xyx^{-1}y^{-2},yzy^{-1}z^{-2},zxz^{-1}x^{-2}\rangle$ of group equal to trivial group What is with words that have forms that end both in -zation and -sation, such as localization and localisation?  Many spell checkers recommend -zation. How to control a motor with only relays and push button? Hello I´ve been working in this project for some days, I need to make a circuit with only relays that can make a dc motor start, stop and a reverse. The thing is that when you push the start or the reverse button the motor need to keep running, I can´t keep it pushed.  This is as far as I can get. SW1 turns the engine on, SW5 turns it off and SW7 reverse the motor but only if it is already running. What i need is that the reverse button also turns the engine on. Please help.  notes: -i can only use three push buttons and as many relays as needed. Yes is a school assignment.  THANKS! How to understand Cauchy's proof of AM-GM inequality(the last step) As I show below, typical alien depictions include an elongated, pale face, two dark elliptical eyes, and a small mouth.  Who first used this design for an alien?  Where does this design come from? 70s UK science fiction TV show with time travel and telepathic cat I'm trying to identify a science fiction series on UK TV during the 70s, but I can only recall the following:   A party of adults and children. There was a woman with a telepathic bond to her cat: she could look into its eyes to see what it had seen. Time travel was involved. I think it may have been inadvertent or uncontrolled time travel. On one occasion the party encountered an Elizabethan seaman who demanded to know if Elizabeth was still queen. They party replied "err... yes" because they didn't want to have to explain time travel. Bridge Rectifier Fails When AC Inputs are Shorted How to install Metasploit and Armitage I want to install metasploit and armitage on my ubuntu 16.04 LTS laptop running kernel 4.6  I've read blogs and posts on how to install it but the problem is is that they are outdated. The packages are not there anymore and or the commands dont work.  If anyone can give me an updated tutorial on how to install it that would be great. I also think other users that see this post and want to install it will also be thankful. Thanks. Suppose that I have a &lt;div&gt; that I wish to center in the browser's display (viewport). To do so, I need to calculate the width and height of the &lt;div&gt; element.   What should I use? Please include information on browser compatibility. How do favored classes work in Pathfinder? I'm working on creating a half-elf paladin character for a Pathfinder game.  Full disclosure: this is the first time I've tried creating a character; I've used ones other people created before.  In the text, it mentions something about "favored classes" I believe.  What is this? There's a web services I want to call in my application, I can use it with importing the WSDL or by just use "HTTP GET" with the URL and parameters, so I prefer the later because it's simple thing.  I know I can use indy idhttp.get, to do the job, but this is very simple thing and I don't want to add complex indy code to my application.  UPDATE: sorry if I was not clear, I meant by "not to add complex indy code", that I don't want add indy components for just this simple task, and prefer more lighter way for that. A geometric reason why the square of the focal length of a hyperbola is equal to the sum of the squares of the axes? When I teach conics, I give a simple geometric argument for the Pythagorean relation for ellipses. (By "Pythagorean relation" I mean the fact that the square of the focal length $c$ is the difference of the squares of the semimajor and semiminor axes, i.e. $c^2=|a^2-b^2|$ for the ellipse defined by $\frac{x^2}{a^2}+\frac{y^2}{b^2}=1$.) The picture argument goes as follows: just draw the obvious isosceles triangle from the two foci to one of the two points at the end of the minor axis. We see two right triangles; in each, one leg is the focal length $c$ and the other leg is the semiminor axis, while the hypotenuse is the semimajor axis. The result follows from the Pythagorean theorem applied to this right triangle.  But when it comes to the Pythagorean relation for hyperbolas, I can't think of an equally convincing picture to draw. Instead I have to give a rather inelegant algebraic argument for the relation $c^2=a^2+b^2$ (for the hyperbola defined by $\frac{x^2}{a^2}-\frac{y^2}{b^2}=1$, with focal length $c$), directly from the definition of the hyperbola. The messy details are below.*  Now, obviously there is an asymmetry between the argument for the ellipse and the argument for the hyperbola: the ellipse argument is almost trivial and doesn't involve working straight from the locus definition, but the hyperbola argument is tedious and does require working straight from the definition. So, my question:     Is there an equally persuasive picture I could draw to convince high   school students of the Pythagorean relation for hyperbolas -- without having to get technical with points at infinity, say?   The natural triangle to draw is the right triangle with legs from the center to a vertex and from that vertex to an asymptote, which has legs of length $a$ and $b$; but then the problem becomes why the length along the asymptote should be equal to $c$. I doubt there is a purely (Euclidean) geometrical reason because this triangle lies on the asymptote, not on the hyperbola itself; I suspect some sort of limiting procedure will be required.    [*] The tedious algebraic details:     The foci lie at $(\pm c,0)$.   Say $a&gt;b$. Then hyperbola is the locus of points $(x,y)$ such that       $$\left|\sqrt{(x+c)^2+y^2}-\sqrt{(x-c)^2+y^2}\right|=2a$$      Assume for simplicity the expression between the absolute value bars   is positive. (The algebra is even more tedious if we don't make such a   simplifying assumption.) Then moving the right root to the other side   and squaring gives      $$(x+c)^2+y^2=4a^2+(x-c)^2+y^2+4a\sqrt{(x-c)^2+y^2}$$      Simplifying gives      $$xc-a^2=a\sqrt{(x-c)^2+y^2}$$      and upon squaring again we have      $$x^2c^2-2xca^2+a^4=a^2(x^2-2xc+c^2+y^2)=a^2x^2-2xca^2+c^2a^2+a^2y^2$$      which reduces to      $$x^2(c^2-a^2)-y^2a^2=a^2(c^2-a^2)$$      Defining $b^2:=c^2-a^2$ brings the equation into the form   $x^2/a^2+y^2/b^2=1$, and the Pythagorean relation follows from the   definition of $b$. Ignore citations in captions in list of figures when numbering When using the \cite command in figure captions and the ieeetr bibliography style, then  BibTeX numbers the citations as appearing earlier than they do in reality (i.e. the first citation in the document ends up being [42]).  Is there any way to get BibTeX to ignore the \listoffigures section when numbering?