How Can There Be Magnetic Force Without Velocity
The equivalent electric field of a magnetic field
I was wondering if $\mathbf{A}\times\mathbf{C}=\mathbf{B}$ and $\mathbf{A}\cdot{\mathbf{C}}= p$, is it possible to express C in terms of A, B, and p?  Note: A, B, C are vectors and p is a scalar.  Sorry if this question has been asked before. If so, can anyone give me a link to it? Is the 5-foot range of the Help action my range to the ally, the enemy, or both? Does Hard reboot damage my Ubuntu How do I get rid of this unwanted bar from Eclipse? I get the following error in my log files every time I try to upload a large file.  a client request body is buffered to a temporary file /var/lib/nginx/body/0000000001   Although the file uploads successfully, I always get the above error.  I increased the client_body_buffer_size to 1000m which is what I expect the largest file uploaded to be. However, this is was just a guess and although I don't get that error anymore I am wondering if this is an appropriate value to set for the client_body_buffer_size?  I would appreciate it if anyone can shed some light on this directive and how it should be used. Problem statement  Prove that if $A$ is an uncountable set and $B$ is a countable set, then $A\setminus B$ must be uncountable.  What I think  The statement does not mention $A$ and $B$ relationship. I think there are two possibilities:   If $A \cap B = \emptyset $, then $A\setminus B$ is trivially uncountable If $A \cap B = B$, then $B \subset A$ and as a bijection can not be made between $A\setminus B$ and $\mathbb{N}$, $A\setminus B$ is uncountable.   And there is where I'm stuck. How can I prove that a bijecton can't be made?  TIA. Do sets, whose power sets have the same cardinality, have the same cardinality? Is it generally true that if $|P(A)|=|P(B)|$ then $|A|=|B|$?  Why? Thanks. What happens when you need to tear-down a hotel but no houses are in the bank in Monopoly? Correct position of "only" Which is grammatically correct?  I can only do so much in this time.  or  I can do only so much in this time. Disk space usage doesn't add up with df & du This tiny little script deletes all small features in all layers. This works fine but the size of .shp files are not reduced. Please tell me why...  canvas = iface.mapCanvas() expr = QgsExpression("$length&lt;100") for l in canvas.layers():     l.startEditing()     it = l.getFeatures(QgsFeatureRequest(expr))     for ft in it:         l.deleteFeature(ft.id())     l.commitChanges() I've been given a proof that shows the following:  If $f:[a,b]\to \mathbb C$ is a continuous function and $f(t)=u(t)+iv(t)$ then $$\left| \int_a^b f(t) dt \right| \leq \int_a^b \left| f(t) \right| dt$$ The proof begins by letting $\theta$ be the principle argument of the complex number $\int_a^b f(t)dt$ and there is one step in the proof I don't understand; can anyone explain to me why $$\int_a^b e^{-i\theta}f(t) dt=\Re\bigg(\int_a^be^{-i\theta}f(t)dt\bigg)$$  Would this not imply that the imaginary part of the left hand side is equal to $0$? I'm not sure why the LHS and RHS would be equal here. I don't understand why su - is preferred over su to login as root. Question Close Updates This week we rolled out a series of changes to  of the question close workflow.  You can read more about the details of these changes .    For context, the overall effort to improve the question close experience will be optimized for Stack Overflow. Given Stack Overflow’s scale, many of the pain points we hear about with question closing and reopening are unique to Stack Overflow and won’t make sense for other network sites. However some  changes are general improvements that should improve usability and guidance overall. Wherever possible, we plan to roll these changes out network wide as well. Any time we roll out a change network wide, we’ll make sure to cross-post here. This first phase is being rolled out to the network and is why I’m posting today.  Let us know if you run into any issues or have questions regarding the network here. "What have you tried" epidemic Statistics  () In Q1 of 2012 - 0.0134 such comments per question (~1.33%) In Q2 of 2012 - 0.0167 such comments per question (~1.66%) In Q3 of 2012 - 0.0169 such comments per question (~1.68%) In Q4 of 2012 - 0.0193 such comments per question (~1.92%) In Q1 of 2013 - 0.0223 such comments per question (~2.22%)  The vast majority of these questions need serious help.  Unfortunately, there is a growing trend towards abuse Along with the good, comes some bad -- as usual in life. These comments are bad when  Posting &quot;what have you tried&quot; multiple times per question is severely obnoxious Posting &quot;what have you tried&quot; when the user has clearly shown that they have tried to solve it on their own  It is becoming increasingly common to see &quot;what have you tried?&quot; comments in these scenarios. Some examples     Workarounds Flagging Comments - It is considered a good thing to be liberal with comment flagging -- that helps us clean up after the fact. &quot;Too Chatty&quot; is a no-brainer here. Unfortunately, that only cleans up after the damage has been done. All signs point toward the continued increase of the trend of using &quot;what have you tried&quot; comments. I spend a lot of time editing and cleaning up bad questions. This trend came up on my radar toward the end of last year and it is getting worse. So, I thought at the very least that I'd raise some awareness. Cool Text Highlighting in LaTeX LaTeX has soul. ConTeXt, of course, , where you can put sweet hightligher under your text for free!    In the pictured example, you can see highlights that not only highlight, but that do it with cool edges that look so good, you can smell fumes! The fearless author says he'll show how that was done in another post; this one shows only how to lay down a straight, yellow box. But, it is possible to do word-wrapped highlighting with cool-shapped boxes within ConTeXt, presumably.  The trouble for me is that getting into the ConTeXt world is steep climb right now, given the state of documentation and the flux of changes, so I need to continue with LaTeX. But the real problem is that now I can't live without the Organic-Looking Highlights of Awesomeness in that picture!  As I said, Non-Organic-Looking Highlights of the Ordinary are easy with the soul package:  \hl{Some awesome text that word wraps fine.}   It just taunts me on the page with its straight lines. So, if I sacrifice word wrapping, I can use PGF/TiKz and create the following macro:  \newcommand\hl[1]{%     \tikz[baseline,%       decoration={random steps,amplitude=1pt,segment length=15pt},%       outer sep=-15pt, inner sep = 0pt%     ]%    \node[decorate,rectangle,fill=navcolor,anchor=text]{#1\xspace};% }%   Which creates something like this:    Now, my color is too dark and my randomness needs adjusting, but you see that a similar effect could be achieved.  My trouble is that I don't understand TeX enough to dig into the Soul package and add the TiKz/PGF goodness that it so sorely needs. So, I leave it to you guys. Anyone want to tackle this?   Or is it yet another piece of fruit, just a touch out of reach, that shall have to wait until ConTeXt settles down? Run MySQLDump without Locking Tables Constants in Objective-C I'm developing a  application, and I'm using constant NSStrings as ways to store key names for my preferences.  I understand this is a good idea because it allows easy changing of keys if necessary. Plus, it's the whole 'separate your data from your logic' notion.     Anyway, is there a good way to make these constants defined once for the whole application?     I'm sure that there's an easy and intelligent way, but right now my classes just redefine the ones they use. Is it possible to assign the same value to multiple keys in a dict object at once? Is there any way to increase the drop rate of items?