frictional force is nonconservative force In nature there are mainly four kind of fundamental forces,electromagnetic force is one of them which is conservative force.If friction force is belongs to electromagnetic force then why it is not conservative force ?
What is the origin of non-conservative force? My understanding about  is a force that its work is independent of path such that we can construct another form of the work called potential to make our life easier.  For friction, if I start from microscopic point of view, it should be the macroscopic effect of the electric force or gravity which are both conservative force.  Why do we initially have description by conservative forces (electric/gravity force) but end up with a macroscopic description of nonconservative force(friction)?
From yesterday, openweather extention on the top panel does not show the weather. Its shows "Please wait Loading".  I tried to find the solution. There was a post of creating account on openweather website and use API id. I found that it is a subscription base plan. There was a free plan which allows 60 calls free for weather data.  I thought that the extention is open source. And may be I deleted any package that is required to work the extention so I reinstall the OS. But there is no difference of doing that.    What could possibly go wrong any guidance? Do you need a comma before a subordinating conjunction (like if)? Are both of the following sentences correct?  a: You can call me, if you need me.  OR  b: You can call if you need me.  Note that in a:, the comma is placed before the "if" and is not present in case b.  From this link, I gather that it isn't necessary because it's a short sentence:   But can someone point out an "official" source on this usage? I understand the proof that  $$Var(aX+bY) = a^2Var(X) +b^2Var(Y) + 2abCov(X,Y), $$  but I don't understand how to prove the generalization to arbitrary linear combinations.  Let $a_i$ be scalars for $i\in {1,\dots ,n}$ so we have a vector $\underline a$, and $\underline X = X_i,\dots ,X_n$ be a vector of correlated random variables. Then $$ Var(a_1X_1 + \dots a_nX_n) = \sum_{i=1}^n a_i^2 \sigma_i^2 + 2 \sum_{i=1}^n \sum_{j&gt;i}^n a_i a_j \text{ Cov}(X_i,X_j)$$  How do we prove this? I imagine there are proofs in the summation notation and in vector notation? I want to prove the  over a field are prime ideals. To be concrete:      For simplicity, let $I=(x_{11}x_{22}-x_{12}x_{21},x_{11}x_{23}-x_{13}x_{21},x_{12}x_{23}-x_{13}x_{22})$ be an ideal of the polynomial ring $k[x_{11},\ldots,x_{23}]$. I have no idea how to prove that $I$ is a radical ideal (i.e. $I=\sqrt{I}$). Could anyone give some hints?   Generally, let $K$ be an algebraically closed field, then $\{A\mid\mathrm{Rank}(A)\leq r\}\subseteq K^{m\times n}$ is an irreducible algebraic set (I first saw this result from ). And I tried to prove this by myself, then I have proved it (when I see the "Segre embedding").      But I have no idea how to show that the "determinantal ideals" are radical ideals (I hope this is true). BTW, is the statement that the determinantal ideals over a field are prime ideals true ?   Thanks. 'Some index files failed to download' while updating Given that   $x^x = y$; and  given some value for $y$   is there a way to expressly solve that equation for $x$? Prove that $\lim_{x \to \infty}\big(\frac{x}{x-1}\big)^x$ is also $e$. Is it possible/sensible to add a suspension fork to an old mountain bike? I have a ~1993 Giant mountain bike.  It's not great, but I am a very new and casual rider.    The bike has a solid fork and it makes for the predictable rough trail ride.  The trails are generally fairly modest (I live in North Texas, a 100' change in altitude makes our ears pop here) and the surface is generally dirt (very few rocky areas).  So, I am thinking that it would be very nice to have a suspension bike.  And seeing that a modest fork can be purchased for a modest price, I am wondering if it makes sense to add a fork to this bike.   asked about going the other direction, and there were some cautionary words about changing steering geometry.  So, on balance, is this something that I would be best off to avoid?  I suspect that if I have to replace a bunch of steering parts, the cost will quickly approach that of a new low end bike. Encountering a FileSystemRights value that isn't defined in enumeration Do the parent directory's permissions matter when accessing a subdirectory? If I have a root folder with some restrictive permission, let's say 600, and if the child folders/files have 777 permission will everybody be able to read/write/execute the child file even though the root folder has 600? What really causes light/photons to appear slower in media? How to redirect a URL to a custom IP address? I have an Ubuntu machine that I'm using as my home Internet router.  It uses a USB mobile broadband modem as its Internet connection, and shares this with another router (via the router's "Internet" port) which I use as my LAN.  How do I redirect requests from within my LAN for a given URL to a custom IP address (ex have google.com redirect to x.x.x.x)? Here's a scenario:  Someone asked about "How to build a SQL query to get count and blah blah blah..."  His only tag is . I added the  and  tags to it.  Someone else answered with a nice SQL statement that has a "group by" statement which seems "logically inevitable".  I proceed to add a tag . I did that on the basis that someone who searches for  will be able to find this post useful, just as I did. My concern is, it wasn't described at all in the question, nor was it implied, unlike  and .  What is the community opinion?  The scenario above is a real one . And the one who answered it nicely is Jon Skeet. One or more of the following things happened to me on a Stack Exchange site:   My post(s) were wrongly deleted or locked by a moderator A moderator posted a wrong comment on one of my posts I was suspended, but did not do anything wrong I asked a meta question, but the question was closed and/or deleted A moderator discredited me in public ...   I think a moderator has abused their power, but I feel powerless against them.  What recourse do I have now? What can I do when I feel that a moderator has abused their power, and has been repeatedly unfair in their dealings with me? Show that the equation, $x^3+10x^2-100x+1729=0$ has at least one complex root $z$ such that $|z|>12.$ Logging into Ask LibreOffice with my Stack Exchange OpenID If I'm on , I click on OpenID and then enter  and click Log In, I get a page that says:  An error occurred on the server  This event has been recorded.  If you believe you encountered this message in error, please report it.   I clicked report it and here I am reporting it! Using Firefox 26 Possible Duplicate:          I live in a small apartment building and we can catch each-others wifi networks. We have different service provides and were wondering if we could amalgamate our networks so that we could use each others bandwidth transparently.   I would assume this would reduce latency.  Does anybody have a tutorial on how to fuse together wifi networks for a throughput increase? The target machine is a Linux computer.   EDIT  My work makes me do lots of 1TB flat file-transfers, so I believe my motivation is sound. com.mysql.jdbc.exceptions.jdbc4.CommunicationsException: Communications link failure Break a previous commit into multiple commits How can I resize (not compress) multiple images in a MS Word document? In a blank document, I insert images (screenshots - all are of same size and same format) from a folder. I want to resize the images to a desired size. All now I am doing is selecting one image by one, setting its size. Word does not seem to have multiselect working for images.  Ideally, I want to select multiple images set its size in one go with out use of a macro.