Why does a helium balloon rise? This may be silly question, but why does a helium ballon rise? I know it rises because helium is less dense than air. But what about the material of the ballon. It is made up of rubber/latex which is quite denser than air. An empty ballon with no air in it falls, so why does a helium filled balloon rise?
Balloons and lifting gases Hydrogen is the lightest element, so it's cable of lifting the most weight in out atmosphere (probably not the best terminology there, but you get the picture)  Would hot hydrogen (in the same sense as hot air) be able to lift even more mass? Would a higher or lower density of hydrogen in a ballon lift more? If you could have a balloon which had nothing in it (it was a vacuum inside) would that lift more than a hydrogen balloon?  Basically what are the physics to balloons and lifting?  (really not sure what to tag this, so if someone else could that'd be great)
Is C strongly typed? To quote :     Two commonly used languages that   support many kinds of implicit   conversion are C and C++, and it is   sometimes claimed that these are   weakly typed languages. However,   others argue that these languages   place enough restrictions on how   operands of different types can be   mixed, that the two should be regarded   as strongly typed languages.   Is there a more definitive answer? Division by $0$ I came up some definitions I have sort of difficulty to distinguish. In parentheses are my questions.   $\dfrac {x}{0}$ is Impossible (  If it's impossible it can't have neither infinite solutions or even one. Nevertheless, both $1.$ and $2.$ are divided by zero, but only $2. $ has infinite solutions so as $1.$ has none solution, how and why ?) $\dfrac {0}{0}$ is Undefined and has infinite solutions. (How come one be Undefined and yet has infinite solutions ?) $\dfrac {0}{x}$ and $x \ne 0$, it's okay for me, no problem, but if someone else wants to add something about it, feel free to do it. apt-get stuck at 0 [Connecting to us.archive.ubuntu.com] When I do upgrade it gets stuck here:  user@plato:~# sudo apt-get upgrade Reading package lists... Done Building dependency tree        Reading state information... Done Calculating upgrade... Done The following packages will be upgraded:   bind9-host coreutils cpio curl dnsutils libbind9-90 libc-bin libc-dev-bin   libc6 libc6-dev libcgmanager0 libcurl3 libcurl3-gnutls libdns100   libdrm-intel1 libdrm-nouveau2 libdrm-radeon1 libdrm2 libisc95 libisccc90   libisccfg90 libjasper1 liblwres90 libplymouth2 libsepol1 libssl-dev   libssl-doc libssl1.0.0 linux-libc-dev mime-support multiarch-support ntp   ntpdate openssl plymouth plymouth-theme-ubuntu-text python3-distupgrade   ubuntu-release-upgrader-core unzip 39 upgraded, 0 newly installed, 0 to remove and 0 not upgraded. Need to get 16.0 MB of archives. After this operation, 12.3 kB of additional disk space will be used. Do you want to continue? [Y/n] y 0% [Connecting to us.archive.ubuntu.com (2001:67c:1562::15)] ░   When I do update it gets stuck here:  user@plato:~# apt-get update 0% [Connecting to us.archive.ubuntu.com (2001:67c:1562::14)] [Connecting to sec░   I'm able to ping us.archive.ubuntu.com just fine:  user@plato:~# ping us.archive.ubuntu.com PING us.archive.ubuntu.com (91.189.91.23) 56(84) bytes of data. 64 bytes from economy.canonical.com (91.189.91.23): icmp_seq=1 ttl=54 time=81.5 ms 64 bytes from economy.canonical.com (91.189.91.23): icmp_seq=2 ttl=54 time=81.5 ms 64 bytes from economy.canonical.com (91.189.91.23): icmp_seq=3 ttl=54 time=81.5 ms 64 bytes from economy.canonical.com (91.189.91.23): icmp_seq=4 ttl=54 time=81.4 ms ^C --- us.archive.ubuntu.com ping statistics --- 4 packets transmitted, 4 received, 0% packet loss, time 3004ms rtt min/avg/max/mdev = 81.425/81.528/81.582/0.355 ms   What can i do to fix this?    What I have tried so far.  I've tried following directions on this blog:   To force apt-get to use IPv4 exclusively   Create a file in /etc/apt/apt.conf.d/99force-ipv4 Add this line to newly created file Acquire::ForceIPv4 false; Apt-get will now use IPv4 exclusively Why is time special? In Special Relativity, the spacetime interval between two events is $s^2 = -(c{\Delta}t)^2+({\Delta}x)^2+({\Delta}y)^2+({\Delta}z)^2$ giving the Minkowski metric $\eta_{\mu\nu}=\text{diag}(-1, 1, 1, 1)$. What is the justification for making time have a negative coefficient, and how closely is that related to the 2nd law of thermodynamics? Sure, by letting $\eta = \text{diag}(1, 1, 1, 1)$, we get a pretty boring spacetime, and the boosts in the Poincaré group become trig instead of hyperbolic functions, but what's the physical reasoning behind this? Find minimum value of $\sum \frac{\sqrt{a}}{\sqrt{b}+\sqrt{c}-\sqrt{a}}$ How long does mana from Burning-Tree Emissary stay around? Does mana added to your pool from  get destroyed when the card that added it is destroyed? I know DES is outdated, but will is is secure in CBC mode? Can anyone help me understand why not? How to call Apex REST API from Javascript? I have two different developer orgs. One contains apex REST web service, and I want to call that REST service from my second org. I have tried to call that REST service using normal ajax post request and forcetk.js, But I got following error     XMLHttpRequest cannot load   . Origin    is not allowed by   Access-Control-Allow-Origin.   I searched about this problem then I got to know that Salesforce never allows CORS (Cross-Origin-Resource-Sharing).  Initially I was using apex to call that REST web service, that time I was able to post data from apex. But as we have callout limitation in salesforce, I want to use javascript.  Can anybody please help me? Invariant Subspace of Two Linear Involutions I'd love some help with this practice qualifier problem:  If $A$ and $B$ are two linear operators on a finite dimensional complex vector space $V$ such that $A^2=B^2=I$ then show that $V$ has a one or two dimensional subspace invariant under $A$ and $B$.    Thanks! I just need to check the reasoning in my proof is correct, I think it is valid although I'm not totally convinced because I can't follow the logic; does proving that $x$ is an integer prove that $ac|bd$?  Theorem: Let $a$, $b$, $c$, $d$ be integers. If $a|b$ and $c|d$, then $ac|bd$.  Proof: $aj=b$ and $ck=d$ for integers $j$, $k$. Then, $ac|bd$ implies $acx=ajck$ and thus $x=jk$ for some $x$. Since the product $jk$ is an integer, $x$ is an integer and thus $acx=ajck$ and thus $ac|bd$. How to change from csh to bash as default shell I have csh as my default shell, as shown by echo $SHELL. I want to switch to bash as my default shell. I tried the following approaches to no avail:   With chsh I get:  chsh: can only change local entries; use ypchsh instead.  With ypchsh I get:  ypchsh: yppasswdd not running on NIS master host ("dcsun2").    I only have .chsrc in my home directory and I cannot find any .profile files in /etc. How can I change my default shell to bash? The junction has a stop sign on each of the four entrances.   The junction has a stop sign on each of the 4 entrances.   The first is preferred, for some reason, by many English texts. Why? I haven't seen this phenomenon in other languages. Limit of $(\cos1/n)^{n^2}$ when $n\to\infty$ I've been trying to solve this limit: $$\lim_{n\to \infty} \left(\cos\left(\frac{1}{n}\right)\right)^{n^2}$$ I solved it using l'hopital but I have to try in another way. I tried to make the expression:  $$ \lim_{n \to \infty}\exp\left(n^2\ln\left(\cos\left(\frac{1}{n}\right)\right)\right)$$  but it didn't work out for me... Split polygons into equal areas within each region I've a set of regions within a city. I'd like to split each region into five subregions and then get the centroid of each one of them.  I know how to make a vector grid, but the problem I have is that the generated points are not taking into account the regions, they're only related to the general map.  I have this:    And I want to split each region in five equal size areas. Something like this:    I'd like to build a visualization similar to this one: . In order to accomplish that, I need to split each region, because the original ones are pretty big. Copying and pasting features using PyQGIS I have created a table join between two shapefiles which I've named 'OSMM' and 'Mastermap'. I want to copy and paste all features (polygons) without a match from the target layer ('Mastermap') into the parent shapefile ('OSMM'). I can't find any up-to-update code for QGIS 3.6. Here is my code so far: #1. loads the SHP files uri = 'C:/Users/xx/Downloads/OSMM.shp' vlayer = iface.addVectorLayer(uri,&quot;&quot;,&quot;ogr&quot;) uri = 'C:/Users/xx/Downloads/Mastermap.shp' vlayer = iface.addVectorLayer(uri,&quot;&quot;,&quot;ogr&quot;)  #2. Joins &quot;InvoiceNum&quot; from OSMM to Mastermap layerToJoin = QgsProject().instance().mapLayersByName('OSMM')[0] target = QgsProject().instance().mapLayersByName('Mastermap')[0] myJoin = QgsVectorLayerJoinInfo() myJoin.setJoinFieldName('InvoiceNum') myJoin.setTargetFieldName('InvoiceNum') myJoin.setJoinLayerId(layerToJoin.id()) myJoin.setUsingMemoryCache(True) myJoin.setJoinLayer(layerToJoin) myJoin.setJoinFieldNamesSubset(['InvoiceNum']) target.addJoin(myJoin)  #3. Selects NULL features from Mastermap params = { 'EXPRESSION' : '\&quot;OSMM_InvoiceNum\&quot; is NULL', 'INPUT' : 'C:/Users/xx/Downloads/Mastermap.shp', 'METHOD' : 0 } processing.run(&quot;qgis:selectbyexpression&quot;, params)  So how do I perform the PyQGIS version of Ctrl+C on 'Mastermap' and Ctrl+V into 'OSMM'? Presumably I will then need to use the following code: #5. Save the changes OSMM.commitChanges() I need to (permanent) redirect all of the following:   www.example.com www.example.com/folder sub.example.com sub.example.com/folder   Basically, all URLs that are reachable without being intended to: www.new-example.com, I want to do this within Apache and using the mod_rewrite module via the .htaccess file. Should I always use a comma after "e.g." or "i.e."? I would like to know if there is some way to share a variable or an object between two or more Servlets, I mean some "standard" way. I suppose that this is not a good practice but is a easier way to build a prototype.  I don't know if it depends on the technologies used, but I'll use Tomcat 5.5    I want to share a Vector of objects of a simple class (just public attributes, strings, ints, etc). My intention is to have a static data like in a DB, obviously it will be lost when the Tomcat is stopped. (it's just for Testing) Compiling documents online I have heard rumors that you can compile documents online, and more specifically that Google has a free online compiler, but I have never been able to find any. Is there a way to compile documents online, so that I can write and compile documents even if I don't have a TeX distribution installed on my computer? Graphics issues after/while installing Ubuntu 16.04/16.10 with NVIDIA graphics I installed or upgraded to Ubuntu 16.04, but I'm having some problems with my graphics card.   The display is blank The display flickers Items on the screen do not display correctly Certain programs complain of graphics issues The computer boots to low-graphics mode Your specific issue may not be here, but try the solutions described in the answers anyway.   This question is very broad, but each one of these issues likely has one solution.    If you are experiencing a login loop, check the last part of my answer.    If you experience one of the listed issues and have your own solution (mine didn't work, or there's a faster way), feel free to post another POLITE answer, describing your problem and how you fixed it.    Those attempting to install Ubuntu, please refer to the section titled: If you are unable to enter a TTY or are trying to install Ubuntu.