I am launching a high-altitude balloon as a part of a physics project I am working on. I know that the amount of helium I need corresponds to about 40 newtons of lift for the launch, which is all I need to know for practical purposes. But if I wanted to account for the ever-changing buoyant force on a weather balloon, I would need a calculus-based model, either an integral or some differential equation.  From the derivation on Wikipedia, I have:  $$ F_b = \oint \sigma dA $$  where $\sigma$ is the Cauchy stress tensor and $dA$ is the surface area in contact with the fluid, or of the latex balloon, in my case. I am an undergrad student and have virtually no knowledge of tensor calculus.   Are there any other calculus-based models for buoyancy? If not, is numeric integration an appropriate strategy for this problem?
For a project into balloon simulation I'd like to know how the force on a balloon changes with altitude:  I know that the Buoyant force on a balloon is:  $F = (\rho_{air} - \rho_{helium})gV$  Using changing air density from the international standard atmosphere and changes in gravitational acceleration I can calculate the buoyant force and add this to the (negative) force of gravity mg.  However I know that it isn't this simple, the balloon changes volume with altitude as the air pressure outside the balloon changes.  I'd like to know how the balloon inflates - this would be a great help!  I think it may be something to do with ideal gas laws -  I can calculate air temperature and pressure from the international standard atmosphere but do I use pressure as the air pressure? - or air pressure minus balloon pressure?
I only get four video output options I have set of temp files created in my folder with .bak extensions. How can I make them hide by default in Ubuntu?  I tried adding a .hidden file with *.bak as entry in the file, but that is not working. Any help is very appreciated... Set myself as owner of /etc with chown command now getting all kinds of errors Missing abstract with amsart class I'm not very good at LaTeX but I've been using it to write my dissertation. Have just come to the end and trying to format everything correctly. Whenever I try to put my Abstract paragraph into the form \begin{abstract} (writing here) \end{abstract} and click play the whole abstract section just disappears. What am I doing wrong?  \documentclass[12pt]{amsart}  \begin{document}  \begin{abstract} (my abstract here) \end{abstract} \end{document} How to determine if string contains specific substring within the first X characters I want to check whether Value1 below contains "abc" within the first X characters. How would you check this with an if statement?  var Value1 = "ddabcgghh";  if (Value1.Contains("abc")) {     found = true; }   It could be within the first 3, 4 or 5 characters. Is it possible to assign numbers generated in an Error-Message? How to hide a(n) (NTFS) partition from ubuntu I need to hide an NTFS partition from listing in the nautilus. How can I achieve that?   Is there any tool to do that?   And how can I hide files in ubuntu (except . operator) How do I create a villager with custom trade offers? I have a large hole in a mesh that I'm trying to fill:    I've been trying an approach like this:    I'm finding it quite tough, since the number of vertices around the perimeter of the hole don't match a grid.  Is there a better approach to filling a hole like this? It's big and empty and I can't get up to the top, what should I do? I can't jump high enough, anti-gravity doesn't seem to do anything either.  My first tactic was to use the octopus summoning crown to create a pile of octopi that I could climb up, but when I teleported to the back of the room they followed me back, trapping me in the entrance. If I want to print multiple figures in figure, I'm using subfigure in the figure environment. This work quit well and is looking smart, if all figures have the similar height. Now there is this figure which is more than two times higher than the other ones. I would like to order the subfigures like its shown in the following picture:     But I don’t know how to get this into Latex.   My regular code for four subfigures:  \begin{figure} \centering \begin{subfigure}{0.32\textwidth}    \centering    \includegraphics[width=\textwidth]{highone}    \caption{}    \label{fig:a} \end{subfigure} \begin{subfigure}{0.32\textwidth}     \centering     \includegraphics[width=\textwidth]{b}     \caption{}     \label{fig:b} \end{subfigure} \begin{subfigure}{0.32\textwidth}     \centering     \includegraphics[width=\textwidth]{c}     \caption{}     \label{c} \end{subfigure} \begin{subfigure}{0.32\textwidth}     \centering     \includegraphics[width=\textwidth]{d}     \caption{}     \label{fig:d} \end{subfigure} \caption{Blabla.} \label{fig:abcd} \end{figure}   Thank you for your help! At the moment the most frustrating part of my brewday is getting the cold wort into the fermenter. Here is my process. I have a 20 litre saucepan that i use as a kettle that does not have a tap. At flame out I add my immerseion chiller that is plumbed into my kitchen sink tap. When I have reached temperature I remove the chiller and whirlpool and leave it to rest to 30 minutes. I am use a siphon to rack directly into the plastic bucket fermenter.  The problem is that the siphon always gets clogged with the hops after i have only transfered about 20% of the wort. I have also tried carefully pouring the wort through a seive, but this sort of depfeats the point of the whirlpool and also the seive got blocked with hops as well. All in all i end up wasting a lot of the wort because i am afraid of getting a lot trub in the fermenter.  What are the best processes to use with my current equpment to get the wort into the fermenter? Install Biber in MiKTeX on a 64-bit version of Windows Update: As of summer 2015 Biber is also included in the 64-bit repositories for MikTeX. There is no need to install Biber manually any more, it can simply be installed via the Package Manager just like other packages.   The question  describes how to use Biber with MiKTeX. But on the 64-bit version of MiKTeX Biber isn't installed.  How it is possible to install Biber on a 64-bit version of Windows?  Can I use this ? Mask and convert color in photo/video I am very new to blender still.  I want to simply change the color of an object in the video.  I want to know if there is good way to do this.  I am including what I have to test. Time-ordered operator in Srednicki , "Note that the operators are in time order...we can insert $T$ without changing anything". This I agree with. But then on the next paragraph he states "The time order operator $T$, moves all...where they annihilate...". How correct is this paragraph? Is equation (5.14) always valid? Is equation (5.14) the reason why the time-ordering symbol $T$ appears all over the place in QFT?  The equations I'm referring to are (braket package available?):   $$\langle f|i\rangle = \langle0|~a_{1'}(+\infty)a_{2'}(+\infty)a_{1}^\dagger(-\infty)a_{2}^\dagger(-\infty)~|0\rangle \tag{5.13} $$  and since time goes from right to left inside the braket we put the $T$-symbol  $$\langle f|i\rangle = \langle 0|T ~a_{1'}(+\infty)a_{2'}(+\infty)a_{1}^\dagger(-\infty)a_{2}^\dagger(-\infty)~|0\rangle \tag{5.14} $$   without changing anything.   After this, the $T$-symbol appears everywhere! I would like to restore LightDM to its default state, because for some reason /etc/lightdm/unity-greeter.conf is now an empty file.  Deleting /etc/lightdm/unity-greeter.conf and then running sudo apt-get install --reinstall unity-greeter doesn't create a new config file as you might expect.   How can I restore a missing config file? Selecting points within polygon and updating attributes using ArcPy? I have two feature classes, a polygon of sales areas for a city, and point layer of addresses. I want to select the addresses based on the sales area they are in and update their attribute table to show the sales area. I have searched python+point+in+polygon for other posts and I know how to select the polygons but I can't figure out how to populate the address points.   I found the following script that works but takes almost two hours to run.  import arcpy import os  # Allow overwrite  arcpy.env.overwriteOutput = True  # Script user input parameters polygonLayer = arcpy.GetParameter(0) pointLayer = arcpy.GetParameter(1) workspace = arcpy.GetParameter(2) copyField = = arcpy.GetParameterAsText (3)   # Add new field to target feature class  fieldList = arcpy.ListFields(pointLayer) fieldNameList = [] for field in fieldList:    fieldNameList.append(field.name) fieldType = arcpy.ListFields(polygonLayer,copyField)[0].type if not copyField in fieldNameList:     arcpy.AddField_management(pointLayer, copyField, fieldType)  arcpy.MakeFeatureLayer_management(polygonLayer, "PolygonLayer")  pntCursor = arcpy.UpdateCursor(pointLayer) for pnt in pntCursor:     rowID = pnt.OBJECTID     arcpy.MakeFeatureLayer_management(pointLayer, "PointLayer", '"OBJECTID" = ' + str(rowID))      # Polygon selection based on CONTAINS selection method     arcpy.SelectLayerByLocation_management("PolygonLayer", "CONTAINS", "PointLayer")      # Copy field values from polygon to point layer         polyCursor = arcpy.SearchCursor("PolygonLayer")             for poly in polyCursor:         pnt.setValue(copyField, poly.getValue(copyField))         pntCursor.updateRow(pnt)      arcpy.Delete_management("PointLayer")          del pntCursor, polyCursor, pnt, poly   I don't think I need to create a new field so I have simplified my script too:  # Import arcpy module import arcpy import os  # Allow overwrite  arcpy.env.overwriteOutput = True  # Script user input parameters polygonLayer = arcpy.GetParameter(0) pointLayer = arcpy.GetParameter(1) workspace = arcpy.GetParameter(2) fsaPoly = arcpy.GetParameterAsText (3) #FSA in FSA_Boundaries fsaPnt =  arcpy.GetParameterAsText (4) #FSA in Addresses   # Select all Address in FSA     polyCursor = arcpy.SearchCursor("PolygonLayer")      for poly in polyCursor: # Select based on CONTAINS      arcpy.SelectLayerByLocation_management(poly, "CONTAINS", "PointLayer")  # Add FSA to Address         pntCursor = arcpy.UpdateCursor("pointLayer")               for pnt in pointCursor:             pnt.setValue(fsaPnt, fsaPoly)             pntCursor.updateRow(pnt)   del pntCursor, polyCursor, pnt, poly   I know my update cursor is wrong but i'm stuck on how to update one feature class using information selected from the other.   I know I can do this manually but I need to do it for multiple cities so I would like to automate the process.   I am using Arc 10.2.2 to run my python script. Where are my Minecraft saves located? Dynamic Object Intellisense Pre-print service like arXiv but with private option, so I can correct my mistakes without publicly advertising my mistakes? As a student, I make mistakes all the time, and stealing my idea will not face heavy retaliation. I don't mind if someone comes up with concurrent works, but I do mind if my key idea is leaked. Many graduate students in my field therefore only use pre-print services if their papers are either publicly presented or conditionally accepted. I am looking for a website that can do most of what ArXiv does, with an additional private option or limited visibility, for example an option to make the paper unindexed by search engines. This way, my paper cannot be seen for at least a few months. By doing so, I can at least claim that I am one of the first few researchers who got the idea right. And most importantly, I can privately correct my mistakes, if any. Publishing mistakes is embarrassing and sometimes harms one's career. Many mistakes will be revealed during the peer-review processes and I have a chance to correct them before publicizing. Please kindly think twice before giving ethically and &quot;politically correct&quot; comments like no one will scoop me, everyone will cite me, and the scoopers will be punished. At least in my field, scooping and idea-stealing is commonly observable, and big-names don't care about publicizing pre-prints because they have resources to punish the scoopers. I do understand that keeping my work private might harm the community as the information becomes less transparent. However, it could also be possible that my un-reviewed flawed works misguide the community, and a paper reviewed will benefit the society more. I personally wasted six months because of someone else's mistake in a published work.  I heard that in molecular biology there is a widely-used service to upload your research data, like protein structure and sequencing, to get a timestamp. The uploaded materials can be set to be private for a period, and fully disclosed later. A lot of people do use this function. This is not exactly what I am looking for but shares some similar ideas. If I remember correctly, there is an interesting paper studying the scooping patterns with the aid of this database.  Solutions found!  and some other university-run pre-print services do have an embargo function. Moreover,  is a widely used repository, that, unlike arxiv, only displays the newest version to the general public, although still keeps a tag on the first submitted date!