On a multirotor, how effective is braiding of power wires from the ESC to the BLDC Motor(s) in reducing or eliminate emf?

Question: Assuming BLDC motors with the ESC positioned at the 'worst case' (furthest distance) with longest power wires between the ESC and the motor, will braiding these three wires reduce the EMF that would be produced by unbraided wires? Or, would shielding these three wires work better? Is it even needed if the wired are inside a carbon fiber strut? After all, the CF is conductive and will act as a shield against EMF.
Comment: Question also here: [On a multirotor, how effective is braiding of power wires from the ESC to the BLDC Motor(s) in reducing or eliminate emf?](https://robotics.stackexchange.com/q/20600/26045)
Answer: The braiding of motor wires should only be done for looks.  It looks cool and that's about it.  It may actually cause problems with smaller motors.  I once had braided motor wires on a brushless whoop size quad and the braiding of the wires caused the motors to behave erratically at start up.  Sometimes they would start spinning.  After removing the braids they started up perfectly every time.
Answer: The BLDC motors we use on quadcopters are 3 phase wiring, so probably the best answer of from the electronics stackexchange Braiding 3 Phase Wires.

Twisting reduces electromagnetic effects (both radiation and reception) in two ways. Firstly if the wires are loose twisting them keeps them close together. Secondly twisting mixes up the direction of the magnetic fields so when viewed from a distance they mostly cancel.
I would expect braiding to have a similar impact electrically to twisting with the advantage that it won't come un-twisted.


A Question about the .NET garbage collector when cyclic references exist

Question: I'm aware that the .net garbage collector detects cyclic references but I was wondering whether cyclic references may cause objects to stay longer than necessary.
I have a cyclic reference in my ASP.NET app (an intended one for performance related reasons), can I get away with that?
Regards
Answer: It doesn't take the CLR any longer to remove a cyclic reference than a non-cyclic reference.  The CLR uses a combination of techniques to garbage collect.  The quick version though is it starts with all rooted objects (objects on the stack, or held with a strong GC handle).  Any object that is reachable from these objects is alive.  Anything else will be collected.  A cyclic reference won't affect the results of this algorithm other than the CLR has to make sure to check for them and not walk in circles
Answer: It doesn't need to detect cycles, because in the algorithm it uses, cycles are simply not an issue.
If there is a path from a root (a variable on the stack, or a static variable, basically, because these are known to be live, and must not be GC'ed) to an object, then that object must be live too, and will not get GC'ed.
If there is no path, then the object is dead and can be GC'ed safely. Cycles simply don't matter. So no, cycles won't make a difference in objects lifetime.
Answer: Basically if any statics, local variables, parameters, or member variables references an object it is not available for garbage collection. It doesn't really matter if cyclic references are present or not. Either the cycle is referenced, in which cases it isn't collected or it isn't referenced in which case the entire cycle may be collected.
If a type implements a finalizer it will survice collection(s) until its finalizer has run.
Answer: The GC will collect any object that is not referenced by a root object. Root objects are typically objects referenced by all appdomains (i.e. loaded assemblies/type objects), global and static object references, all object references on each thread's respective stack plus any object reference currently loaded into a CPU register.
At collection time, the GC walks the references of all known root objects and marks any object it finds along the way as "in use". When done, any objects not marked can be safely collected.
So if none of your objects are referenced (directly or indirectly) by a root object, it does not matter if there are cyclic references. They will all be eligible for collection regardless. I say eligible, because the GC uses three different strategies for collecting eligible objects, and for performance reasons only one of those collects all eligible objects when run.
Normally you won't have to think about this, it just works. But there is a lot more to garbage collection and you still need to understand the fundamentals of it in order to understand memory management and write bug free code that does not leak resources etc. Therefore its something every .NET developer should take a look at. Here are a few resources that explain more about how the CLR does garbage collection.
MSDN - Garbage Collector Basics and Performance Hints (Rico Mariani)
MSDN Magazine - Garbage Collection (Part 1): Automatic Memory Management in the Microsoft .NET Framework (Jeffrey Richter)
MSDN Magazine - Garbage Collection (Part 2): Automatic Memory Management in the Microsoft .NET Framework (Jeffrey Richter)
Answer: The Java GC can detect cycles and deal with them.  I'd bet that the C# GC can as well.


Preventing water from entering carport

Question: How can I prevent water from entering my carport? My entire front yard and driveway slope toward my house. Negative draining issue. I have french (or trench) drains installed across the entire width of the front of my house. However, I don't have anything preventing the water running down the driveway from entering my carport. If I could just redirect it, I think the water would bypass my carport and head down my backyard if there were something to redirect it. I'm attaching a picture to show you. There is a drain on the left side of my carport (if you're facing the carport). One option might be to install another similar drain across the entire width of my driveway right before the carport entrance and then connect that drain to the existing drain. OR, is there a less invasive and less expensive option that would work? I've been considering whether a burm (sp?) or speed bump would solve the problem. Any ideas? I can't figure out how to add a picture...
Comment: Hello, and welcome to Stack Exchange. If you edit the URL of your pictures into your question, someone will be along to edit the pictures in.
Answer: Most people in your situation trench the driveway, drain it off somewhere, and put a grate on it.
The speed bump could work if it was big enough, but chances are (1) some water would come over it, and (2) you'd probably hate it enough that you trench in the future.
Answer: You might try a drain channel in front of the door, pulling any water to the side and into you french drain lines.  Easier to cut one of these in then redoing the entire driveway. Here's an example on

Amazon. Drainage Trench & Driveway Channel Drain

Many other options are available, different widths, plastic grates, ect...Just shop around a little.

Preventing water from entering carport


Can a Pakistani citizen transit through Netherlands or Switzerland if traveling with an official passport?

Question: I am going to UK to attend an official meeting and have UK visit visa. Can I go to UK through Switzerland or Amsterdam. I have an official passport of Pakistan but I do not have a Schengen visa.
Comment: Rules for entry with official and diplomatic passports are set by the individual Schengen member states, add are rules for airport transit.  Answering this question therefore requires checking the rules for both Switzerland and the Netherlands, I believe.  I don't have time at the moment but will do it later if nobody else has done so by then.
Comment: Official, diplomatic and special are all different things.
Answer: As stated in Timatic, the database used by Airlines:
For the Netherlands:

Visa required, except for Nationals of Pakistan, holding
  onward tickets transiting by the same or first
  connecting aircraft AND meeting one of the following
  conditions:
[...]

holding a visa issued by Bulgaria, Canada, Croatia, Cyprus,
  Ireland (Rep.), Japan, Romania, USA or United Kingdom,
  traveling to any non-Schengen Member State;
returning from Bulgaria, Canada, Croatia, Cyprus, Ireland
  (Rep.), Japan, Romania, USA or United Kingdom after having
  used the visa;

For Switzerland

Visa required, except for Holders of confirmed onward tickets in transit only through Geneva (GVA) or Zurich (ZRH)

So yes, you can transit in the Netherlands or Switzerland if the trip is in a single booking.
Comment: There is a difference between a Pakistani official and diplomatic passport.
Comment: @greatone Edited. Timatic has a "Normal", "Seaman" and "Government duty" Option. This is for "government duty" which applies to both official **and** diplomatic passports
Answer: The other answer references Timatic, I will, however, try to answer your question with direct references to official governmental sites.

Question 1:

Can I transit through Netherlands without airport transit visa?
Answer:

You only need an airport transit visa if your nationality appears on
the list of countries whose nationals require a visa (Pakistan is in this list). However, an
airport transit visa is not required if you:

have a visa or residence permit for a country in the Schengen Area, the EU (UK is here) or the EEA.

So yes, you can transit through Netherlands without airport transit visa. (Source).

Question 2:

Can I transit through Switzerland without airport transit visa?
Answer:

Citizens of the following states are required to hold an airport visa:

...
Pakistan

However, holders of a valid visa issued by one of the following states are exempted from the requirement to hold an airport transit visa:

...
Great Britain (They mean UK)

So yes, you can transit through Switzerland without airport transit visa. (Source).


What type of material can I use to make a model used for medical ultrasound practice?

Question: I'm attempting to make a model for medical professionals to practice ultrasound guided procedures. What substances or materials would be adequate to project a picture similar to human tissue? I have a few ideas but want to pick your brains. Thank you
Comment: Ballistic gel?....
Comment: My guess is that the ultrasound will respond to impedance mismatches in the subject. In this case that would be the speed of sound. At the threshold between 2 substances with different speeds of sound, there will be some reflection, which is what I believe the ultrasound picks up.
Answer: There is a substance called ballistic gel that is used by bullet manufacturers to shoot bullets into, to test their behavior when passing through flesh. Since it is designed to mimic the bodies of large mammals, perhaps it would work for your purposes.
it is sold as a dry powder to which water is added, after which it sets up like very stiff jell-o. By adding lumps of stuff like barium sulfate to it, you can vary its density locally and thereby get ultrasound reflections off it.
Comment: This was the first thought that came to my mind, too.
Comment: Came to me much earlier...


Tutorial on stochastic simulation in Haskell

Question: I'd like to use Haskell for stochastic simulation, but I don't know how. I've read Hutton's 'Programming in Haskell', and I'm comfortable writing deterministic functional programs. However, I don't know how to start writing stochastic simulations of the sort that are easy in imperative languages like R or python. Is there a tutorial or primer on this that I could read, or can anyone provide some tips on getting started?
Answer: There's a nice self-contained paper Erwig and Kollmansberger: Functional Pearls - Probabilistic Functional Programming in Haskell on this topic. I used this as a starting point for writing a natural language processor based on Hidden Markov Models in Haskell. There's a package that is based on this paper, which also seems to provide a basic interface to R plotting.
There's also an entry on the HaskellWiki with more links to hackage. In particular, the ProbabilityMonads package might be useful for you.
Comment: Thank you - that looks interesting.
Answer: http://learnyouahaskell.com/a-fistful-of-monads#the-list-monad
This small section in Learn You a Haskell talks about using the List monad and functor functions to easily deal with non-determinism. May be a little simplistic depending on what your needs are, but make good use of the tools that are already in the standard library.


Add tiles in the app launch

Question: Is it possible to add other tiles besides newsfeed, OneDrive, sites in the app launch in the above blue bar in SharePoint 2016?
Answer: In short, Yes can add your own custom tiles that point to other SharePoint sites, External links...etc.

Note: Your farm must be patched with at least Feature Pack 1

To add custom tiles in APP launcher

Open SharePoint Shell.
Run the below cmdlets.

Cmdlets
Get-SPFeature -Identity CustomTiles

Enable-SPFeature -Identity CustomTiles -Url http://web_app -Force

Browse to http://web_app/lists/custom tiles and add your tiles!

Note: After you add a new item in the custom tiles list due to caching, it may take up to 24 hours before you can see it appear in
  the app launcher. If you want to see it immediately, you can run
  ClearSuiteLinksCache() function in the F12 console

For more details, check  Custom Tiles in SharePoint Servers 2016 and 2019
Comment: Thanks for your answer!  I can add new custom tiles now, but it is not displayed in the app launcher yet!!
Comment: Please check my updated answer!
Comment: Yes, it worked for me , after 24 hour worked for all, thank you!

Add tiles in the app launch


How does Liferay compare to Weblogic and other portal servers?

Question: I need to select between Liferay, Weblogic and other portal servers on the market?  Has anyone done a performance test amongst them?
Answer: Not sure where did you get that liferay is the best of breed. I have actually used on many sites and I am trying to getting rid of it. The documention is really bad and out of date and the portlet out of the box are ridiculous.
Blogs and wikis portlet are not up to any nice web2.0 site and the framework is rather clumsy than nice to work with.
The config mode for portlet has to be developed using liferay framework...good luck.
It is easy to start but a nightmare to extend and to use. Be aware if you are looking for an enterprise portal.
Comment: I 2nd that the documentation is nearly 'non-existant' for Liferay 5.2.  We are trying to run with Liferay and my company just forked out money for training, so that might help.
Answer: I have used several and you will find that Liferay is the best of breed. It is usually first to implement new industry standards, is the most secure and is even certified to run on thousands of CPUs concurrently.
Answer: Gartner reported at one time that WebLogic Portal was 2 years ahead of everyone else technology-wise. It is being used by companies like Verizon, Wells Fargo, Level 3, CitiGroup, etc. who spend quite a bit of money doing their homework about which technologies they invest in. My expertise is primarily in WebLogic portal (full disclosure: I worked for BEA Systems in their portal group) but since leaving I have worked with LifeRay and WebSphere. Frankly, they are just not even close to the kind of maturity, technical feature set, and ease of development you'll find in WebLogic Portal.


Government debt held by the Federal Reserve

Question: The Federal Reserve delivers its profits, after expenses, to the US Department of the Treasury. The Federal Reserve also holds several trillion dollars' worth of government debt. So when the securities that they hold reach maturity and the government pays the maturity value on them, does the Federal Reserve just pay it right back to the Treasury Department, effectively canceling the debt?
Comment: The repayment of principal is not profit except to the extent that they were not purchased at par.  The Federal Reserve System created liabilities when it originally acquired the Treasury bonds (typically by increasing commercial bank deposits/reserves at the Fed) and can use the repayment of the bonds either to extinguish some of those liabilities or to acquire new assets.
Comment: Thanks, I think I understand everything in your comment except for the part about extinguishing the Fed's liabilities. Where does the money go in that case? The Fed owes that money to itself, correct?
Comment: The Treasury pays off its bonds by "sending money" to the bondholders, in effect instructing a credit to the bondholders' accounts and a debit the Treasury's accounts.  So this would either increase the Fed's assets or, if the Fed has an existing liability that this offsets, reducing the Fed's liabilities
Comment: Yes I know the Treasury pays the money to the Fed. But when the Fed purchased the bonds from the banks, it paid for them with money which it created, right? So are you saying that the money which the Fed receives from the Treasury Dept at the bonds' maturity date is used to pay back the money that the Fed owes itself for the bond purchase?
Answer: The repayment itself is not part of the profit. That is just treasury returning back to Fed the money they borrowed. For example, when that treasury was originally issued and sold at \$1000 (assume face value is also \$1000 the government received \$1000 dollars from Fed, and when the government repays the money to the Fed, that is what effectively 'cancels' the original loan. If Fed would forward that money to treasury that would effectively be equivalent of Fed just giving \$1000 forever.
However, the interest that government paid on those treasuries will be part of Fed's profit. So any interest government pays will go back to the treasury, meaning that effectively the government pays the interest rate to itself.
Comment: Does the Fed ever purchase directly from the Dept of Treasury? I thought they always bought securities from commercial banks when conducting OMO.
Comment: @student it does not, it does it on secondary market, but that is the same thing with just one extra step. The reason why they do it on secondary market is that in US when institutions like fed were created people were very scared of big government so almost all US institutions have extra walls erected between executive branch of the government and themselves. However, from an economic perspective there is no significant economic difference between fed directly buying it from the treasury or buying it on secondary market, the money just pass via intermediary before reaching gov.
Comment: Well my understanding was that they buy/sell on the open market in order to affect the money supply and interest rates.
Comment: @student even if they did it directly on primary market it would have effect on money supply (through government) and on interest rate since interest rates on primary and secondary market closely follow each other.


Is there a word for the point where the optical axis meets the ground plane in a scene?

Question: In aerial photography, or photography in general: Is there a term for the point where the optical axis intersects the ground plane( scene)?
Answer: In engineering we call the optical center on the image plane "principal point". But that would be where the point you are looking for is projected. We dont have a name for that point in the natural domain.
Comment: Thank you! At least then I know I am not doing anything wrong in just creating a name for that point.
Answer: I think what you are looking for is the focal plane or plane of focus, which is distinct from the image plane that contains the film or sensor recording the scene. However, not all scenes contain elements that are all the same distance from the camera. Even in aerial photography unless the optical axis is perpendicular to the ground then some parts of the scene will be different distances than others. This is usually overcome by using a narrow enough aperture to provide a fairly deep depth of field.
Comment: I might have misunderstood my own question in that case. The word that I am looking for will be identical to the ground nadir for a vertical photo; that is, the point on the ground in the middle of the photo, seen through the camera. For a non-vertical photo, what would the equivalent to "ground nadir" be?
Comment: @TormodHaugene - Maybe just a "center point", when you look at the photograph. I guess you must mean the point in the subject area, for example in a landscape photo you might have a certain tree in the center point. I have not heard a name for that point, but perhaps technical photography, machine vision, or suchlike, has a name for it. We are a bit on the artistic side of photography here :)
Comment: Okey! Well, thank you for you help. I guess I will just give it some name then.. :)
Comment: @Tormod: In normal photography, there isn't anything that corresponds with a "ground nadir" as in arial photography. Depending on where your subjects are in the scene, there could be multiple points across the area of the frame where important subjects are in clear focus. Those points will not necessarily be in the center of the frame, either. All there really is is the focal plane, the point within the real-world scene where optical focus is sharp enough to not otherwise be determined as out of focus, and that all depends on the aperture.


AWS Cognito: cross region user pool replication (moving or sharing)

Question: Just came to my attention that there are now more regions supporting Cognito. Is it possible to move a user pool from one region to another?
Answer: AWS Cognito, does not replicate ( moving or sharing) the user pools across the regions at the moment, if your users / clients are closer to the region then there will be a minimum propagation delays and users will have a better speed to get the resources from the data center.
If you want to authenticate your users across the region that will be possible by the API they have provided (For example, in javascript sdk you need to mention the region while setting up API for authentication and accessing resources).
Answer: For the folks tuning in now , As of June 2020 cognito supports multi-region user pools.
https://aws.amazon.com/about-aws/whats-new/2020/04/introducing-multi-region-user-pools/


Can you mount GRX FC-RX600 110/80 chainrings on a Tiagra 4700 chainset?

Question: Has anyone mounted these https://www.bike-discount.de/it/comprare/shimano-grx-fc-rx600-2x10-11-speed-chainring-961783 on a Tiagra 4700 crankset?
If they will fit it's a LOT cheaper than buying the whole GRX crankset.
Comment: Note that the GRX cranks have a more outboard chainline if you do end up going that route. I believe you would need to use a GRX front derailleur as well.
Comment: If the chainrings mount on to the Tiagra 4700 chainset though then the chainline will not change? The question is whether these rings will fit, they *look* like the same bolt standard?
Comment: I meant "in the event you DO end up buying the entire GRX crankset..."
Answer: No.
The Tiagra FC-4700 crankset has 110mm bolt circle diameter (BCD). The 30 teeth chainring of the GRX FC-RX600 has a 80mm BCD. It's a "sub-compact" or "super compact" crankset compared to the "compact" Tiagra.
The larger 46 teeth chainring has a 110mm BCD and might fit on the Tiagra, but then you can just get the Tiagra chainring.
Comment: Awesome thanks Michael, I hadn't realised that the 34t chainring wasn't 80mm I assumed it was 110 outer and 80 inner. Having 46/34 on the front isn't really what I was looking for, I'm trying to get 32 or lower with the small ring. Looks like I need a new crankset as the only other chainrings I can find that will fit the Tiagra 4700 with 48/32 or 46/30 are the expensive Absolute Black oval rings.
Answer: The GRX chainset is 3.4mm (or possibly 2.5mm, depending on the source you read), outboard of the road chainset. The purpose of this is to allow wider tyres.
In addition, it's worth noting that road freehubs were traditionally 130mm wide, and MTB were 135mm wide. When discs were added to road bikes, the disc freehubs went to 135mm as well, but the non-disc freehubs stayed at 130mm.
At the same time that this happened, there were no new 'disc' chainsets released. They kept with the same chainline, which was 43.5mm for road double. This compared to 48.8mm for MTB double, which is the extra (5mm/2), plus another 2.8mm to cater for big fat tyres.
The chainline measurement at the front is from the middle of the BB to the middle of the chainrings (so around 2.5mm further out to the large ring, and 2.5mm further in to the big ring). The chainline at the back is the distance between the middle of the centreline of the bike, running from the middle of the bottom bracket to the rear hub.
The relevant measurement is then the chainstay length, which is the distance between these two points.
A compact road bike such as this one
https://www.giant-bicycles.com/int/tcr-advanced-1-disc-pro-compact
has 40.5mm chainstay length
whereas an endurance bike such as this one has 42.0mm chainstay length
https://www.giant-bicycles.com/int/defy-advanced-2-2021
Meanwhile an MTB such as this one is barely longer at 42.5mm
https://www.giant-bicycles.com/int/xtc-advanced-29-3-2021
which turns out to be the same as this GRX-equipped gravel bike
https://www.giant-bicycles.com/int/revolt-advanced-1-2021
It should be obvious that a shorter chainstay means you will get a bigger variation in chain angles when cross-chaining, so that with a particularly compact chainstay you could get rubbing when using the small/small combination.
That said, they still managed to release wider frames and wider disc hubs without having to make new road chainsets with chainline further out, and everyone was pretty much happy with their new disc road bikes.
Anyways, there was nothing particularly different about GRX as a groupset. You got:

rear derailleurs that were compatible with road shifters but perhaps supported larger cassettes
brifters which were largely the same as hydraulic road brifters, but with slightly different ergonomics
identical brake calipers (except for the paint job) to those sold as 'road'
the exact same bottom brackets used for 'road'

and finally you got this new chainset which had smaller chainrings because 'gravel'. Although Shimano in the past have used various funky chainring bolt spacing, all their current road double chainsets use 110mm BCD. This 110mm means that the smallest possible chainring is 33t, while there isn't really a largest one, but stiffness might dictate for a very large chainring you would use a bigger BCD.
Since the standard road chainset is now 50/34, to go any smaller (which makes sense, off-road), Shimano need to use a smaller BCD, which here is 80mm, for reasons that aren't clear, but probably for aesthetics and/or to make them incompatible with the millions of MTB chainrings around this size which are 96mm BCD or 104mm, and could have been fitted.
Note, btw, that the q-factor (width between the cranks) of the GRX chainset is only 5mm wider than the Ultegra/Dura-Ace/105 chainsets, and 1mm wider than 4700 chainset, whereas MTB chainsets tend to be around 30mm wider than road. This is of course not related to chainline but is influenced by the tyre width, in that big tyres will mean big q-factors.
If you look at this supposed 'GRX' bike:
https://masoncycles.cc/products/insearchof-grx then it boasts HUGE 27.5 x 2.8" or 29 x 2.4" tyres, but it uses a Deore XT 1x chainset, which obviously has massive q-factor and eliminates Shimano's incompatibility between its MTB front derailleurs and its road brifters.
The maximum tyre clearance of GRX 2x is officially 42mm, which is nominally 1.6"
If you look at a 'gravel' bike like this then it's using the a bog standard 50/34 road chainset
https://www.decathlon.co.uk/p/triban-rc-520-gravel-riding-bike/_/R-p-302303
and has 42.5mm chainstay length, and the bike works well with 38mm and possibly 40mm tyres
https://thesweetcyclists.com/widest-tire-decathlon-triban-rc-520/
(note that the 'RC 520 Gravel' and the 'RC 520' are the same frame and fork, it's just finishing kit that differs)
An apparently purpose-built-for-gravel frameset such as this one
https://www.decathlon.co.uk/p/men-s-gravel-bike-grvl-900-shimano-grx-titanium-frame/_/R-p-325632
has the same 42.5mm chainstay length and will go up to 45mm tyres, so a bit wider frame than a modern disc road bike that might also do for gravel.
Specailized fit the same frame with both standard road compact double chainset
https://www.specialized.com/us/en/diverge-base-e5/p/175291?color=290931-175291&searchText=96220-7344
and with a Praxis 1x road crankset
https://www.specialized.com/us/en/diverge-comp-e5/p/175289?color=290949-175289&searchText=96220-5349
and claim 47mm of tyre clearance, though supplied is 35 or 38mm. Given that this is accomplished with a 43.2mm chainstay length and a bog standard road chainline crankset then it's difficult to see that GRX brings much to the table.
It appears to me that with discs taking over, and manufacturers taking the opportunity to widen the resulting freehubs by 5mm, then road cranksets should probably have got 2.5mm wider anyway. But as I show above, this 2.5mm does not seem all that critical.
More critical tends to be induced incompatibility, e.g., the use of different pull ratios on MTB and road, so it is difficult to use road brifters with an MTB FD.
Note that front derailleur chainline specs are not exact numbers - the GRX chainset is specced at 46.9mm (I am not sure if this a misprint for 46.0mm), the FD is specced at 46.0mm, but plenty of people use road FDs (43.5mm) with the GRX chainset, and they work fine.
If you examine  a GRX chainset

you can see that the driveside assembly is merely 'FC-RX600' - it is not FC-RX600-1 or FCRX600-10/-11, it's just FC-RX600, so the difference between the three models is the chainrings.
In addition, you can clearly see there the hole for the bolts smaller BCD inner chainring - which in this particular crankset has not been fitted. This is rather like a MTB triple chainset, which bolts the big/middle ring into one hole, and the small ring into a separate hole.
An alternative way of mounting a smaller chainring is like this:

With this the small chainring piggy backs onto the large one, and is not connected directly to the spider (in this case the large chainring is a direct mount (onto the spindle, rather than bolt on) type, though that doesn't really affect the connection to the small ring).
So your FC-4700 spider is designed to have two chainrings with the same BCD bolted to it, whereas a GRX spider has two separate sets of bolt holes for the different BCD chainrings.
I would further note that the bolt holes for the GRX inner ring are spaced inbound of the outer holes, whereas the spacing of road compact rings is achieved by placing the rings either side of the spider, such that the spacing is a function of the thickness of the spider, as well as of the hole thickness of the outer ring.
If you consider the chainline of the different rings:

GRX 1x 49.7mm quoted
GRX 2x 46.9mm quoted, which means approx 49.4mm outer and 44.4mm inner chainline, given typical 5mm spacing
Road 2x 43.5mm quoted, which means approximately 46.0mm for the outer and approx. 41mm for the inner

then it's clear that the GRX spider bolt holes must be physically outbound of the road spider bolt holes. So even if you could fit the GRX chainrings to the 4700 chainset, then it wouldn't really make it more 'gravelly', especially as we've noted above, nearly all the difference between 'gravel' and 'road' in terms of the drivetrain & brakes is 'marketing' (there is a difference in the frameset and fork in that for gravel these must be have at least around 35mm of clearance, but that is something that has come with many 'road' frames and forks with the movement to disc brakes, which eliminate the 'max clearance' that you had with caliper brakes), and you can make a very good 'gravel' bike with Tiagra 4720 hydraulic, with no real need for GRX at all.
Well ok, yes, the GRX does have slightly smaller chainrings but that's nothing 'Gravel' specific. The GRX chainring gap of 16t or 17 is designed so that they could reuse road front derailleurs, which are 16t gap. There are plenty of 46/30t chainsets, but they'll all use smaller BCD, by necessity - a Tiagra 4720 'gravel bike' with all Tiagra components except a 46/30t third-party chainset would not be much different from a GRX 2x10-speed bike.
Another popular option is to take a triple road crankset, remove the outer chainring (can be replaced with a chainguard), and then use a road front compact derailleur with inner rings at 16t spacing (e.g., 26t/42t). This gives 40mm/45mm/chainguard (50mm).
But any and all such smaller chainrings will REQUIRE a new chainset that supports a smaller BCD for sub 34t (or 33t) chainrings.


Should I install all plugins to my new installation

Question: I am new to QGIS and I am preparing for first time use. Should I install all plugins that are not installed?
Comment: do you need them? some are now core to QGIS installs (referring to 3.8 and the up and coming 3.10).
Answer: No, there's no need. I've been using QGIS for years and I'd guess that I've used maybe 3-5%  of the plugins that are available. Loads of them are for niche processes, locations, or are in a different language. Just install them as you need them. I occasionally have a browse through them to see if there are any that might be useful as my uses change.
Comment: `"Plugins Choices"` is a subjective and disputable topic. You may check this source [QGIS Python Plugins Repository](https://plugins.qgis.org/plugins/?sort=-downloads) and filter it by the total sum of downloads, or by votes. IMHO those might be interesting: QuickMapServices, MMQGIS, QuickOSM etc.
Comment: Thanks TeddyTedTed. Are there any basic ones I should install?
Comment: These plugins : autoSaver, changeDataSource, Memory Layer Saver and QuickMapServices  may be useful regardless of what kind of work you intend to do
Answer: No need, it will make QGIS slower to load/start also if you have too many plugins installed.


Does the circumference of light from a star expand?

Question: If photons do not split, and light travels in straight lines then at x distance from a star there should be gaps in the straight line light tangents (which would make the star unobservable) However this doesnt happen which suggests the light field expands but how? Put another way if a light wave is present then this suggests that with wave particle duality the number of photons should increase to maintain the perimeter of the expanding field of light. With light speed at around 300,000 kms is there a formula for light wave frequency (photon multiplication) expansion?
Answer: The light from a star is distributed uniformly among the surface area of a sphere enclosing it, much like how a Gaussian surface enclosing a point charge has an equal $\vec E$ field everywhere on its surface. As such, it does "expand" in the sense that a larger Gaussian surface means that there is a lower luminosity per unit area (i.e., flux) from the star.
The light field also expands for two more reasons. The first is what is called "seeing" in astronomy, which is the blurring of stars' light because of turbulence in the atmosphere. The second reason for the "field" expansion is because the light must pass through some kind of aperture detector (either a telescope or your eye), which creates circular diffraction patterns on the viewing surface.
Comment: Thank you. So does this mean that photon density decreases hence resulting in a decrease in luminosity? If so then the distance between photons should be greater as the distance from the sun increases. I like your answer though i had thought photons must split and lose luminosity.
Comment: If by photon density, you mean the number of photons per unit area, then I think that makes sense.
Comment: Ok so if the distance between photons increases over distance then the constant emission of light would explain why at a distance we dont miss the observation of a star when the distance between photons is greater than the width of an eyeball.


How did Salazar Slytherin build the Chamber of Secrets without the other founders knowing?

Question: Hogwarts was founded and built about 1,000 years ago by Godric Gryffindor, Helga Hufflepuff, Rowena Ravenclaw, and Salazar Slytherin. Slytherin additionally built the Chamber of Secrets beneath the school in secret (pun intended). This is obviously a very large chamber, and its creation would have taken a considerable amount of time and labor. With that in mind, how is it possible that he was able to hide its construction from the other founders?
Comment: Um........ Magic?
Comment: I don't have the detailed knowledge of the Potterverse that some have. Is there any specific reason to suppose they didn't know? They appear to have deliberately created a school that full of secret rooms. The assumption is that they just allowed him to create it.  Or maybe they had an agreement where each could create some secrets, and the others would not try to find o0ut.
Comment: @InvisibleTrihedron Care to elaborate?
Comment: @Pete Good thinking! My answer below was too straightforward. You're right, the four could have agreed not to watch as Salazar Slytherin performed his work. (By magic.)
Comment: Imperius curse placed on those building Hogwarts?
Answer: We don't know, but it must have involved magic.
We don't actually know how Hogwarts was built, do we? But four wizards could not have done the job all by themselves without magic, so they must have used magic or supervised laborers, or quite possibly both. One scenario is that the school was raised magically overnight, in which case Salazar Slytherin could have tampered with the spell, or with the instructions to the magical creatures who did the physical labor, in such a way as not to be noticed. Or, if the process were a more protracted one using ordinary workmen and tools, Slytherin could have cast a spell of deception over the others so they would not notice what was happening in front of them. Or, then again, Slytherin could have waited until the foundations were complete, and then created the Chamber of Secrets secretly, again using magic. What does not seem possible is that Slytherin could have pulled the task off secretly without the use of magic, if all four were involved in the construction. Of course, if the others delegated the task of building the foundation to Slytherin, they may not have noticed workmen constructing the Chamber without magic, but it's hard to believe that they would have absented themselves long enough for him to get away with it. So magic was involved in some way, but we don't know how.
Comment: Thanks! That answers my question as best as it possibly could be, so I've marked it as correct.


LinkedList: Remove the kth node form the end

Question: Question: Write a function that takes in the head of a Singly Linked List and an integer k and removes the kth node from the end of the list. The removal should be done in place, meaning that the original data structure should be mutated (no new structure should be created). Furthermore, the input head of the linked list should remain the head of the linked list after the removal is done, even if the head is the node that's supposed to be removed. In other words, if the head is the node that's supposed to be removed, your function should simply mutate its value and next pointer. Note that your function doesn't need to return anything. You can assume that the input Linked List will always have at least two nodes and, more specifically, at least k nodes.
Each LinkedList node has an integer value as well as a next node pointing to the next node in the list or to None / null. if it's the tail of the list.
The Solution:
class LinkedList {
  constructor(value) {
    this.value = value;
    this.next = null;
  }
}

function removeKthNodeFromEnd(head, k) {
  let first = head;
    let second = head;
    let i = 1;
    while(i <= k){
        second = second.next;
        i++
    }
    if(second == null){
        head.value = head.next.value; // Why can't I do: head = head.next
        head.next = head.next.next;
        return
    }

    while(second.next !==null){
        second = second.next;
        first = first.next;
    }
    first.next = first.next.next;

}

I have understood the above algorithm for the most part. Except why I need to have:
        head.value = head.next.value;
        head.next = head.next.next;

Why can't I just replace the above lines with a single:
head = head.next

Answer: head = head.next

would accomplish nothing. It would only change the local variable head.
Read the question again:

Furthermore, the input head of the linked list should remain the head of the linked list after the removal is done, even if the head is the node that's supposed to be removed.

Since you cannot change what node is the head, you change the contents of that node.


Vector Calculus - Gradient Fields

Question: To show that a function $$F(x,y,z)=(f_1,f_2,f_3)$$   is a gradient/conservative field, is enough to show that $$\frac{\partial f_1}{\partial z} = \frac{\partial f_3}{\partial x} \ and \ \ \frac{\partial f_1}{\partial y} = \frac{\partial f_2}{\partial x} \ \ and \ \ \frac{\partial f_2}{\partial z} = \frac{\partial f_3}{\partial y}  \ \ ????$$
To show that is NOT it's enough to show that one of this conditions are not true. But my question is: if these 3 conditions are true can i conclue that the function F is a gradient field?
Comment: if those three conditions are true then $\nabla \times F = 0$
Answer: It's a bit complicated/delicate:

If the domain $D$ of the vector field $F$ is a non-empty, simply connected open set, then every $C^{1}$ vector field whose curl vanishes throughout $D$ is a gradient field in $D$. Particularly, if $F$ is defined in some rectangular box, or on a ball, or on all of $\mathbf{R}^{n}$, then the following are equivalent:
(i) $\nabla \times F = 0$ throughout $D$.
(ii) There exists a $C^{2}$ function $f$ in $D$ such that $\nabla f = F$.
The vector field
$$
F = \frac{(-y, x, 0)}{x^{2} + y^{2}}
$$
is curl-free on the complement $D$ of the $z$-axis in $\mathbf{R}^{3}$. Because the integral of $F$ around the unit circle is $2\pi$, however, $F$ is not a gradient field on $D$.
If $D'$ denotes the complement of an arbitrary closed half-plane containing the $z$-axis, then $F$ is a gradient field in $D'$. (!) For example, if $D'$ is the complement of the half-plane $x \leq 0$, $y = 0$, and if $\theta:D' \to (-\pi, \pi)$ denotes the branch of the cylindrical angle function, then $F = \nabla \theta$ in $D'$.

In summary, whether or not a particular curl-free vector field $F$ is gradient in a domain $D$ depends not merely on the component functions of $F$, but on the topology of $D$.
Answer: If the domain $D$ is simply connected, then the answer to your question is YES.
Added:- Since $\nabla ×F=\begin{vmatrix}\hat i&\hat j&\hat k\\\delta/\delta x&\delta/\delta y&\delta/\delta z\\f_1&f_2&f_3\end{vmatrix}=0$
$\implies \hat i(\delta f_3/\delta y-\delta f_2/\delta z)-\hat j(\delta f_3/\delta x-\delta f_1/\delta z)+\hat k(\delta f_2/\delta x-\delta f_1/\delta y)=0$

Vector Calculus - Gradient Fields


What can I do if autofocus does not work but manual focus does?

Question: Recently, the autofocus on my Nikon D3100 stopped working. The ring of the lens wouldn't even turn when I tried to focus. However, when switched to manual focus, the camera works fine.
When I used a different lens on the camera, the autofocus would still not work.
Is this fixable? Or do I have to change camera?
Comment: The D3100 has a settings reset feature - try resetting the settings to get it back to factory default and start from there
Comment: Are you sure you have autofocus turned on on the camera, independently of the lens switch? The rear display should show AF-A, AF-S or AF-C. If it is set to manual, it will show MF.
Comment: If the lens you are trying on does not have A/M indication in Auto mode you cannot turn the focus ring. Kit lens does not have it.
Comment: Are the photos you are taking while manual focusing ok in terms of brightness? Is your lightmeter working properly? Also is your aperture working properly? If not you have problems about lens coupling elements(diots). In any other case reset the machine to factory settings as Rob said and update the firmware, if the latest update, overwrite the firmware again.
Comment: Try cleaning the electric contacts on the lens mount.
Comment: Thanks for the Answers, I am also have the same problem but now it got resolved by Resetting the Options. Now it's working fine.
Answer: There are multiple possible scenarios here.

Lens without internal motor (AF line, including AF-D series and a minority of G-type lenses): Modern low-end cameras can't focus these, even though they can assist you if you enabled the option found in Settings>Rangefinder. You must flip the on-lens switch to M to disconnect the AF coupling gears before the focusing ring will turn (assuming the lens isn't broken!!)
Cheapest AF-S lenses (all kit lenses except the 18-105) and old AF-I lenses: Can autofocus on every Nikon DSLR if the focus style in the quick menu (<[i]> button) isn't set to MF. They still feature a traditional focusing ring which is rotated by the motor and thus disabling AF with that option won't enable manual focus unless the mechanics are disengaged via the on-lens switch.
Modern mid-grade to high end AF-S lenses: Soft touch, "endless", not-turning-during-AF focusing ring and M/A-M switch: can focus manually in any mode, and automatically if AF is enabled on both the lens and the camera (see above). Rotating the ring during AF operation will stop the motor.
AF-S DX VR 18~105:3.5~5.6G ED IF: An odd breed of the previous case where the AF disable switch selects A-M: focus can be adjusted manually at any time but will not cancel motor operation and enable shutter release even after AF fails. It becomes possible for the user to literally fight against the motor (not recommended!)
Top of the line AF-S lenses: A combination of the two previous scenarios, with a 3-position focus switch alternating between A/M, M/A and M.
Answer: Steps to take:

Try a different lens. (You mention that you already have, so that's good.)
Use the camera's "reset to defaults". (See this for Nikon, or your camera's instruction manual.)
Gently clean the electronic contacts on the camera. (Use a very tiny amount of isopropyl alcohol or camera cleaning solution, and cotton swabs meant for this kind of thing.)

If these things don't work, it's time to call the repair shop.
Answer: The D3100 doesn't have any internal focusing inside, it relies on the Lens internal motor entirely, so if you tried two lenses, the only explanation is that you have disabled focusing in camera sofware. Try resetting the camera to defaults.
Answer: Check the settings for the AE-L/AF-L button on your camera and make sure you didn't set that to AF-ON as that would remove the ability of the shutter release button to affect autofocus. See page 146 of the manual (English version) for more details on this, but you can also verify this by pressing that button (at the back of the camera, top and to the right of the viewfinder) to see if it focuses, if it does, that's your issue.
As a note, I prefer this setup, it's what I use on my D800.


Error exporting SQL Server 2008 R2 database to Excel 2007

Question: I was trying to export SQL Server 2008 R2 database to Excel 2007. I used the export wizard in SQL Server Management Studio. I used a query to export some columns of a table. I checked the query (saw the result of parsing it) and it was good. But on the REVIEW DATA TYPE MAPPING it showed error notifications. It was converting varchar to LongText. But when I executed the package it failed to run showing fail status for steps like prepare for execute,pre-execute,execute . the error message is like this

Error 0xc002f210: Preparation SQL Task 1: Executing the query "CREATE TABLE Query (
  id Long, sku LongText..." failed with the following error: "Unexpected error from external database driver (1309).". Possible failure
  reasons: Problems with the query, "ResultSet" property not set
  correctly, parameters not set correctly, or connection not established
  correctly.
  (SQL Server Import and Export Wizard)

Someone has a clue whats wrong?
Answer: Not an ideal answer (I have no idea what the cause of this problem is) but I was able to bypass this error by saving the excel sheet to 97-03 format and extracting to that.


Somewhat convoluted ssh issue. Can ping, but not ssh, but only from on-site

Question: I am trying to ssh into my server at work (CentOS) from my laptop (Ubuntu). When I am at home, I do it by running the following script on the server (I start it while I am physically at work):
ssh -R (port #):localhost:22 (name@home ip)

I do this because it doesn't accept connections from external IPs. Then, I can ssh into the specified port on my laptop, and it works fine.
However, when I am actually at work, I cant ssh to the server. The other people in my office can. They do this very simple command (only works while they are at work, since they need an IP from inside the system):
ssh (username)@(work ip)

And they are automatically logged in. When I do that, I get no response; no public key denial, no wrong username, no response at all. Eventually it times out. But I know the server has ssh running, because everyone else can do it.
Additionally, if I do this on my laptop, while at work:
ping (work ip)

I do get response packets, with no loss, almost no lag. But ssh doesn't work.
I can ssh to other places from my laptop, both while I am at home and at work. So my laptop is properly configured to ssh to things, it just doesn't reach the work server for some reason. I talked to the guy who set  it up and he insists there is no whitelist; the only security from internal IPs (and I am physically on site, so I have an internal IP, so there should be no need to do the ssh -r like I do at home) is the private/public RSA key system, and I know the keys must be already set up, because it works when I do the ssh -R from home. Plus, if the keys were not set up, I would get a public key denial, instead of no response at all, right?
If I do ssh -vv, this is the last line before it times out:
debug1: Connecting to ccny6 [work ip] port 22.

I see other people have posted similar questions, but the responses they seem to generally be "is the server running the ssh service" etc, which I know it is as other people can ssh to do (as can I if I'm using the ssh -r tunnel), so none of those responses did me much good, unfortunately.
Summary: I can ssh to a server from OFF site via a ssh tunnel, but can't ssh to it while I am right next to it using direct ssh, even though I can ping it, and others can ssh to it.
Answer: The most possible reason for the same is your ISP. I too had this issue few months back. They had closed ssh ports. Ask them and get it released.
Just a confirmation, try to do ssh to other IP's as well. It wont work either if your ISP has blocked it.
Comment: I can ssh to other places (from my laptop, both at work and at home) such as bitbucket or github. Just not to the work server.
Comment: Its a different Issue then. in my case, I was unable to do ssh anywhere.


How to remove background size from svg file made with illustrator?

Question: I am experimenting with making svg files in illustrator and using them in my html website. But the problem is when i paste my svg code in my html file it has the background size of the artboard of illustrator. And when i adjust the stroke in html, the size of the svg cuts the strokes of the path.
Answer: As far as I know, there is no way to 'turn off' the viewBox (which I'm guessing is what you are talking about) when exporting as SVG from Illustrator.
I'm not sure you would want to, as it is the reference for positioning in you entire SVG file. Deleting it would pretty much always result in a broken image. See here for more info.
If you want SVG files that scale automatically when scaling your surrounding element (e.g. a div, a paragraph, ...), you need to use Export as... from the File menu, then choose Responsive in the options.


Shoes "Natural Sole" meaning

Question: When buying a decent pair of shoes, I often see the statement "with Natural Sole" ("Loafers in Tan Leather with natural sole.") written in the description of the shoe. Does this mean that the shoe's sole is made out of natural materials e.g. various types of wood or does it refer to something else in particular?
Answer: Natural Sole means a sole made of natural materials. Leather is a natural material, so "Loafers in Tan Leather with natural sole." are a natural footwear. By adding "natural" the manufacturers mostly imply "allowing the wearer's foot to breathe."
Natural Sole are footwear made of (from) wood, papyrus, palm leaves, (cowhide) leather, ostrich skin, anaconda skin, elephant skin, and even cotton.
Comment: As a challenge, can you define "natural"?
Comment: @Tᴚoɯɐuo There's no need in that... Any dictionary states that **"natural"** means *existing in or derived from nature; not made or caused by humankind*
Comment: What is the difference between "derived" and "made"?  Is rubber derived from nature?  [The challenge is to present a definition that needs no refinement or clarification :)  ]
Comment: @Tᴚoɯɐuo And are you mentioning **natural rubber** or **synthetic rubber**? The first is derived from nature while the second one is made by humankind.

Shoes "Natural Sole" meaning


How laminar or turbulent is air?

Question: Consider an outdoors scenario, with good weather and no sensible air currents at the floor level. How turbulent or laminar is the air surrounding this environment?
Comment: Turbulent state is defined for a flow, not a fluid. So you have to define what the air flow you want to consider? Flow around you as you walk? Flow around a candle or an incense stick? Cannot answer the question satisfactorily without more details.
Answer: Use an incense stick to test. You will find that the only turbulence is from the heated air (smoke) rising from the stick, or from moving the stick around. If you can cool the smoke to room temperature without heating the area of interest, you are likely to get layers unless the room or outdoor area has no temperature gradient with height.
Where is gets interesting is a calm day at ground level and being able to see two layers of clouds moving in different directions. What the heck is going on where the layers meet?
Answer: Assuming absolutely no wind there will still be areas of turbulence, things will still be transferring heat to the air causing it to move, it doesn't take much motion for air to become turbulent because of its low viscosity. Viscosity will dampen any instability, so low viscosity means its easy for instability and turbulence to occur
Answer: If you would just calculate the Reynold number to Troposphere, say, 10 000 m thick.
Kinematik viscosity of air is, say, 1.2 x10^-5
so you found that with Re 4000 you would have just need to have a 0.0000048 m/s velocity
In praxis, though, this is not the case, but it's a stratified fluid system;
https://www.youtube.com/watch?v=9hmjcIfy8wE&list=PL0EC6527BE871ABA3&index=22
You can often see a stratified layer on a cumulus cloud base, Which might be just 600 m above ground. This doesn't change too much; for Re 4000, you still need only .00008 m/s.
So i'ts obvious that the air is Turbulent near the ground most of the time. But it's as obvious that it's not so Turbulent up there, or how you can explain that the cumulus clouds are not completely mixed away immediatly?
In Finnland, at night there is also a notable phenomenon on calm lakes. You hear even the smallest sounds far away. To me it's a sign of a laminar air. Just found this example video about the issue;
https://www.youtube.com/watch?v=6oatyxA3n0g
To conclude; It's Turbulent most of the time near the surface. It's mostly (Jet streams etc. exluded) laminar on the upper layers, and sometimes it's Laminar even near surface.

How laminar or turbulent is air?


Why didn't the Clone army capture Dooku and the Separatists Leaders?

Question: A subsequent question of this one. When the Clone Army arrives at Geonosis, why didn't they capture the Sith Lord and the Separatist Leaders? They had the advantage, because they almost destroyed the Droid Army.
Comment: Because they escaped
Answer:   They could not. Advantage of Clone Army was not so great, it was very hard battle, leading to second battle some time after that. Note that Clone army throughout the war rarely had numerical superiority over Droids. They would usually had to destroy large number of enemies before achieving strategic goal. In this case there were simply no time to protect Jedi in Arena, battle droids and capture leadership. Especially hard would be to capture powerful Sith Lord like Dooku when even Yoda could not do that.


How to get the current workflow status from within itself?

Question: I have a workflow that start by setting the workflow status to, let's say, "Awaiting Manager Approval", after the manager has approved and the workflow status is changed to "Awaiting Director Approval", how can I check this new status in a condition to determine which tasks to execute next?
Comment: helped my answer to you?
Answer: Workflow Status is inner state of Workflow and cannot be requested from Workflow 2013.
I recommend to create list column called 'Approval Status' which you will fill with the same value and checking the state with this value.

Create list column
Set Field in Current Item (Workflow Action)
If any value equals value (Workflow Action) select: CurrentItem:ApprovalStatus and fill the condition.


Which of these two forces is stronger: static friction or kinetic friction?

Question: Which of these two forces is stronger: The force of static friction or the force of kinetic friction? I am having a hard time understanding this concept of friction, so please explain your answer!
Comment: See the answer to [this question](http://physics.stackexchange.com/q/541/) and [this hyperphysics page](http://hyperphysics.phy-astr.gsu.edu/hbase/frict2.html).
Comment: This will depend very much on the materials/systems you are considering. But in general, it is harder to put an object in motion than to keep it moving.
Comment: This is the part I don't understand. Why is it harder to put an object in motion than to keep it moving ?
Answer: Looking very close at the surfaces that touch at friction, this is an illustration

Both surfaces are rough. They have ticks, holes, gabs, pits, spikes, and edges on the microscopic level. The smoother, the lower the coefficient of friction $\mu$. This constant is thus to be considered as a combined "roughness" between these two surfaces.
Intuitively and in general:
Kinetic friction is the situation where the surfaces are moving relative to each other. The surfaces are not being allowed to relax and "sink" into the depth of the gabs. At this point they slide over each other a bit higher than if they "fell" into the holes completely.
For static friction the surfaces relax and spikes fall into nearby pits as deep as possible. From here it is harder to start again, and friction holds tighter.
Answer: When two surfaces are at rest, the only friction between them is static friction.
Static friction is a force, and takes a value between 0 and $F_{max} = \mu_\mathrm{s} F_{n}$
When two surfaces are in relative motion, the only friction between them is kinetic friction.
Kinetic friction is a force and has a constant value of $F_{k} = \mu_\mathrm{k} F_{n}$
Generally we find that the coefficient of kinetic friction is less than that for static friction for most substances, i.e. once you get an object moving it's easier to keep it moving. However, there is some research into cases where the opposite may be true.


What physical effects do the other planets in the solar system have on Earth?

Question: I am interested in the following question. What physical effects do the other planets in the solar system have on Earth? That is do they change the Earth's orbit significantly or any other physical aspect of the planet?
Comment: Look at Jupiter effect here http://hyperphysics.phy-astr.gsu.edu/hbase/tide.html
Answer: One hypothesis is that planets affect each other in such a way that over a long period their parameters adjust to certain numeric sequences:
"The logical conclusion is that feedback is present via perturbations between the planets and Sun which arranges the planets into an order which minimises work done, enhances stability and maximises entropy. This calls to mind the constructal law, stated by Bejan in 1996 as follows: 'For a finite-size system to persist in time (to live), it must evolve in such a way that it provides easier access to the imposed currents that flow through it.'"
All Solar system periods fit the Fibonacci series and the Golden Ratio.
Answer: all the planets and the sun interact via gravity- that is, as they orbit the sun, they exert an attractive force on one another, the strength of which varies with distance. Since the distance between any two planets varies as they proceed around in their orbits, those gravitational pulls are periodic and given enough time, those orbits will shift into what are called gravitational resonances, in which the diameters of their orbits adjust into a fixed mathematical ratio. this is most apparent in the orbits of the moons of Jupiter and Saturn and much has been written about their orbital resonances.


What's the difference between bidialect and diglossia?

Question: According to what I have studied before, when a person speaks a dialect just "in the street" and uses one dialect else "in the school", he is a bi-dialectal person. Now, I have been faced with "diglossia". I searched and found that in some Arabic countries, for example, high variety of the language is used for important matters such as political events and religious discussions, and a low variety of the language is used for everyday affairs. How I can distinguish bi-dialect and diglossia?
Comment: Dear Otavio, I know what is the definition of diglossia,however, I don't know how I can distinguish it from bi dialect. In this specific question you mentioned to help me, diglossia is discussed but nothing is found for its difference with bi dialect. I look froward to hearing from you.
Answer: Diglossia typically refers to the situation you've described when the two forms are named, 'recognized' varieties. The distinction is a culturally reified one, while changing register is not usually considered to be a clear shift from e.g., 'Work English' to 'Street English'.
Comment: Sorry, would you mind  explaining what's your mean to say the distinction between diglossia and bi dialect is culturally? In both, there are two kinds of varieties, as you said "work or street English", that are used by people.
Comment: Actually, I misread part of your question, so my answer addresses a question different from the one you've asked, sorry.


How do I save my betta fish from dying because of a dirty tank?

Question: I have a betta fish and I keep him in a gallon tank with a water filter. There has been algae growing in his tank but I thought it was normal and didn't pay mind to it. Today I see the water is cloudy and found my fish laying on the bottom of the tank- almost dead. I see he is still breathing and I saw him swim up for air once. But I am worried he is going to die. I moved him to a jar temporarily with 60% clean conditioned tap water and 40% his tank water. But I know I can't keep him in the jar for long. Help! I really don't want him to die he is barely 2 years old.
Comment: Any time I have seen cloudy water , it is from over feeding.
Answer: 1) Put him in something larger than a 1 gallon. I'm sure you've heard this before, but I promise it's true.
2) Add a snail - snails eat algae and will clear up your tank fast. Only do this if you actually use a larger tank.
3) If you haven't had the water in your tank for very long, the cloudy water could just be good bacteria forming.
4) Algae can also form because of overfeeding (decomposing food). I would reduce your feeding to once a day. A betta's stomach is only about as big as it's eye, so feed accordingly.
All of the above are from personal experience.
5) Since it's just a 1 gallon tank, I would replace a standard coffee cup worth of water every week with an equal amount conditioned water.
Comment: Can you rewrite this without the questions so it's more of a complete answer than an invitation to discussion? It's a great start, but could use more detail and fewer questions.
Comment: @AllisonC done!
Answer: Keep a first aid kit. Pet stores often will not carry Betta fish medication, meaning that you will need to order it online. If you do this after your Betta fish gets sick it will likely be too late.
Prevent illness. Most Betta fish ailments are caused by improper feeding and cleaning. These will be examined in greater detail later.
Know how to recognize the first signs of illness. The most obvious way to tell that a Betta fish is sick is to observe whether or not it wants to eat. If it is not eating or appears unexcited at the sight of food, it is likely sick. Other signs of illness include a less vibrant color or strange discolorations.


If God is perfect; why was the first covenant faulty?

Question: In Hebrews 8, the author discusses the two covenants God has made. Why the need for two? Surely God knew that the first could not be kept by man. Why didn't He just start with the covenant of Christ?
Comment: Does Hebrews say that the first covenant was faulty? Christians don't say it was faulty, just that its purpose was not the salvation of the world.
Comment: "By calling this covenant "new," he has made the first one obsolete; and what is obsolete and outdated will soon disappear." (Heb 8:13)
Answer:
There is therefore now no condemnation to them which are in Christ Jesus, who walk not after the flesh, but after the Spirit. For the law of the Spirit of life in Christ Jesus hath made me free from the law of sin and death. For what the law could not do, in that it was weak through the flesh, God sending his own Son in the likeness of sinful flesh, and for sin, condemned sin in the flesh: That the righteousness of the law might be fulfilled in us, who walk not after the flesh, but after the Spirit.
For they that are after the flesh do mind the things of the flesh; but they that are after the Spirit the things of the Spirit. For to be carnally minded is death; but to be spiritually minded is life and peace. Because the carnal mind is enmity against God: for it is not subject to the law of God, neither indeed can be. So then they that are in the flesh cannot please God. - Romans 8:1-8

It is important to remember that the law was given to an already corrupted humanity.  Adam had already fallen, had already willfully chosen to usurp and take for himself God's lone prerogative in determining good from evil; for this was the temptation:

... your eyes shall be opened, and ye shall be as gods, knowing good and evil. - Genesis 3:5b

This disposition of sin is part and parcel of each individual human and humanity in general and it's wages are death.  In Adam all die.  Retaining this intrinsic disposition all die.  How to convince a people who are intrinsically predisposed to discern between good and evil for themselves that they do not actually possess such an ability or right and, therefore, cannot actually ever be righteous?
The first step in getting someone to accept a rescue is convincing them that they need rescuing.  Any addict in recovery knows this to be true.  We are addicted to our sinful disposition.  We love darkness rather than light because our deeds are evil.  Our sinful nature would have us believe that we are just fine.  The Law is, if you will, God's intervention not to rescue us itself but to demonstrate that we need rescue (unless one is bold (and foolish) enough to claim absolutely flawless obedience):

For whosoever shall keep the whole law, and yet offend in one point, he is guilty of all. - James 2:10

The law (old covenant) was given to a humanity at enmity with God and intended to drive us to that faith that admits we are sinners whose only hope is the mercy and grace of God which was ultimately and fully revealed in Christ:

Is the law then against the promises of God? God forbid: for if there had been a law given which could have given life, verily righteousness should have been by the law.
But the scripture hath concluded all under sin, that the promise by faith of Jesus Christ might be given to them that believe.  But before faith came, we were kept under the law, shut up unto the faith which should afterwards be revealed. Wherefore the law was our schoolmaster to bring us unto Christ, that we might be justified by faith. - Galatians 3:21-24

The righteousness that a person needs does not come by the law but the law came to show us that we do not have nor can we attain to the righteousness that is needed.  Once that was made clear, then Christ came to provide.
Those who still operate under the assumption that they can attain or maintain righteousness through law-keeping have no use for Christ.  They are in rejection of or blind to the purpose of the law and, until that veil is lifted, cannot be saved because they do not perceive their need:

Seeing then that we have such hope, we use great plainness of speech: And not as Moses, which put a vail over his face, that the children of Israel could not stedfastly look to the end of that which is abolished: But their minds were blinded: for until this day remaineth the same vail untaken away in the reading of the old testament; which vail is done away in Christ. But even unto this day, when Moses is read, the vail is upon their heart. Nevertheless when it shall turn to the Lord, the vail shall be taken away. - 2 Corinthians 3:12-16

Generally speaking, if the law had not come first no one would have perception of needing salvation.
Comment: Very edifying, Michael. Thank you. Up-voted +1. _Any addict in recovery knows this to be true. We are addicted to our sinful disposition._ How true that is ; I know it my own self.
Answer: God warned humankind from the beginning that for naturally created creatures to attempt to live ('life' being more than a physical, animal, existence) by his own strength, his own ability, his own mental apprehensions and, above all, by his own (supposed) inherent 'rightness' - was a fatal mistake.
Humankind having failed utterly, by their own disobedience to God's word, by the deceptive nature of a spirit other than God's spirit, and by their natural weakness, God instituted a covenant - an agreement between two parties - that highlighted the law that humankind had actually chosen to live by.
The failure of humanity was brought into sharp focus by the full revelation (at Sinai) of what the law really was and how far-reaching it was and how deeply it reached and how beyond the ability of flesh and blood it truly was.
What was the answer ?
A new birth ; a new humanity ; another covenant altogether.
These things are evident in the gospel and very evident upon a full reading of the epistle to the Hebrews.
It was impossible to 'start' with the 'covenant of Christ', sic, (the bible does not mention these precise words in this way) for created mankind is not capable of receiving it. Ye must, says Jesus of Nazareth, be born again.
There is no other way.
We must repent - first - and receive truth and believe. Only thus do we enter into a relationship with God, in Christ. And thus partake of the covenant that is between the Father and the Son.
Answer:
OP: Surely God knew that the first could not be kept by man. Why didn't He just start with the covenant of Christ?

While God knew it, man didn't.  Man had to be taught our inability to be righteous.  Still today, some believe they can successfully work to merit right standing with God.
Paul put it this way.

And be found in him, not having mine own righteousness, which is of the law, but that which is through the faith of Christ, the righteousness which is of God by faith: Phil 3:9

The Mosaic Covenant was an IF/THEN.  If you can do XYZ, then I (God) will do ABC.

And it [Mosaic Law] shall be our righteousness, if we observe to do all these commandments before the LORD our God, as he hath commanded us. Deut 6:25

The Law was to show everyone that we can't do it.  We all fail.

But the scripture hath concluded all under sin, that the promise by faith of Jesus Christ might be given to them that believe. Wherefore the law was our schoolmaster to bring us unto Christ, that we might be justified by faith. Gal 3:22, 24

So, God didn't fail.  He showed we were all sinners in need of a Saviour.
Comment: More important than all burned offerings and agreement on Last Suppers and Jewish Calendars! +1.
Answer:
For if that first covenant had been faultless, then should no place have been sought for the second.
For finding fault with them, he saith, Behold, the days come, saith the Lord, when I will make a new covenant with the house of Israel and with the house of Judah:
Not according to the covenant that I made with their fathers in the day when I took them by the hand to lead them out of the land of Egypt; because they continued not in my covenant, and I regarded them not, saith the Lord.
For this is the covenant that I will make with the house of Israel after those days, saith the Lord; I will put my laws into their mind, and write them in their hearts: and I will be to them a God, and they shall be to me a people:
And they shall not teach every man his neighbour, and every man his brother, saying, Know the Lord: for all shall know me, from the least to the greatest.
— Hebrews 8:7–11 (KJV)

The "fault" with the first covenant was that it failed.
One of the parties (the Israelites) failed to live up to their part of the agreement:

… fault with them …
… they continued not in my covenant …

The new covenant will not be one of physical obedience, but one of spiritual obedience.
People will have God's spirit to individually guide them.

… I will put my laws into their mind and write them in their hearts …
… for all shall know me …

This is clearer in the NLT version:

If the first covenant had been faultless, there would have been no need for a second covenant to replace it.
But when God found fault with the people, he said:
"The day is coming, says the LORD,
when I will make a new covenant
with the people of Israel and Judah.
This covenant will not be like the one
I made with their ancestors
when I took them by the hand
and led them out of the land of Egypt.
They did not remain faithful to my covenant,
so I turned my back on them, says the LORD.
But this is the new covenant I will make
with the people of Israel on that day, says the LORD:
I will put my laws in their minds,
and I will write them on their hearts.
I will be their God,
and they will be my people.
And they will not need to teach their neighbors,
nor will they need to teach their relatives,
saying, 'You should know the LORD.'
For everyone, from the least to the greatest,
will know me already.

The Israelites were "the chosen people", but they were chosen not as a special people that is somehow better or more favoured than everyone else.
They were chosen to set an example to the world, to show that when the leaders and the people followed God's Law, the nation prospered, and that when society turned to idolatry and other gods, the nation suffered.
(This observation was the reason that the Pharisees added to God's Laws, so that obeying their version of the law would guarantee that God's Law is also followed, and hence so that Israel would prosper.)
During this current age, only a relatively small number of people will become part of God's new covenant.
These "saints" or "elect" will become the leaders and teachers during the Millennium, when everyone will be called to receive God's salvation and to have his Law "in their minds, and [written] on their hearts".
But people must freely and individually choose this process.
Having ancient Israel as an example helps people to see and understand how essential God's laws are, and what happens when they are disobeyed.
See my (and other's) answers to:

How do Christians that believe in creatio ex nihilo answer the question of why human beings are not created with a perfect character from the outset?
What is the basis for the belief "It is possible to have free-will and be incapable of sinning, but it is not possible to be created in that state"?
Comment: " the Pharisees added to God's Laws, so that obeying their version of the law would guarantee that God's Law is also followed, and hence so that Israel would prosper." If this is true then why did Jesus condemn them so strongly for adding to the Law.
Comment: So then, they didn't add to ensure a safety net around God's Law so that it would be sure to be obeyed.  That was not their motivation ...It was a power grab.  They shut up the kingdom of heaven against men: for they neither go in themselves, neither suffer they them that are entering to go in.  At least that's what Jesus said.
Comment: This request for a mediator leads on to God promising Christ, Deut 18:15-19
Comment: @MikeBorden, they were more concerned with the letter of the law than the spirit, and because they treated infractions of *their* additions as just as, or even more, important than God's law. ¶ A modern analogy is traffic signs that define who has right of way at an intersection. A "yield" sign is like God's law, while a "stop" sign is like the Pharisees' addition. Both perform the same function. Jesus's reaction is like how you feel when you get a ticket because your wheels didn't completely stop turning, even though you waited until it was safe to go before driving into the intersection.
Comment: I'm sorry to say I think you miss the point of God's law and why he gave it.  You may hit upon a minor point, but not the most vitally important reason. -1.  When Moses led them out of Egypt and the people said "All that the Lord has spoken we will do", Exodus 19:8, the Lord was not pleased.  But when they were awakened to the vast gulf between themselves and the holiness of God and asked for a mediator God was pleased, saying "they have well said all that they have spoken"  Deut 5:25-28.  The law was given to show human _inability_ to gain blessing by obedience, and the need for a mediator.


libGDX - perspective camera and field of view

Question: I have problems to understand the field of view of the perspective camera in libGDX (or maybe my calculations are wrong).
I want to draw a box (for example 800px wide, 480px high and 20px deep). The box is placed between x- and y-axis (in other words: the width on x-axis and the height on y-axis). Now I want to stand behind the box on the x-axis (camera position) and look in a direction in such a way that I see the box on my very right screen side.
I made some sketches:

On the last sketch, the box is on the right side of the field of view.
Moreover, the height of the box must fit exactly in the screen. In other words: the top edge of the box must be on my screen top and the bottom edge of the box must be on my screen bottom.
Here is some code and calculations to achieve this:
public class Box implements ApplicationListener {

public PerspectiveCamera camera;
public float SCREEN_WIDTH;
public float SCREEN_HEIGHT;

@Override
public void create() {
    SCREEN_WIDTH = Gdx.graphics.getWidth(); // 800px
    SCREEN_HEIGHT = Gdx.graphics.getHeight(); // 480px

    // Create camera (90°) and set position
    camera = new PerspectiveCamera(90f, SCREEN_WIDTH, SCREEN_HEIGHT);
    camera.position.set(SCREEN_WIDTH + SCREEN_HEIGHT/2, SCREEN_HEIGHT/2, 0f);
    camera.lookAt(0f, SCREEN_HEIGHT/2, 0f);
    camera.near = 1;
    camera.far = 2 * SCREEN_WIDTH;
    camera.update();
}

@Override
public void render() {
    Gdx.gl.glClearColor(Color.WHITE.r, Color.WHITE.g, Color.WHITE.b, Color.WHITE.a);
    Gdx.gl.glClear(GL20.GL_COLOR_BUFFER_BIT | GL20.GL_DEPTH_BUFFER_BIT);

    // Build the box - width is screenwidth and height is screenheight - one corner in origin
    ModelBuilder modelBuilder = new ModelBuilder();
    modelBuilder.begin();
    MeshPartBuilder mpb = modelBuilder.part("ID", GL20.GL_TRIANGLES, Usage.Position | Usage.Normal,
            new Material(ColorAttribute.createDiffuse(Color.GREEN)));
    // bottom left corner of the box should be in (0,0,0)
    mpb.box(SCREEN_WIDTH/2, SCREEN_HEIGHT/2, -10f, SCREEN_WIDTH, SCREEN_HEIGHT, 20f);
    Model model = modelBuilder.end();
    ModelInstance instance = new ModelInstance(model);

    // Build some light
    Environment environment = new Environment();
    environment.set(new ColorAttribute(ColorAttribute.AmbientLight, 0.4f, 0.4f, 0.4f, 1f));
    environment.add(new DirectionalLight().set(0.8f, 0.8f, 0.8f, -1f, -0.8f, -0.2f));

    // Draw the box
    ModelBatch modelBatch = new ModelBatch();
    modelBatch.begin(camera);
    modelBatch.render(instance, environment);
    modelBatch.end();
}
}

To fit the box height in the screen I must step back SCREEN_HEIGHT/2, because x = SCREEN_HEIGHT/2 / tan(45°) = SCREEN_HEIGHT/2 / 1 = SCREEN_HEIGHT/2, that's why I added this length to the x-component in the position setting.
The code represents sketch 2 and give me that correct screenshot:

In order to achieve sketch 3, I must rotate my field of view about 45° to the left. But in this connection I must step back a little, because the sides of the field of view are longer than the centerline. I made two hand-drawn sketches:

Where h = SCREEN_HEIGHT. First of all, I have to compute x. x = h/2 / sin(45°) = h/sqrt(2). Now I must step back a length of x and look in a new direction (45° left from box). To calculate the point at which I will looking, I calculate the length y on this sketch:

y = sin(45°) * h/2 = h * sqrt(2)/4. So, the point is for example (SCREEN_WIDTH + (x-y), SCREEN_HEIGHT/2, y). Now I change the position and looking point in my code:
public void create() {
    SCREEN_WIDTH = Gdx.graphics.getWidth(); // 800px
    SCREEN_HEIGHT = Gdx.graphics.getHeight(); // 480px

    // Create camera (90°) and set position
    camera = new PerspectiveCamera(90f, SCREEN_WIDTH, SCREEN_HEIGHT);
    float x = (float) (SCREEN_HEIGHT / Math.sqrt(2));
    float y = (float) (SCREEN_HEIGHT * Math.sqrt(2)/4f);
    camera.position.set(SCREEN_WIDTH + x, SCREEN_HEIGHT/2, 0f);
    camera.lookAt(SCREEN_WIDTH + x - y, SCREEN_HEIGHT/2, y);
    camera.near = 1;
    camera.far = 2 * SCREEN_WIDTH;
    camera.update();
}

And I get this screen:

But the box isn't on the very right side :(
So now the question: what is wrong? Is my thinking of the field of view wrong or my calculations?
Best regards!
Answer: Your understanding of the PerspectiveCamera is a bit off, specifically what it does with the second(viewportWidth) and the third(viewportHeight) argument.
The only thing the PerspectiveCamera does with them is get the aspect ratio and use it to set the projectionMatrix. For the specific code it uses see this line that calculates the aspect ratio and this method that sets the projectionMatrix.
Now this is all well and good, except that you also do the math to deal with the aspect ratio, and doing it twice causes the camera to stretch again. The solution is quite simple, either redo your math to not adjust for the aspect ratio or replace
camera = new PerspectiveCamera(90f, SCREEN_WIDTH, SCREEN_HEIGHT);

with
camera = new PerspectiveCamera(90f, 1, 1);

to make the aspect ratio that libgdx calculates be 1.
I'm not so good with sine and cosine so I can't provide solutions to the first option, but you don't seem to be having any problems with difficult math.


Finished studying and just waiting for graduation what should i put on my resume?

Question: I studied BS Computer Science in a trimester college school and i already finish all of my subjects this sem (September 2 2016) my problem is my graduation is next year April 2017 but i already want to apply for a job my question is what should i put on my resume to explain i finish my study but because of our school system i still haven't graduated? And Is finishing all subjects but not graduated yet is common?
Comment: "Completed September 2016, graduating April 2017"
Comment: @gnat I think it's different problem because i already finished all my subjects and i am no longer studying i am just waiting for my graduation.
Comment: You are a "graduand" if that helps...
Answer: Don't overthink this. Something like "September 2016 - BS Computer Science diploma (graduation : 04/17)" will be perfectly fine. Though it's not the most common it's definitely not unheard of, and it's unlikely to confuse the HR guy who will read your resume.
Answer: Ero's is a good answer

"September 2016 - BS Computer Science diploma (graduation : 04/17)"

Since it's somewhat unusual, you could add some more detail

Have you met ALL graduation requirements already? Is there any possibility you won't graduate?
Are you available for full time work right now? Is there anything left at school that you need to wrap up or that you need to take time off for?
Are you grades already set in stone? Can anything happen that impacts your final grades?
Answer: I have done this a few times, I have not yet finished by BA and have it as:

Coolguy University - BA/BS In Cool Guy-ism - Project Grad Date: June 2017

You can add an extra modifying blurb saying you Completed Studies/Study Completed, or whatever wording you prefer, and give that date. The schooling experience is what is more or less what is important.
Just be prepared to backup those claims in case an HR Manager happens to ask, that may or may not vary by Country and/or internal Company HR Policies/Directives.


Simple logistic regression

Question: I tried to solve a simple linear regression problem of two-dimensional classification.
Here I have a feature matrix X_expanded, which has a shape of [826,6]. To classify objects we will obtain probability of object belongs to class '1'. To predict probability we will use output of linear model and logistic function.
And here is the function of compute probability.
def probability(X, w):
    """
    Given input features and weights
    return predicted probabilities of y==1 given x, P(y=1|x), see description above

    Don't forget to use expand(X) function (where necessary) in this and subsequent functions.

    :param X: feature matrix X of shape [n_samples,6] (expanded)
    :param w: weight vector w of shape [6] for each of the expanded features
    :returns: an array of predicted probabilities in [0,1] interval.
    """

    # TODO:<your code here>
    X = X_expanded
    m = X.shape[0]
    w = np.zeros((m,1))
    Z = np.dot(w.T,X)
    P = 1./(1+np.exp(-Z))

    return P

For a simple test:
dummy_weights = np.linspace(-1, 1, 6)
ans_part1 = probability(X_expanded[:1, :], dummy_weights)[0]

But it always returns array([ 0.5,  0.5,  0.5,  0.5,  0.5,  0.5]).
Any suggestion?
Comment: w = np.zeros((m,1)) returns array of zeros, Z = np.dot(w.T,X) will always return 0.So P = 1./(1+np.exp(0)) will be always 0.5.
Comment: So how should I initialize the weights vector?
Comment: np.random.rand(m, 1)
Answer: Since you've initialized your weights as zeros, Z = np.dot(w.T,X) will be 0 and the sigmoid function would return 0.5 always. You need a random initialization of weights. It can be done by :
dummy_weights = np.random.rand(m, 1)

Simple logistic regression


About capacitor

Question: If i have 20uF capacitor with charging 3V .
Now i m gonna attach 1 ohm resistor and  2 V led in series with capacitor .
Now current will flow through the led is 1Amp max For a while , but if 1 Amp current will flow through the led it must be burn out theoretically so why it will not burn out practically ?
Comment: can you please add a circuit diagram to your question? There's this handy "add circuit diagram" button...
Answer: You're assuming a simplified model of the forward biased diode (constant Vf forward voltage in series with constant "on resistance"). This model leads you to believe that the initial current would be (3V - 2V) / 1ohm = 1Amp. Your maths are correct but the simplified diode model is giving you an incorrect result.
A general procedure for estimating how a source and a load will interact, is to plot a graph of the source's V/I characteristics and the load's V/I characteristics, and see where the two plots intersect.
A real diode follows an exponential curve. Check the datasheet for your "2V LED" (which I assume must be a green LED). Most likely the datasheet specifies Vf=2V at a current of maybe 10mA or 20mA. From this Vf,If point you can determine the exponential curve values. (This is assuming the Shockley diode equation and a constant temperature.)
Draw an exponential curve through that given Vf,If point, and you have the characteristic V/I curve of that diode as a load.
Next, draw another line on the graph, representing the source characteristic of the 3V source. You're only asking about what happens initially, so for now let's not worry about how fast the capacitor discharges. The source V/I curve for a 3V source in series with 1 ohm, is a straight line that passes through (3V,0A) and also passes through (0V, 3A). In other words, if you shorted out the 3V capacitor through 1ohm without the load, then the initial short-circuit current would be 3V/1ohm = 3Amp.
The place where the diode's V/I curve intersects the 3V source's V/I curve, is the point that represents the initial voltage and current that will flow when the 3V 1ohm source is connected to that diode.
Answer: You may not know this but a 5mm LED rated for 75mW has an internal ESR of about 15 Ω so adding 1 Ω is not significant. ( but at 3V ESR is probably closer to 10 Ω then rises above 20 Ω below 1.9V )

Recalulating without 1 OHm

Assuming Vf=2.1 (Red)
Vi= 3V on 20uF cap
Imax= (3-2.1V)/15Ω =60mA

calculating light pulse Decay time to 1.8V (dim)

If T = 20uF * 15 Ω = 300 µs  decay time to < 40% of 3V
even though LED turns off below 1.8V
this will not be visible

Try again
Answer: As a minor addition to Tony's answer. The energy stored in the capacitor is probably also not sufficient to generate enough heat to destroy the diode.
The energy stored in the electric field of the capacitor is:
W = 1/2 * C * V²
W = 1/2 * 20uF * 3V² = 0,0009J
That's not much, but in the end it also matters how fast this energy is discharged.
Answer: A current of 1 A will not directly destroy your LED attached to the capacitor. But it is still recommended to stay around 20 mA (if it is a red one of 5 mm).
It is not that because it works for a while that it will keep working.
Besides, there is no interest to keep a LED at 1 A. The luminous intensity (cd) does not upgrade as much as from 0 to 20 mA.
Comment: I'm pretty positive that 1A for **seconds** through a typical 20mA diode will destroy it. However, OP didn't mention 20mA, so this is all guesswork, isn't it?
Comment: You are right for the first part (I changed my post) but not second part, it isn't guesswork. The capacitor is in serial with the resistance. And from what I did understand from OP post (lack of precise information) there is also charger (battery) of 3V in serial. The fact that these are in serial will make the led dim and then turn off. Because the current stops when the capacitor gets charged up to the battery voltage (3V). His circuit is probably a simple current detector. The LED starts at 1 amp but will go to 0 very fast. This, in my experience, does not destroy the LED.

About capacitor


Would an interior inflation affect the size of the event horizon of a black hole?

Question: Lets imagine that the interior of black holes are not singular, and they undergo an inflation.
I know no information can escape a black hole, but the event horizon can grow in size, for instance, by accretion.
Could this inflation inside make the event horizon increase in size?
Comment: When you say "interior of black holes" are you referring to the inside of the event horizon? Because that region is not singular.
Comment: I was talking about just GR, in which they are singular.
Answer: No, it could not.
The horizon, is not in the casual future of any point in the black hole interior (by definition). Consequently, nothing that happens in the interior can affect the horizon.
Comment: What if the interior inflates?
Comment: @Manuel Doesn't matter, it still would not be able to affect the horizon.
Answer: The case you are referring to in a way is similar to the formulation of Wheeler's bags of gold paradox. I will discuss two issues here and clarify why such situations don't influence the horizon.
Case 1: Consider a spacelike slicing of an eternal black hole (you can consider single sided as well, eternal in AdS are easy to visualize and have nice properties) which end at Kruskal time $(u_L, 0)$ and on $(0, v_R)$ on the horizons. This slice stays away from the singularity at all points, and does not have large curvature invariants. Let us consider "maximal volume slices" in the interior of the black hole, i.e. you impose that the volume of such slices is maximized. This calculation is performed in Appendix A of this paper and the expression for the volume grows as:
$$V \propto \frac{\beta}{2\pi} \log{(u_Lv_R)}.$$
This is a slicing whose Kruskal time evolved volume keeps increasing in the interior, and can be thus thought of as an inflating spacetime. Note that there is absolutely no reason here why the horizon should change, because you are considering different spacelike slicings of the eternal spacetime.
Case 2: Consider an inflating spacetime which is directly glued to the black hole interior using Israel junction conditions. This is nicely discussed in this paper and leads to the bags of gold problem. Here since the interior region by construction itself undergoes inflation, the volume of the spacelike slicings going inside the interior increase (analogous to but not the same as in Case 1). Again since you have imposed junction conditions properly in the interior, there is no reason for the horizon to increase.
The basic reason for why the interior can undergo inflationary behaviour while the horizon remains unchanged is because any changes in the interior won't be able to influence the horizon via causality. This is obeyed in both Case 1 and 2, where any sources or excitations in the interior on these slices lead to bags of gold like scenario, but their effect is restricted to the interior itself as they cannot causally propagate to influence the horizon.
Aside: Note that even in the formulation of the bags of gold paradox, the coarse grained Bekenstein Hawking entropy remains the same even if slices in the interior on which the excitations live grow with time. This is again due to the fact that excitations in the interior cannot causally influence the future horizon(s).


Are requests for problems of a certain type on-topic

Question: Is it on-topic to ask for problems concerning a certain subject matter?  For example, could I ask for something like "counting problems involving the Euler totient function," or "problems involving the Vandermonde determinant"?
Comment: You might browse the tag ([tag:big-list]) (and perhaps you might add some keywords like [problems](http://math.stackexchange.com/search?q=%5Bbig-list%5D+problems) or [exercises](http://math.stackexchange.com/search?q=%5Bbig-list%5D+exercises). I guess you can find a few similar questions in this way. You will see how they were received.
Answer: Historically this is not how the site has been used, but it is an interesting idea that would be useful to a lot of people.
A major challenge in self-learning of mathematics is finding good problems to work on. Even when taking classes, sometimes the problems assigned are not what the student needs to understand the material. Some problems are more effective than others at promoting learning, and it is hard to know which ones are the best a priori. One can always pick up several books and work all the problems, but this takes so much time that realistically it is rarely done. This is especially true of self-learners who are not full-time students and have limited time to study.
Also, there are many places throughout the world where getting access to high quality books with problems is not so easy. While there is a great deal of material online now that explains mathematical concepts (e.g., wikipedia articles, recorded videos of lectures on youtube, course notes pdfs on university webpages, etc), there is a surprising lack of coherent well-thought-out problems sets online for many subjects.
There would definitely be value in having an online website where people can pose problems that can be upvoted and downvoted based on their usefulness to someone at a given stage of learning. Whether or not math.stackexchange is the right site is not clear. For this reason I think such questions should be tentatively allowed.
Answer: In principle, yes, however a specific instance could easily be "too broad."
If you ask such a question, make sure to be as specific and detailed as possible to explain what you are looking for precisely. In particular, it can be helpful to explain why you are asking something.


Console command for completing Valuable minerals quest

Question: In Mass Effect is there a console command to add minerals to complete the Valuable Minerals quest?
Comment: well if you are going to cheat, why not just edit your minerals in me2, since completing that quest in a carry over just gives you some mineral bonus in me2
Answer: No, there isn't a console command to complete this quest. However, here is a full guide with the specific locations of each of the minerals.
The alternative is to use a saved game editor for Mass Effect 2 to edit the number of each mineral that you have.
If the purpose of completing this question is to maximise the experience you gain in a single playthrough, you can use the GiveXP x command (for example: GiveXP 100 to add 100 experience) in the conosole.