Why is the second kaf hard in שעשה לי כל צרכי?

Question: In the daily berakhah for having shoes, שעשה לי כל צרכי, why is the letter kaf pointed with a dagesh kal? I.e., why is the word pronounced tzorki rather than tzorkhi? My sense is that similar forms of the word are pronounced with a soft khaf, e.g., צרכי ציבור.
Comment: You can shift the question onto Tana"ch, if the _dagesh_ was fossilized from Divrei Hayamim (cf. #5 [here](http://hebrewbooks.org/pdfpager.aspx?req=20312&st=&pgnum=111)).
Comment: כל האומר דבר בשם אומרו מביא גאולה לעולם
Comment: The credit for my awareness of that _sidur_ belongs to @DoubleAA [linking it in an answer](http://judaism.stackexchange.com/a/47618/3).
Comment: What is the source of vocalization of צרכי ציבור?
Comment: Thanks for that source, @WAF. It has a full answer to my question, I think.
Comment: @Argon, I guess, for me, the source would be the mi she-berakh after yekum purkan. also, probably, hearing that phrase in Israeli Hebrew. It's also in a mishnah in Avot, but of course the mishnah doesn't come with nikkud.
Comment: I can find it in the siddur but not in the Mishnah. Do you know where I could find it there?
Comment: @Argon, sorry, I made a mistake about that (the phrase there is כל העמלים עם הצבור).
Comment: Looks like there's a Tosefta that mentions the phrase, in Brakhot chapter 1: אמר ר' יהודה פעם אחת הייתי מהלך אחר ר' עקיבא ואחר ר' אלעזר בן עזריה והגיע זמן קרית שמע, כמדומה אני שנתיאשו מלקרות מפני שהן עסוקין בצורכי ציבור, קריתי ושניתי, ואחר כך התחילו הן וכבר נראית חמה על ראש ההרים
Comment: some old versions of the blessing are in plural: kol tzerachai.
Comment: @DoubleAA, yes, I think that might have been very common. I think that's also common in today's eidot ha-mizrah versions.
Comment: @Argon, I came across another source for vocalization of צרכי-- the very short version of prayer, to be recited in emergencies, צרכי עמך ישראל מרובים ודעתם קצרה -- which is provided vocalized is some siddurim.
Comment: A manuscript of  וארץ איכף לאלוהיי ואתחנן אשיר עוזו רגעיי וארנן written before 1089 has the line: "צָרְכֵי הַזֶּבַח כְּדַת הִשְׁלִים וּמִלָּא" and "צָרְכִּי יוֹם חֲסַרְתִּיו וְחֹק טֶרֶף מְזוֹנַי".
Comment: I'm not sure that צרכי ציבור is a valid proof. One is a first person possessive object ("my needs"), with a chirik under the kaf, and one is a plural possessive subject ("needs of"), with a tzeirei under the kaf. Perhaps that alone explains the difference. Likewise for צרכי עמך ישראל - it's of the second category.
Comment: Pretty sure that the policy is that questions about Tanach Hebrew are on-topic, contrary to close voters' belief.
Answer: The vowel symbol kametz is a single symbol which covers two sounds, namely the kametz gadol (which is the usual kametz, in most places we encounter is) and the kametz katon, which is a cholam which is reduced to a kametz katon when the stress shifts to later in the word. The kametz gadol is a long vowel (tenuah gedolah) and the kametz katon is a short vowel (tenuah katanah).
In general, the rule is that a long vowel appears in an open syllable (that is, consonant vowel) and a short vowel appears in a closed syllable (consonant vowel consonant). A sheva nach will close a syllable, and the letters bgdkft, after such a sheva, will be the plosive (hard) kind, with a dagesh, rather that the fricative (soft) kind, without the dagesh.
So, for instance, in the word
כָּתְבוּ
the kametz under the kaf is a kametz gadol, a long vowel, so the sheva under the thav is a sheva na (moving sheva), and the bhet is fricative. This is the usual case, which you would expect.
Certain times, the kametz is the kametz katon. For example, in chochma, or ozni. Many cases we know it to be a kametz katon on the basis of the full form, which has a cholam. In such cases of a short vowel, the syllable needs to be closed, and so the sheva is a sheva nach, and so the bgdkft letter afterwards has a dagesh.
This does not answer your question about tzorchei tzibbur. Assuming the spelling is indeed with a kametz katon vs. a cholam, I would suppose that this is a result of the weirdness of sheva merachef, which is inconsistent. (See here, about birkat vs. birchat, after the short vowel of chirik chaser.)
Comment: A kamatz gadol is NOT a cholam. It is a reduced vowel which *originated* in a cholam **before** reduction. This resulting reduced vowel is a short vowel. See e.g. here where a tzeirei (a long vowel) is "reduced" to a sheva, or depending on context, sometimes segol or chirik, as short vowels.  https://books.google.com/books?id=OOkNAAAAYAAJ&lpg=PA92&ots=1DBtwKGfYp&dq=tzere%20segol%20reduction%20hebrew&pg=PA92#v=onepage&q&f=false
Comment: The cholam is always a tenua gedola,  and the Kamats katan is a Tenua ketana.  I am confused
Comment: what is the confusion? Indeed, the cholam is always a tenua gedolah. But the kamatz katan is always a tenua ketana, as opposed to the kamatz gadol, which is always a tenua gedola. Did I say otherwise?
Comment: I don't understand how it can be both holam and tenua ketana
Comment: OK thanks. BTW I asked myself if in ashkenazic keria we have a specific sound for kamats katan
Comment: I'm not sure, but I tend to pronounce it as midway between a cholam (as in the word "road") and kametz gadol (as in "bud"). It sounds kind of like the non-diphthong 'o' in Tajiki.
Answer: The word is "tzarki" rather than "tzarchi" because the kaf begins a syllable. And the rule is that the "beged kefes" letters (beis, gimmel, dalet, kaf, pey and sav) have a dagesh when they begin a syllable. It seems to me that this rule is widely known, but in my 15 minute search in a beis medrash, I could find no great source. Here's what I did find. (1) ArtScroll's publication of Rashi's commentary on the Chumash, at Numbers 7:89, footnote 10. The author of that footnote gave no source. (2) A Wikipedia article to be found here: https://en.wikipedia.org/wiki/Dagesh
Comment: Hi Chaim. What about the Kaf in Tzorchei Tzibbur? Doesn't that also begin a syllable?
Comment: Double AA, responding to your last question. Words of that form - like tzorech, qodesh, melech - have a unique plural form i.e., tzorachim, not tzorkim; melachim, not malkim. (Note that it's actually malKin in Aramaic, but not Hebrew.) In the semichut form, the /a/ phoneme gets reduced. However, the underlying vowel phoneme is still there and hence it's pronounced rafe (since beged kefet consonants are rafe after a vowel). Similarly, the masculine imperative form for "write" is /kotob/ [kethov]. When an object is added to the word, it becomes kotvenu, not kotbenu, since the vowel phoneme is st
Answer: When I got home I looked in the books I've got there. A Grammar For Biblical Hebrew by C.L. Seow contains the following pages 4 to 5. My understanding of his analysis is that in the word Tzarki, the Kaf is medial and preceded by a consonant (that is, a Reish with a Sheva). Therefore, following rule 4.b.iii, the Kaf is a stop; and then following rule 5, it takes a Dagesh.

Introduction to Hebrew by Moshe Greenberg contains the page 13 attached. It comes out pretty much the same.

I also found instructions at the front of a Tikkun that were bewildering in their complexity, so I will not even try to summarize or apply.
I am not too familiar with the subject; I just happened to find a few sources. I don't know if complete explanation of the Dagesh is really possible, and I had not thought about Tzarchei Tzibor last night when looking at the books, only this morning when you posed the question. Just off the cuff I would say that perhaps the change in the vowel FOLLOWING the Kaf is relevant; and perhaps there's no strong explanation.
In Exodus 15:11, the song Mi Chamocha... Mi Kamocha, the first Kaf has no Dagesh, and the second has a Dagesh, although they appear in exactly the same phrase. I really don't know if that's explicable, 'tho I see now that there's a Mi Yodeya question on this last point from 2014.
-Chaim
Comment: Chaim, the question already mentioned צרכי ציבור. He's asking why they are different. This wasn't just my addition.
Comment: You're right. I overlooked the last two words of the original post. Greenberg's last complete paragraph seems to mean that Kaf should have a Dagesh if it follows a consonant; or if the Kaf has a mobile Sheva and follows a vowel; or if the Kaf is Long and follows a vowel. Otherwise (if it is a short Kaf with no mobile Sheva following a vowel) is should have no Dagesh. So he seemingly would put the Dagesh in the Kaf each time.
Comment: @Chaim, thank you for the answer and for the scans. It does look like that's the rule, and the form tzorkhi is to expected.


Bad pulley or Belt?

Question: Okay so I had my car break down about a week ago and from looking at the car myself and using my common sense, after Turing the car on for about 8 seconds all I heard was squealing and smoke coming from the belt, I know it's from the belt because after touching everything after those 8 seconds the belt was the only thing that was able to burn me, what would you guys recommend me do at this point to further investigate or help me narrow down the problem. Also not sure if it had to do with whatever is burning the belt but after the break down a decent amount of oil from a what looks like a bolt on the side of the engine and also around the injectors. Let me know what you would do thank you!!  https://ibb.co/cDNQFLS https://ibb.co/WfXr7g1
Comment: My first check would be to make sure all the pulleys are turning as @SolarMike says.
Comment: Has the idler or a/c pulley seized?
Comment: The oil seems to be unrelated to the main issue here, I would first check the belt tension, to make sure it has the correct amount of slack, if it does, I would remove the belt and check all the pulleys if they turn freely as the guys above suggested. At any rate, if the belt tensioner was lose it wouldn't produce smoke and that much heat, they usually squik for a while and that's all.
Comment: So I took a deeper look and moved some things out the way to look at the belt and the pulleys, I was easily able to get the belt off till it was the alternators turn, the belt was melted onto the alternator, and the pulley was the one that couldn't spin almost at all took a lot of force, am I able to change the pulley or do you guys think I'm going to have to change the alternator?
Answer: From your comment, it looks like the alternator has seized.

I was easily able to get the belt off till it was the alternators
  turn, the belt was melted onto the alternator, and the pulley was the
  one that couldn't spin

Unfortunately you will need to either get the alternator reconditioned or get a new one fitted.
Answer:
Take the belt off, noting every pulley it turns (aside from the crank pulley).
Try to turn every pulley you noted in step 1 (including the tensionor).
Any device with a pulley you cannot turn by hand, should be inspected.

Many air conditioner compressors have a clutch that takes the load off the engine when not in use.  So with the A/C off (like when the engine is not running), there will be very little resistance.  When you ran the engine for 8 seconds, was the A/C on?
Many shops can test your alternator, and some will do so for free.
Answer: From your description of the alternator, I would suspect the rotor has been in contact with the stator and it is well-past reconditioning.
Well, if you have access to new stator, new rotor and new bearings it is possible, but at that point a replacement unit that comes with a guarantee is a better option.

Bad pulley or Belt?


Where to get description of SE webites?

Question: I see a lot of website down in the footer from the Stack Exchange group. Where can I get a brief description of each of them rather than going on each website and look what it talks about.
Answer: You can see the full list of Stack Exchange sites, which displays the short site description at a glance when in "list" view.
Answer: You can also click on the "stack exchange" icon on the upper left corner and click on the down faced arrow. You would get a list of all the sites along with short descriptions in the "all sites" tab.

Where to get description of SE webites?


General approach to randomized algorithm for equality problem

Question: Using randomized approach we can guarantee that the equality problem has O(1) complexity (in communication).
With other definitions of of equality (not strictly equal), is there a general approach to design a randomized algorithm with bounded error? Or how to determine if there is no way such an algorithm can be designed?
Comment: Please do not delete your question once someone has answered it. If you think the answer fully addresses your question, please mark it as accepted by clicking the check mark next to it. If you think the answer is helpful, please upvote it by clicking the `^` button on the left of the answer.
Comment: You ask for the case of 1-bit difference, but I suspect once we answer this you will ask about some other case. Please fix your question in advance and stick to it.
Answer: Suppose that the inputs are $x,y \in \{0,1\}^n$.
Consider the following protocol:

The parties decide on a random string $z \in \{1,2\}^n$.
Alice sends Bob $\alpha = \sum_{i=1}^n x_i z_i \pmod{3}$, and Bob sends Alice $\beta = \sum_{i=1}^n y_i z_i \pmod{3}$.
The parties compute $\gamma = \alpha - \beta \pmod{3}$.

Suppose that the Hamming distance between $x$ and $y$ is $d$. Then $\gamma$ is the sum of $d$ random values in $\{1,2\}$. Using linear algebra, we can calculate
$$ \Pr[\gamma = 0] = \frac{1}{3} + \frac{2}{3} \left(-\frac{1}{2}\right)^d.$$
(You can also prove this formula by induction.)
In particular, when $d=0$ the probability is $1$, when $d=1$ the probability is $0$, and otherwise the probability is in the range $[1/4,1/2]$ (corresponding to $n=3$ and $n=2$, respectively).
The suggests a protocol of the following form:

The parties decide on $N$ random strings $z_1,\ldots,z_N \in \{1,2\}^n$.
Alice sends Bob $\alpha_j = \sum_{i=1}^n x_i z_{j,i} \pmod{3}$ and Bob sends Alice $\beta_j = \sum_{i=1}^n y_i z_{j,i} \pmod{3}$, for $1 \leq j \leq N$.
The parties compute $\gamma_j = \alpha_j - \beta_j \pmod{3}$ for $1 \leq j \leq N$.
If all $\gamma_j$ are zero or all are non-zero, output "Hamming distance is probably at most 1", otherwise output "Hamming distance is definitely more than 1".

As in the case of equality, this protocol has only one-sided error: whenever the algorithm outputs "Hamming distance is more than 1", it is always correct. It remains to determine the connection between $N$ and the error probability $\epsilon$. Let $p = \Pr[\gamma = 0]$. If $p \neq 0,1$, the probability that the algorithm outputs the incorrect answer is
$$
p^N + (1-p)^N \leq (1/2)^N + (3/4)^N.
$$
This shows that $\epsilon$ is exponentially small in $N$, and so in order to guarantee an error probability of $\epsilon > 0$, it suffices to take $N = O(\log(1/\epsilon))$.


Who should log bugs?

Question:
In an agile environment where project priorities constantly change meeting deliverables, how healthy is the situation where bugs are logged by project members other than the actual qa member who is the dedicated personnel for quality? especially where no communication is done with the qa personnel on this matter?
Does this interfere with the test plan of the quality analyst? Especially if the logging method does not coincide with qa method?
Does this situation increase the time invested to fix them? Especially if they are not well described with steps to reproduce?
Does it decrease the qa personnel's learning curve and morale on the long run? Especially if the project involves complex scenarios that need analysis and on the spot decision making?
Does it impede qa's test design strategies to consider unexpected behaviours for the next run?
Comment: This is quite the loaded question/rant. Perhaps consider splitting each one, organizing your thoughts, and coming back with something more "answerable". Some helpful reference: http://sqa.stackexchange.com/help/dont-ask
Comment: Doesn't sound like an agile environment, just chaos. Do you think you are agile?
Comment: I second Meesh proposal to split this into separate questions.
Comment: I definitely agree - this should be multiple questions, and each one should be expanded more to describe what the business problem actually is.
Comment: I think that the partitioned question has been answered already. My question rorated around the who. Now it would be interesting to elaborate more on how to manage an environment where bugs are logged by non QA members and find a solution of how to not interfere with the QA personnel's test plan, in the specific case where logged bugs do not follow any agreed methodology.
Comment: +1 for the splitting. I would be glad to answer some parts of it with examples. But these 5 is too much for me, sorry :)
Answer: I tend to prefer allowing anyone in any capacity to log defects on any team I am a part of.  It helps build a sense of ownership of quality in the entire team, which is as it should be, every team member regardless of role should equally own and care about quality.  It is a different story when people outside of the project team are entering defects, I would draw the line there since the quality level of those defects could potentially be much lower and you can end up spending way too much time trying to triage and chasing down issues that don't really exist.  There should be another way for other people - including customers and people on other teams who have dependencies or are dependent on your application - to provide feedback though there are a lot of options on how you handle that feedback and would probably be an entire other question/answer.
To keep things from getting out of control, and from losing sight of defects coming in, I typically dedicate some time every single day to defect triage.  Often, I set it up directly after stand up, so stand up finishes and we transition into defect triage which often has a few less people involved, but allows myself and the rest of the team to keep on top of defects.  Any team member who entered a defect should be required to stick around for the triage and speak to the defect that they entered and answer questions.  If a defect is missing details, or is a duplicate of an existing one, or is a misunderstanding and should be deleted, or is really a new story/feature request, we figure it out first thing in the morning.  We can also make a decision immediately about whether it is related to current story work and we need to address it in the current sprint, or if we should prioritize it in the backlog.
I am confused at how other people entering defects could/would impact the QA team's learning curve or morale?  If the QA team members are territorial and feel like they are the sole owners of quality and defects and anything related to that, then perhaps that would be the case, but I feel like you would have even larger problems if that were the case, because quality truly should be a team effort.
I'm also not sure what you mean by your question about impeding QA's test design strategies.  How would learning about unexpected behaviors in your application do anything but help your team understand how the app works, and build even better, more robust test cases around those scenarios?  The more knowledge you have about the application you are testing - especially from exploratory testing and feedback from other team members - the better equipped you will be to test it.
Comment: Whenever a valid defect is filed, regardless of who entered it, the QA team should ensure that there is a test case to cover that scenario in the regression test suite.  This allows you to "learn" from any defect entered.  This is also the first mention of test cases.  I did not say that other people should be executing test cases, they would likely be finding these defects through their own exploratory testing and QA should still run through their entire suite of test cases.
Comment: Thank you for the answer, it provides a good insight on how to handle the situation as a product owner or project manager. My question did not clarify how a qa member can handle this case, so now I have widened it to the qa member itself. By 'decreasing the learning curve' I mean decreasing the chances that a qa member can learn from mistakes he makes or learn about cases he did not include in his test cases, if bugs are logged by another member not part of the qa team. Also, as qa team runs test cases and closes them, it would interfere with the end report if they are not communicated with qa
Answer: Some thoughts here:

how healthy is the situation where bugs are logged by project members
  other than the actual qa member who is the dedicated personnel for
  quality?

This is normal - anyone can log a bug. The team decides which bugs are in scope for the project and which need to go into the main backlog. The team decides which bugs get fixed when they're planning the next iteration/sprint/cycle.

especially where no communication is done with the qa personnel on
  this matter?

I'm not entirely sure what you mean here - if you mean that the bug gets logged without the testers confirming that it's a bug, that's normal. If you mean that the testers aren't in communication with the development team, whatever your company is doing it isn't agile (there's a lot of that - organizations adopt some of the forms of agile development but turn it into a kind of mini-waterfall or worse).

Does this interfere with the test plan of the quality analyst?

In a word, no. A good tester is constantly monitoring what's happening in the project anyway and adding notes to the test planning to ensure that bugs and potentially problematic interactions get covered at some point. It doesn't matter where the information comes from, and since in agile development all team members are in constant communication, the information is always available.

Especially if the logging method does not coincide with qa method?

I'm not sure what you mean here - do you not have a means of tracking reported bugs that's available to people other than the test team?

Does this situation increase the time invested to fix them?

Not if you have a company-wide tracking system that explicitly asks people for the information testers and developers need to fix the bugs. What increases the time invested for a fix is a bug report that doesn't have enough information to identify the problem. "It's broken" is useless. "I get this error (with screenshot or stack dump) when I do this (with description of the actions)" is a lot easier to identify and fix.

Especially if they are not well described with steps to reproduce?

That does increase identification and fix time - this is why a tracking system that guides people to give the information testers and developers need is a good idea. At my last position I worked with a home-grown system that had these prompts (paraphrased because I haven't been there in over a year) in the problem description section:

What actually happened?
What did you expect to happen?
Why did you expect this to happen?
Can you reproduce the problem?
Describe how you reproduce the problem?

Does it decrease the qa personnel's learning curve and morale on the
  long run?

No. A good test team is checking reported bugs anyway and learning from them that way. It doesn't matter who reports the problem: the goal is to produce the highest quality software possible (for whichever definition of quality your organization uses) with the time and resources available to you.

Especially if the project involves complex scenarios that need
  analysis and on the spot decision making?

There are projects that don't? Seriously, anything non-trivial is likely to involve complex scenarios and so forth. As long as everyone in the project team is communicating - and the team itself is communicating with the wider development, test, and project management teams - it doesn't matter who reports a problem. What matters is triaging it and deciding whether it needs to be fixed for the next sprint.

Does it impede qa's test design strategies to consider unexpected
  behaviours for the next run?

It doesn't impede anything. It can impact strategies, but it doesn't impede them. A good tester has already checked the bug reports before the next round of regression or the next sprint and knows to look for the reported problem.
I'd suggest that it sounds awfully like you don't have anything remotely agile going on here: instead your questions imply a completely isolated test team, no consistent issue reporting system, and functionally ad-hoc development - if this is the case, the first thing you want to do is to try to get more communication going, then you want to try to get one issue tracking system that everyone uses.
Comment: Thank you very much for the elaborative answer. I came up with another thought. Should quality assurance members insist to follow clearly defined methodologies or should they adapt and try to fit within any methodology? As all answers clarify that quality should be the concern of all, should methodologies be also of concern?
Comment: @user5529 - my view is they should adapt and fit with any methodology. The end goal is the highest quality software the team can produce with the resources and time they have. It doesn't matter how they get to that goal - although it tends to work better if everyone on the team is using the same methodology.
Answer: Anyone who has access to the application should be able to file bugs/defects.

Defects should be filed in a bug tracker with a status new.
New defects should be verified and extended (With at-least a reproduction path)
Defect should be added at the top of the backlog (Keep a no defect policy when being Agile)

Products Owners should monitor new incoming defects on their backlog and prioritize or reject as needed.
During the plannings sessions (This meeting include the whole team, including the testers of the team!) all tasks should be clarified. So everyone should be aware and understand the work for the upcoming sprint.
In a active Sprint "blocking" issues can be fixed adhoc, but should be put on the sprint board and the bug tracker if needed. The tasks should follow the normal processes including testing of-course.
Test scenarios should be added and run during the cycle of a task and be part of the definition of done, preferably they are automated to keep the cycle as short as possible.
Agile environments should include everyone in all the meetings and the work should be transparent and visible. I do not understand how you could miss anything :) unless your not part of the team. Separate QA teams is NOT agile.
The QA members of an Agile team should coach on better quality practices and identify room for improvement here, they should be the goto guy not "the you do all the testing" guy. Often the ratio testers vs developers is very low and thus you need to teach developers to use processes that lead to high quality software.
Comment: This clarifies my question very well. Also the previous answer I think asserts that the qa personnel should be an indispensable member within the quality goal, regardless of the status of the pfoject. The lack of this approach was what I was missing in my question.
Answer: I agree with the others, anyone should be able to enter bugs. One comment on:

Especially if they are not well described with steps to reproduce?

If the bugs being entered are not productive, I would invest some time those people to train them in the art of great bug writing.  You just might make a great tester in the end.
Answer: This is going to be a bit unpopular, but I am going to have to buck the tide on the answers so far. I agree that it is important to allow everyone to submit potential defects and that system should be as clear, helpful, and accessible as possible. However, I have found that it is important to separate the "official" bug list that the team is working from the "potential bug" list.
Before a "potential" gets promoted, it should be verified by someone (usually a tester). That person should be adept at posing follow-up questions to insure that the the classification of the bug as duplicate, new feature, etc. is correct. That is not a "territory" issue, it is simply a means of cutting through any confusion before the team addresses it.
This is especially true for reports from customers. I have found that prompt, intelligent follow up questions are always appreciated by customers. Also, no matter who submits the "potentials", it is important to follow up with status every once in a while. On a previous team, my desk resided next to the customer support lead. I would be called in on incoming calls where defects were triaged almost immediately on receipt when possible.
Comment: We implemented your approach by defining a proper bug lifecycle in JIRA. Before a bug is considered in bugs triage it remains in a state "New" (or "potential", as you called it). If the bug report is unclear during the bug triage session, it is moved to the "Further Discussion Required" state. Only when enough information is collected, it will be promoted to the "official" state.
Comment: Another reason for having a separate customer bug list is that the customer-created issue should not be closed until the customer verifies it. (An "internal" bug is closed when the team verifies it.) In addition, keeping a separate list allows the original issue description to be maintained and a separate "team" interpretation be created for implementation. Of course, (as with most testing issues) context within each team is everything!

Who should log bugs?


Options for lubricating wires for conduit?

Question: I am trying to pull four #8 stranded wires through a flexible PVC conduit that is 25 feet long. I can get the wires just a few feet into the conduit before they bind. I know there is a commercially available wire lube, but I do not have any. Can I safely substitute another product like dish soap or something? Or would that just be a bad idea and I should just get the real deal?
Comment: only a few feet? are you sure there's not another problem?
Comment: I pulled/pushed 100+ft of #10x3 wires through 3/4" without using anything for about 50% of the way. Towards the end I started using some dish soap and it went through pretty easily
Answer: I'd agree with the comment that you may have another problem.
What size is the conduit?
Are you pulling or pushing (pulling is what works, pushing won't.) If you need something to pull with, use a shop-vac to get a rope through the conduit first. Braided hollow rope can be nice as you can expand the end and use it like a Chinese finger cuff to grab the ends of the wires (use tape as well.)
If the conduit is exterior and the wires are correctly rated for exterior conduit, you could use water as a lubricant. It's not a good idea to come up with a random soapy product, since the commercial products are tested for long-term compatibility with wire jacket material, some lube will remain in the conduit, and the wires are generally expected to be there for decades - over which time it's possible that the seemingly innocuous dish soap might have a deteriorating effect on the wire insulation, due to some component inconsequential in washing dishes (say, for example, the scent...)
The real deal is quite affordable if you can find it in quarts, which you generally can at most electric supply houses or the internet if you have difficult electric supply houses and your home improvement stores don't stock it.
Comment: That is what I was thinking, but I just wanted to confirm it. The conduit is the flexible PVC and is offering up quite a bit of friction, even at 3/4". I will get the real lube just to be safe. Thanks.
Comment: Good Answer, And i totally agree about the shopvac and use of water, I did however use soapy water, and it did help. the pull action is the best, but if you have a friend that can help, its worth both pushing and pulling at once.
Comment: Dawn dish soap is a great pull lube the only problem is it drys out after a while and trying to pull the wires out at a later date is almost impossible, but this also happened with commercial lubes, they do make non drying lubes, but in a residential I usually save a few bucks as these are not pulled as often as commercial or industrial.
Answer: The nominal diameter of #8 stranded copper with insulation is just over .21 inches.
You are trying to pull 4 of these wires through 3/4 inch pipe. I'll bet the 4 wires together have a diameter of .5 inches (in the best case where the wires are still straight and have no bends.) That's a tight fit over 25 feet.  I don't know if you are using a mechanical pulling device, but that would be very helpful if one is at hand.
It is pointless to try to push the wires that far.  It would be like a shovel with a rope handle.  As suggested before.  Send a pulling rope down the conduit and attach it to the wires solidly.  The trick suggested of using a rope like a Chinese finger puzzle is a good one.  As for friction.  A quart of the real pulling compound costs about 6 bucks and you will have plenty left for your next job.  I agree that using dish soap is not a good idea.  The insulting materials vary and the stuff will be in place for decades.  Pinholes in the insulating material are a very real possibility and copper will start to corrode with soap.  It's not likely there would be enough to degrade #8 wire, but why chance it.
Also, soap solution does not have as low a coefficient of friction as the pulling compound. After paying what you had to buy that amount of copper wire, spending the six bucks for a quart of the right stuff is chump change.
Comment: FYI The finger puzzle thing is called a pull sock.
Answer: In regards to the ANSI/NFPA 70 National Electric Code (NEC) is your flexible PVC referring to

Article 356 Liquidtight Flexible Nonmetallic Conduit (LFMC), aka smooth walled Sealtight or
Article 362 Electrical Nonmetallic Tubing (ENT), aka corrugated "Smurf Tube"

The link to the Elliott Electric Conduit Fill Chart only addresses metal conduit: RMC, IMC and EMT.  Not PVC.
You need to look at Tables C2, C5 or C6 in the NEC to find what is legal for your wire type and flexible PVC conduit/tubing type. However, in all likelihood, unless you're using some thick 2-hour fire rated RHH, you are allowed, by code, to install 4ea THHN/THWN insulation, 8-AWG wires in 3/4" "flexible PVC".
Run a fish tape and pull line through your conduit/tubing (especially if smurf tube), then pull your bundle of wire through, making sure you wrap the head well with electrician tape so no bare wire sharp edges dig into your pipe. You shouldn't need lube if your conduit is straight.  If you have more than 360 degrees of bends total, by code, you need to add a pull box or junction box.
If you have a kink in the conduit with an angle greater than 90 degrees or a tear or rip, or drywallers put a few screws into it, or someone mangled it with  a forklift, decreasing pull resistance with lube probably won't solve your problem.
Stranded wire is easier to pull through bends than solid.
Comment: Pushing in flex is tough but can be done if there are only a couple of bends tape the ends of the wire tightly so that the tape is stretched and beyond the end of the wires. If it dosent go quickly blow or vacuum a string into the conduit have 1 person push the wire with a light pull on the string and you would be amazed how easy it will go.
Answer: Talk about code violations. This is way past the allowed fill rate for a 3/4 piece of conduit. The minimum conduit size for 4 #8's is 1.0, with a 75% fill or 1.5in with a 50% fill rate. That is so the cables can dissipate heat. Yikes! Hope your conduit/house hasn't burned down yet.
Comment: Do you have a source for that? All the conduit fill tables I've looked at say 3/4" conduit can take 6 #8s. e.g., http://www.elliottelectric.com/StaticPages/ElectricalReferences/ElectricalTables/Conduit_Fill_Table.aspx
Comment: @littleturtle It all depends on the type of #8 you specify. THHW normal conductor will let you have 4 in Schedule 40 3/4". It jumps to 7 in Sch. 40 1". Source is 2017 NEC Informative Annex C Tables (after Chapter 9).


Is there a generalization of the Sylvester Inertia Law for the symmetric generalized eigenvalue problem?

Question: I know that in order to solve symmetric eigenvalue problem $Ax = \lambda x$, we can use the Sylvester Inertia Law, that is the number of eigenvalues of $A$ less than $a$ equals the number of negative entries of $D$ where diagonal matrix $D$ comes from the LDL factorization of $A-aI = LDL^{T}$. Then, by bisection method, we can find all or some eigenvalues as desired. I wish to know if there exists a generalization of the Sylvester Inertia Law for symmetric generalized eigenvalue problems, that is solving $Ax= \lambda Bx$, where $A$ and $B$ are symmetric matrices. Thanks.
Answer: Yes, if the pencil is definite, i.e., if $A$ and $B$ are Hermitian and $B$ is positive definite. Then the signature of $A-\sigma B$ has the same interpretation for the eigenvalue problem $(A-\lambda B)x=0$ as in the case $B=I$. A more general result of this kind holds for any definite nonlinear eigenvalue problem $A(\lambda)x=0$. See Section 5.3 of my book
Arnold Neumaier,
Introduction to numerical analysis,
Cambridge Univ. Press, Cambridge 2001.
For $(A-\lambda B)x=0$, the proof of my assertion can be deduced from  the argument given by Jack Poulson upon noting that $C-\sigma I$ and $A-\sigma B$ are congruent, hence have the same inertia.
In particular, one can directly compute the inertia of $A-\sigma B$, and doesn't need a Cholesky factorization of $B$ to form $C$. Indeed, if $B$ is ill-conditioned then the numerical formation of $C$ degrades the quality of the inertia test.
Comment: Good point about the ill-conditioning of B; I think that your approach is better if one truly is only interested in computing the inertia. The approach I suggested is typical for actually solving the eigenvalue problem (in the case where $B$ is well-conditioned).
Comment: @JackPoulson: The inertia test is usually applied to get the eigenvalues in a specific interval when $A$ and $B$ are sparse and their joint sparsity pattern generates not too much fill in. But your $C$ will be dense already when $B$ is tridiagonal, hence using it is never suitable for finding the eigenvalues of a large sparse generalized eigenvalue problem. (Whereas if the problem is not large, there is little point in using inertia, as finding all eigenvalues is usually fast enough.)
Comment: Certainly; it seems that I mistakenly left the word "dense" out of my comment.
Answer: In the case where $B$ is Hermitian and positive-definite, a Cholesky factorization of $B$, say $B = L L^H$, gives that
$$
Ax=LL^H x \lambda,
$$
and this equation can be manipulated to show that
$$
(L^{-1} A L^{-H})(L^H x) = (L^H x) \lambda,
$$
where it should be clear that $C \equiv L^{-1} A L^{-H}$ preserves the symmetry of $A$, and also has the same spectrum as the pencil $(A,B)$. Thus, after forming $C$, with a Cholesky factorization followed by a two-sided triangular solve, you can directly apply the Sylvester inertia law to $C$ to glean information about the eigenvalues of the pencil $(A,B)$.
Note that, since Sylvester's Law of Inertia is invariant with respect to congruence transformations, e.g., $S \cdot S^H$, then the matrix $C$ is congruent to $A$ through the transformation $L^{-1} \cdot L^{-H}$, and so $C$ has the same inertia as $A$. However, if the inertia of $C-\sigma I$ is desired, for some nonzero shift $\sigma$, then we can no longer simply consider $A$.
Comment: I haven't logged out on my office's computer,  and my officemate happened to run into this tab in my browser and downvoted the answer, I apologize for the misunderstanding and will ask him why he downvoted this.
Comment: A downvote without any constructive criticism?
Comment: You were absolutely correct when $B$ is an spd matrix, for the pair $(A,B)$, we could simply look at $A$ to get what we want. However, my officemate said that you didn't answer the question if $B$ only has symmetricity. Sorry for the confusion.
Comment: @Jon: Sigh. That is not what the downvote is for.
Comment: I know! I already told him "please read the rule" after I found that he used my account to downvote a relevant answer!
Comment: Of course, there is the alternative of eigendecomposing $\mathbf B$ to form an equivalent regular Hermitian eigenproblem.


What is the Bitcoin spot price and how is it calculated?

Question: I was offered to buy Bitcoin for today's spot price (MtGox to be specific). So, I am wondering, what is the spot price and how is it calculated? Is there a standard tool to look it up?
Answer: Just like with other commodities, there can be multiple "spot price" numbers.
With gold, for instance, there can (and usually will) be different price on the Comex exchange versus the price the at the auctions in London for local delivery there.
So when referencing the spot price, that is ambiguous until the specific market whose prices will be used are clearly defined.
Even then there is the issue of the the bid/ask spread at any point in time.  Since there is no regulations defining how spot price is computed, there is ambiguity between whether the highest bid offer should be the spot price, or the lowest ask offer.  Others might consider the value from last trade as being the spot price.  Another approach is to find the value exactly in the middle of the bid and ask and deem that as the spot price.
BitPay uses an algorithm that attempts to be fair in determining "spot price" by using multiple exchanges:

https://bitpay.com/bitcoin-exchange-rates

And Coindesk provides a proprietary spot price service where "spot price" also is computed using data from multiple exchanges:

http://www.coindesk.com/price
http://www.coindesk.com/coindesk-launches-proprietary-bitcoin-price-index/
Answer: I believe on the precious metal markets, the spot price is fairly uniform because of the large amounts of money involved and arbitrage.  The spot price it the average of the bid and ask of the highest valued traders (i.e., the lowest spread which may be only a few pennies).  During some times of the day 3 or more major exchanges around the world are trading.  If the price on one exchange is significantly higher than another, a big trader will buy on the lower exchange and simultaneously sell on the higher.   Instant profit with almost no risk.
Stephen Gornick is correct.  I deal with several metal brokers, some run their own exchanges, so when they quote spot they are quoting their spot, which is always higher than New York, London, or whichever major market may be open at that time. They also set their own spread rather than market makers.  When Bitcoins trade in New York and Hong Kong, then spot may really be spot.
Answer: Spot pricing is calculated by looking at amounts and prices in the bids/offers that everyone has posted, and adding them up until they meet your what you are willing to sell/buy.
I'm not sure if that's a great explanation, but incidentally, I created an application that illustrates this a while ago: http://btc-spot-pricing.johnhenry.c9.io/
Comment: I'm having some issues with c9.io, so that app may not be working properly. Sorry. :/
Answer: There is not single "spot price" for bitcoin. There are only bid and ask prices on various exchanges, for various currencies.
You have to factor in how much BTC you want to buy and then look at the order book to see what price you will pay to buy or sell.
There is a tool to view all markets for a given currency, see http://bitdango.com/currencies to view all currencies. Then pick a specific one (USD, EUR, PLN, etc) you are interested in. For example, here is the USD URL:
http://bitdango.com/markets?quoteCurrencies=USD - that gives you all the bid/ask prices for Bitcoin Markets that support USD.
Comment: I downvoted, and let me explain why: First, this answer is arguably false, because there are several services that offer spot-prices for Bitcoin (e.g. see the accepted answer). Second, your answer doesn't sufficiently address the main question "What is a spot-price?", third, the linked tool provides something similar with the bid/ask prices, but doesn't show a spot-price, which is explicitly the topic of this question.
Comment: @Murch - ok, I think this is what you are looking for: http://bitdango.com/currency/BTC/rates - the "spot" price of BTC, against other currencies.


What are the ground and flight requirements for high performance endorsement?

Question: I'm told ground is proficiency based but that in the plane you have to fly up to 25,000 feet! I don't know anything short of a turboprop capable of doing that, let alone have the ability to rent it. I'm finding different answers everywhere I look! Can anyone cite the FAR on this?
Answer: You seem to be confusing a high-performance endorsement with a high-altitude one. Both are in 14 CFR 61.31. They're too long to quote here in full, but the high-performance one comes down to this:

(i) Received and logged ground and flight training from an authorized
  instructor in a high-performance airplane, or in a flight simulator or
  flight training device that is representative of a high-performance
  airplane, and has been found proficient in the operation and systems
  of the airplane; and

The high-altitude one is where your 25,000' number comes from:

(g) Additional training required for operating pressurized aircraft
  capable of operating at high altitudes. (1) Except as provided in
  paragraph (g)(3) of this section, no person may act as pilot in
  command of a pressurized aircraft (an aircraft that has a service
  ceiling or maximum operating altitude, whichever is lower, above
  25,000 feet MSL), unless that person has received and logged ground
  training from an authorized instructor and obtained an endorsement in
  the person's logbook or training record from an authorized instructor
  who certifies the person has satisfactorily accomplished the ground
  training. The ground training must include at least the following
  subjects: [...]

As well as the ground training, you have to do (at least) the following in an aircraft or approved simulator:

(i) Normal cruise flight operations while operating above 25,000 feet
  MSL;
(ii) Proper emergency procedures for simulated rapid decompression
  without actually depressurizing the aircraft; and
(iii) Emergency descent procedures.
Answer: There is not a whole lot of difference between flying a 160 hp Skyhawk or Cherokee and a 200+ hp "high performance" Skylane or Cherokee 6. About the only thing I can think of is that you need more right rudder on takeoff. They are usually heavier airplanes so ground control is a little different as well.
I suspect the high-performance endorsement is a holdover from the days when students learned to fly in 65 hp Aeronca Champions since the definition of high-performance from 61.31 is:
(

f) Additional training required for operating high-performance
  airplanes. (1) Except as provided in paragraph (f)(2) of this section,
  no person may act as pilot in command of a high-performance airplane
  (an airplane with an engine of more than 200 horsepower), unless the
  person has—

Most airplanes with more than 200 hp also have a prop control as well as a throttle. So for most people, the biggest thing you need to learn is how to manage the manifold pressure (MP) on the engine with the throttle and the prop speed (RPM) with the prop control.
If you haven't used a constant-speed prop before, it might take an hour of ground instruction and an hour of flying. If you already know how to use a constant speed prop and how to read the flight manual pages regarding various MP and RPM settings then it might take 10 minutes on the ground and 3 takeoffs and landings.
Comment: The FAA used to define a high performance aircraft as having more than 200 horsepower or having retractable gear, flaps, and a controllable propeller. In Aug 1997 they they created the complex endorsement (retractable gear, flaps, controllable propeller) and redefined high performance as simply more than 200 horsepower.


Luggage when travelling with children

Question: When travelling with small children we strongly prefer to have few pieces of luggage, so they end up being rather heavy (unless driving). This is fine until you need to check-in for a flight, where many airlines (e.g. KLM, BA) insist on every bag being at most 23kg. The total weight of our luggage is always well within the sum of the limits for all the passengers (because kids get the same allowance), but every piece is too heavy. Did anyone figure out some reasonable way to work-around this limitation?
Comment: Stackable / attachable suitcases? So you have the same number of items until checkin, when they come apart?
Comment: ^ this. Duct-tape the bags together for ease of transport (unless you got some fancy equipment), cut the tape at check-in and have a neat way to recognize your bags upon arrival.
Comment: @Gagravarr, I have never come across these kind of suitcases...
Answer: I think your best bet is to get some luggage that can be attached together. That way, each adult can wheel along 2-3 pieces together, whilst still being able to take them apart to check in individually.
As an aside - the reason for the 20-25kg limit (varies slightly between airlines) is that beyond that the airline normally has to pay extra to the airport for handling it. Cases much beyond about 20kg need special handling, such as multiple people to lift them safely. If you check a heavy bag, it'll almost certainly have warning labels added to it about the weight.
So, luggage you can attach together. There are two broad types here, those that were made with this in mind, and those where you can make it work!
A lot of airline crew luggage is designed to attach together, and you'll often see them wheeling a suitcase with another on top through the airport. Here, the aim is usually to keep both pieces within the cabin baggage size limit, and if it's a long trip they then attach the second one. Typically these attach by one joining on to the handle of the other, so you end up with a very tall suitcase. That said, these are normally smaller cases, so you may struggle to find large ones that'll do the same thing. You can also get ones where they attach with the largest sizes touching, to effectively give a very deep suitcase. A quick google only shows a luxury brand doing this, but I'm fairly sure I've seen others about.
Otherwise, you just need two fairly similarly sized suitcases, and some straps. Put one strap on the floor, stand the two cases together, wrap it around and tighten, then add a 2nd one the other way. Carefully tip forward onto the wheels of the bottom one, being aware that the centre of gravity is now likely off, and wheel away! (Note - don't just attach them at the top, otherwise the first time you corner or hit a bump, it'll probably go horribly wrong...)
Oh, and there is another option - many airlines allow their frequent travellers to check in heavier luggage without fees. Generally only the higher tiers, so you'll need to be doing a lot of flying to hit it... Oh, and many airlines also wave heavy bag fees for premium cabins, but that means expensive tickets!
Comment: I couldn't find any CC licensed pictures of any of these, if someone could track some down and edit them in that'd be great!

Luggage when travelling with children


Does more "Power" (W) means better motor?

Question: If a motor has more Power (W) from another it means that it is better?
Comment: It is better if you need more Power. It is worse if you don't need more power and it uses your fuel source up too fast.
Answer: The short answer is no.  The longer answer requires more information from you, regarding how the motor is to be used.
Since you have tagged your question with "quadcopter", I'm going to assume that you plan to put the motor on a UAV, in which case more power definitely does not mean "better".
More power (W) will draw more current (A) from the battery (meaning a larger battery and higher all-up-weight (AUW) of the craft).
You need to choose the correct motor and prop for your particular quadcopter frame.  There is no one-size-fits-all motor for quadcopters.  In general, you should be looking for a brushless outrunner motor with low kv, and pairing it with a slow-fly prop.
Comment: why low kv? and what is a slow-fly? thanks. btw +1 for noticing the quadcopter tag ;)
Comment: i believe that you typically want a relatively high KV for a multi rotor vehicle.
Comment: No, you want to use low kv motors for a multicopter, paired with long diameter, slow-fly propellers.  The reason being that it is more efficient to generate thrust using long propellers rotating slowly than it is to use short propellers rotating quickly.  Slow-fly propellers are used because they generate static thrust (used in hovering) more efficiently than propellers designed for high-speed aircraft.
Answer: The wattage rating is usually (read the manufacturer's specs very carefully) the measure of how much power the motor can draw.  It doesn't tell you anything more than that.
Wattage does not tell you whether the motor is efficient at converting the input electrical energy into output mechanical energy.
Wattage does not tell you whether the motor's power-to-weight ratio is reasonable.
Wattage does not tell you whether the motor's size is reasonable for your application.
Wattage does not tell you whether the stall torque or maximum speed is correct for the load you wish to drive.
Wattage does not tell you whether the motor's duty cycle can support the needs of your application.
If you are comparing 2 motors that differ only by their input power rating, and more power is important to you, then yes: the motor with more power will be better for you.


What makes a good entry level road bike?

Question: I've been riding a mountain bike for years, but I prefer to go longer distances on paved ground, so I think a road bike would be better. When I ask what is a good bike, I don't need brands or models - more like what components of the bike should I look for, and what should I avoid? Additionally, is it necessary to "go big or don't go at all" as far as spending - getting a real good quality road bike for $500 to a few thousand, or should I cheap out on a $100-200 road bike, as it is my first, then get a better one when I'm more experienced?
Comment: you could consider just putting slicks on your mountain bike
Comment: how far and how often will you be riding
Comment: I'd prefer a longer ride than a shorter, tougher ride. On weekends, I'll go on 30-40 mile rides on my mountain bike, just on bike paths. On week days, i go around 10-15 miles, and go about 3-4 days a week through the spring/summer/fall. I don't know anything about putting slicks on a mountain bike. My current bike (somewhat shamefully) is a walmart schwinn that I got 2 years ago before going to college, and have used it a lot.
Comment: @srank I approved the edit because its technically correct, but really a minor edit for stuff like that isn't adding any value.  We know what OP means, even if the formatting and capitalisation is not perfect.  Minor edits detract.
Answer: Things to consider are that when you buy a new bike it is a mix of different components, which are then packaged together and you receive a discount on the sum of the parts. This means that different manufacturers attempt to save money by using cheaper items for certain pieces of the overall puzzle.
The major headline items are the frame, gear system, brakes and wheels - everything else is (relatively) cheaper. So consider the things that are easy to change when you feel like upgrading. It's trivial to change the wheels, it's harder to change the brakes, harder still the gears and a new frame means a total rebuild.
So that's the priority when considering a bike.

Make sure the frame fits well - ideally it will be perfect, but different uses require different frame sizes/geometries (time trials, road racing, training, commuting, leisure rides) so perfection means different bikes.
Choose the gearings and brakes - you don't want to get into a huge Shimano vs. Campag Vs SRAM debate, though - frankly they're all much of a muchness unless you know a lot more than I do
Wheels are supremely important, but also easier to change. Deeper rims are faster, having fewer spokes is generally faster, lighter is better. And you can spend as much on a pair of wheels as a second hand car.
Everything else is a bonus and easier still to change (saddles, handle bars, cranks, etc.)

At each stage spend as much money as you feel you can, you will always be able to spend more. But ultimately for a new machine it isn't worth building your own, you will get a better deal buying it whole and complete, but you should purchase with half an eye on what bits they've skimped on and which you can upgrade down the line.
Comment: Shifters are also expensive to upgrade, and I'm not sure whether or not you include this under your "gears" category. Thus, it's hard to change the number of gears (say, from 7 sprockets to 10), but replacing a cassette is relatively cheap.
Comment: This is the answer.
Answer: At the beginning of the summer I just bought an old Norco bike.  I have had tons of fun on it.  I watched Kijiji (similar to craigslist) every day for a month and eventually this bike came up.  It had never been ridden (there was not even a scratch on it) and the price was only $75 Canadian.
Comparing my Norco to a friends $3000 Trek, here is what I found:

The difference in weight between the bikes was less dramatic that I expected.  The newer bike weighed maybe 70% of what the older bike weighted.
The newer bike had a much better shifting system.  The shifters where on the brake levers.  The older bike has shifters on the downtube.  To shift gears you need to remove your hand from the bars and reach down to the down tube.

The hoods on the newer bike are more comfortable for longer rides.
The newer bike has significantly more gears.  This is a pro and a con.  The older bike has only 6 gears on the back and 2 on the front.  One advantage to this is that the gears can be substantial thicker.  I have heard that these gears will never wear out.
I am not scared to ride the older Norco to school and lock it up while I am working.  I would never do that with the more expensive bike.
I had a crash on the older Norco the second day I was riding it that resulted in the the front wheel being destroyed.  Replacing it cost me $30.  On the expensive bike god only knows what that crash would have cost.
The newer bike has some carbon in the frame.  In theory this should reduce the road vibration (and other things??).  The older bike is made entirely out of steel.

In conclusion, if you want to race you will need the better bike.  If you want to have fun, be active and go fast I think that the older road bike is an excellent entry point.  Probably I will invest in a better road bike at some point but I will defiantly still keep this bike to make grocery runs and ride to school.  (Maybe I will convert it to a single speed).
Answer: The most critical thing is that it fits you well.  You don't need to spend a lot of money -- in fact, I'd recommend against it for your first road bike.  But a basic bike that fits you well will feel fun and will get used.  And as you gain more experience with it, you'll know what to change/upgrade if you feel the need to buy a more expensive bike down the road.
Comment: Then...  See my post a by an older $75 bike on Craig's list!  http://bicycles.stackexchange.com/questions/1121/what-makes-a-good-entry-level-road-bike/1125#1125
Comment: This makes a lot of sense. Its just hard for me to justify spending <$1000 on a bike that I'm not sure I'll even like, because I haven't ever ridden this kind of bike before.
Answer: Typically the cutoff point (I think) for a quality bike is in the $1000 - $1200 range. You're often looking at the same frame as on the $2000 model but with lower grade components. It's always seemed easier on my bank account to slowly upgrade components.
Since I'm talking retail you may be able to find something in the $500 range on a site like Craigslist. In fact I did that this summer. I purchase a Trek 1.5 from last year for $650 (retail is about $1200 Canadian).
I would say that $100 - $200 is much too low to purchase a bike of any type of reasonable quality unless it fell off a truck (stolen) or you happen to know someone that has a bike fetish and is willing to part with one to keep their significant other around.
Another good question was asked specifically about purchasing from Craigslist that would probably inform you if you decide to go that route.
What should I look for when buying a used bicycle?
Comment: Are you talking USD or dollars from somewhere else?
Comment: I haven't shopped for bikes in quite a while, but took a quick look at some major brands. There are some very decent bikes to be had in the $1200 - $1600 price range.
Answer: Find a good LBS (Local Bike Shop) that deals in road bikes and has a used bike section.  They might not have something for you right now, but if you talk to them, tell them what you are looking for, try some new bikes to get a really good handle on the size you are needing, they may just give you a call when the right trade-in appears.
Answer: I've done exactly this - having broken an old MTB frame after 2500 km on it, I decided to buy a road bike.
Searching and researching and browsing suggested 105 level components would be a good floor, and I had a preference for a disc brake on the front.  Pricing worked out around $2200 NZD for an aluminium and about $2500 for the same thing in carbon.
Instead I bought a 1998 road bike for $100 from trademe (the local ebay) and its great.   I ride further and faster.
You'll end up riding 5 km/h faster on average, and your climbing times will drop by ~10-20%.
Answer: Comfort makes for a good road bike.
Comfort comes from a good body position, so bike fit is more important on a road bike than a MTB.   On an MTB you're up and out of the saddle a little more, on a road bike you maintain a fairly similar position a lot.
Smoother tyres help - you can go full slicks on a road bike, but it will be terrible off sealed road (gravel roads are not fun) and a patch of gravel on the road can surprise you.
Road bikes have hard tyres too - tube air pressures are 100+psi and make for a hard ride.  In a new bike I'd look for steel or carbon forks, to take the rattly edge off the ride.  My aluminium forks are "buzzy" and that leads to hand numbness.
Consider hanging out for something with Brifters (integrated brakes and shifters), if that's what you want to try.  Downtube shifters were good back in the day, but you're trying ideas for a new bike - nothing has downtube shifters any more.
I suggest you look for a good used road bike, ride it for some months, and refine your expectations and requirements.  I still want a front disc brake, but it can wait.
Comment: About a year later, update.   I have done 6,500 km my "cheap" road bike, and have learned a lot.  My belly has shrunk to the point I can use the drops for 5-10 kilometres at a time.  I have competed in a 100 km road race and turned in a time of 5 hours 10 minutes (there were lots of hills, ok?)  and have entered for this year's race.  New tyres, shorter quill stem, and the bike fits me nicely.   And there's something about passing carbon-fibre cycle riders on an almost 2 decade-old bike, specially where the design looks every bit of its age.


Java on FreeBSD

Question: Is it possible to get the latest Java (Java 7u3 or Java 6u31) on FreeBSD? Everything i found on the internet is outdated. I currently have 6u24 from the packagemanager which works for now but may get problematic really soon.
Answer: I am sorry but there is no Oracle (ex: Sun) java support for FreeBSD.
On the other side, you can use the FreeBSD port of OpenJDK.

Java on FreeBSD


Does Tor entry point store my ISP information?

Question: Does Tor entry point stores my ISP addresses after I disconnect from the Tor system?
Answer: The entry point into the Tor network (so called guard node) sees your real IP address. Depending on the policy of the relay operator it could save every connecting IP address. This can be done with or without Tor. In general it is considered good practise to not save any data.
Comment: No. Tor works also with malicious relay operators. It has privac by design. So as long as there is one good acting operator or operators with conflicting goals the privacy can't get degraded.
Comment: IMHO storing *any* logs in Tor node breaks the entire concept of anonymity online.
Comment: sadly - yju are right here. That's why I'm in my project incorporating other darknets and tools like Perspectives project. I even was offered by russian FSB to help them to set up a network of malicious relays...


Does P2PKH substitute P2PK in any circumstances? Why?

Question: Does P2PKH substitute P2PK in any circumstances? Why? I know address(public key hash) was invented later than the official implementation released. Is it the reason?
Comment: Or did you mean "P2PKH instead of P2PK"?
Comment: Did you mean P2SH instead of P2KH?
Answer: Both P2PK and P2PKH were part of the first bitcoin 0.1 release, so that second point isn't true.
P2PKH is theoretically safer because the pubKeyScript only reveals the hash of the public key rather than the public key itself (which only matters in the case that the elliptic curve crypto involved is broken). So as long as you don't reuse the address, no-one else will know the public key before the money is spent.
All bitcoin addresses encode public key hashes so only P2PKH can be used with an address, as the public key is not known. P2PK were mainly designed for IP-to-IP payments, which is no longer possible in Bitcoin Core since version 0.8.0
Comment: Pay to IP was disabled by default in 0.3.20 (feb 2011!). It was only some leftover code that was removed two years later in 0.8.0 (feb 2013).
Answer:
Pubkey scripts are created by spenders who have little interest what
  that script does. Receivers do care about the script conditions and,
  if they want, they can ask spenders to use a particular pubkey script.
  Unfortunately, custom pubkey scripts are less convenient than short
  Bitcoin addresses and there was no standard way to communicate them
  between programs prior to widespread implementation of the BIP70
  Payment Protocol discussed later.
To solve these problems, pay-to-script-hash (P2SH) transactions were
  created in 2012 to let a spender create a pubkey script containing a
  hash of a second script, the redeem script.
The basic P2SH workflow, illustrated below, looks almost identical to
  the P2PKH workflow.

Read More Bitcoin.org


What's a good substitute for amchur?

Question: If I'm using amchur primarily just for the tanginess, are there any good substitutes? I'd rather avoid things like lemon juice, since often it's very convenient that it's a powder, not a liquid.
Comment: (For the record, I personally live down the street from an Indian store and have plenty of amchur but sometimes I share recipes with people with less well-stocked spice racks.)
Answer: Amchur doesn't give much of a flavor other than sour, at least, not in the amounts I use of it. It does have a bit of a distinctive aroma, but really, to me, it is just a fairly neutral sour flavor.
The easiest, most neutral substitute would be citric acid powder. Unfortunately, they don't usually give concentration/strengths on the packaging, but comparing Ball Citric Acid for Canning to the powdered amchur from the Indian store, it's pretty close. I think the citric acid tastes a tiny bit lemony, but not enough to make a difference.
Also, the amchur powder I buy is generally quite finely ground, and the citric acid is generally in crystals. If you are doing something like a spice blend (e.g., chat masala powder) you would probably want to grind the crystals up to match the texture of the rest of the spices.
Depending on what you're making, there are quite a few substitutions, but most of them would be more obscure than (or at least as obscure as) the amchur, and all would be less neutral. Also, amchur is dry, so adding something liquid might affect your recipe. (Easy to adjust if you are adding to something liquid, of course.)
In many Indian dishes, most of the Indians I know simply do use lemon juice when they want to make something sour, especially for northern dishes. This really is quite common, and nearly every Indian cook I know in the US has a large bottle of lemon juice in the fridge for every day use.
If you're making something South Indian, tamarind pulp or paste is often the souring agent of choice, but it is distinctly not neutral. It has a distinctive flavor and will also darken things (in a somewhat yellow-orange-brown range).
Some regions also use dried kokum fruit in the same way (classic Maharashtrian aamti, for example), but I would say it is even more distinctive than tamarind, and it also will darken it (with a dark grey-green-brown range of color).
For both kokum and tamarind, you soak the pulp in water and add the soaking water without the pulp to the dish. You can also usually find tamarind concentrate (a tarry paste) but I've never seen anything like that for kokum.
One more option I just thought of is using some other unripe or sour fruit. Amchur is made by powdering dried green mangoes. You can actually sometimes find dried green mangoes at places like Trader Joe's and if it is unsweetened, a small piece of that can certainly be used, but it seems that any fruit that is firm and sour should work, such as an unripe plum or a very tart apple. In that case, I would suggest either cooking with a small piece or two of the fruit in the sauce (if there is one) and removing it later or you could try to crush it and press some juice out. The problem here would be in figuring out the correct amount to add, but I think you could start from the dried-to-fresh conversion rates for herbs.
Comment: I see one problem with lime/lemon juice: it could degrade in case you decide to store and reheat the food, or cook it further. After all, you are supposed to add it only in the finishing phase of cooking. In some cases, a neutral/distilled vinegar (not wine, malt, apple cider or flavored vinegar!) might work better...
Comment: Vinegar could be used, but in my experience it is not used very much in Indian cooking. I'm sure your concerns about lemon juice breaking down are valid, but I'm not sure that it detrimental to the dish. Maybe that is part of why leftovers sometimes seem better? ;-)
Comment: There are some vinegar based dishes (admittedly, not nearly as common as other souring agents).  My favorite is a particular gajar ka dish (I don't recall off hand where I got the recipe) - it's essentially carrots and spices that have been caramelized in malt vinegar.  Really amazing stuff.  I've also bottled several chutneys that use vinegar as a souring agent.
Comment: I'm not saying it isn't used at all, just that my experience shows it to be less common. It could also be a regional thing, for example, vindaloo does use vinegar and I think that is more from the Portuguese influence.
Answer: I suppose citric acid powder might work (i.e., if you want the convenience of a powder vs a liquid souring agent).  That being said, I've never tried this myself and I suspect it's about as difficult to find as amchur powder.
Comment: It's not difficult to find in the same way, though; a lot of bigger grocery stores will sell it for canning but you'll have a hard time finding amchur if it's not part of any of the more prominent local cuisines.
Comment: In the US I have seen citric acid in Asian markets, grocery stores, seasonally in home goods stores (and sometimes even craft stores) where they sell canning jars. It depends on where you are -- here in California there is stuff for canning available year round, but in NY/NJ you're lucky to find it even in the fall at a regular store.
Answer: I also use crushed dried pomegranate seeds for tangy flavours in Indian curries. And once cooked, you don't get the crushed texture, rather smooth curries.
However, for making mint chutney, I use fresh green mangoes and I understand that they do have liquid content in them, but pulp as well, which balances out the liquid.
Answer: If you don't want the liquid from lemon juice, you could use the zest instead. Lemon or lime zest would work, I think.
Comment: I don't think citrus zest is particularly tangy/acidic?

What's a good substitute for amchur?


I have a criminal record in Canada. Can I enter Germany?

Question: I am an Indian citizen and I was charged with criminal harassment in Canada. I am looking to marry a girl who is a German citizen. Can I enter Germany after marriage and get German citizenship?
Comment: What was the crime? What was the punishment? Do you speak German? Do you plan to live in Germany?
Comment: "charged with criminal harassment": were you convicted?
Answer: NO, and yes.
Germany is a member of the Schengen Area, whose law is relatively lax.
Questions about criminal convictions are not asked when applying for a Schengen visitor/business visas, and border agent and landing cards (there aren't any) don't ask this either.

If any officials or forms ask you if you have a criminal history, you still have to answer truthfully, but in general if its not more than 3 years of imprisonment, or crimes involving alien smuggling or drug offences that resulted in more than 2 years of imprisonment, then generally they will not refuse you entry or visa on that ground.
Germany has specific rules that state anyone convicted of an offence relating to public order with a sentence of more than 3 years, a drug offence with a sentence of more than 2 years, and any offence related to alien smuggling is deportable (a "must deport"). Like the UK they are more concerned with offences committed in their country, rather than outside of the EU.

If you lie on any visa application or question by officials, that is a "can deport".
So, in other words, you can ENTER on a vistor visa.
However, if you want to marry a German girl, and stay in Germany, you'll need a residence permit. And a residence permit, you won't get with a recent (or serious) criminal history. So while you can enter, entering makes no sense.
Also, it could be difficult finding work with a criminal record.
Comment: Furthermore, a bit of searching does not turn up support for your claim that one cannot get a residence permit with a recent or serious criminal history, for example at https://www.berlin.de/labo/willkommen-in-berlin/dienstleistungen/service.493516.php/dienstleistung/328191/en/.
Comment: Could you link to, or at least identify, the source of the quoted material?  Also, it might be worth pointing out that the couple could move to any other EU country with minimal trouble unless the criminal history were very serious indeed.


Can I make custom fonts show up in iBooks?

Question: Is it possible to include custom fonts in an ebook and have them show up in iBooks, or can you only use the various default fonts listed on the device?
Answer: It is certainly possible to embed custom fonts and have them be used in iBooks, as well as on other EPUB reading systems. The process is known as embedding. Embedded fonts are simply included in the .zip archive that comprises the EPUB file, and then referenced properly in all the relevant content files. Places you will need to reference them include:
CSS file
This is just like using fonts on the web: the first thing you need to do is define @font-face:
@font-face {
  font-family: roboto;
  src: url('fonts/Roboto/Roboto-Regular.ttf')
}

and then use that declaration in for whichever styles you'd like to use that font:
p.intro {
  font-family: roboto;
  font-weight: normal;
  font-style: normal;
}

content.opf file
Just like all the other files in your EPUB, you'll need to include any font files you're embedding in the <manifest> of your .opf file:
<item id="Roboto-Regular_ttf" href="fonts/Roboto-Regular.ttf" media-type="application/x-font-truetype" />

The above steps are sufficient for most reading systems, but iBooks has one more hoop to jump through:
com.apple.ibooks.display-options.xml
This is a file that goes in the META-INF folder of the EPUB, rather than the OEBPS folder, and as the name suggests it's iBooks-specific. Neither this file nor the container.xml file need to be declared in the <manifest>, so don't worry about that. Here is the file in its entirety:
<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<display_options>
<platform name="*">
<option name="specified-fonts">true</option>
</platform>
</display_options>

A more in-depth writeup can be found on Liz Castro's excellent blog. As noted elsewhere, be aware that there are some potential hurdles, both legal and practical, to embedding fonts.
Answer: To extend on Tom's answer I have to disagree with how the media type is written for Truetype and I would also like to show how its written for Opentype.
In the .opf file <manifest>:
Truetype fonts
<item id="Roboto_Regular" href="fonts/Roboto_Regular.ttf" media-type="application/x-font-ttf" />

media type for Truetype is: "application/x-font-ttf".
Opentype fonts
<item id="Roboto_Regular" href="fonts/Roboto_Regular.otf" media-type="application/vnd.ms-opentype" />
media type for Opentype should be: "application/vnd.ms-opentype" but I have seen "application/x-font-opentype" used and pass validation.
Prior versions of Epubcheck I want to say used to check for this but I haven't tested it within EpubCheck-3.0.1
Be careful on how you code your paths when declaring the src:url in the .css file for the font.  I have seen some people use absolute paths when they should have used a relative path and it throw a validation error of an unused font.
Note you cannot just include a master .css file that calls for a ton of fonts.  If you code for a font to be used in the .css file and include it in the package it must be used and declared in the .opf file or you will throw a:
<Error>
<Message>'fonts/foobar.ttf': referenced resource missing in the package.</Message>
<Line>2</Line>
<Column>71</Column>
<Resource>foobar.css</Resource>
<Class>com.adobe.common.ReportElement</Class>
</Error>

If you are not going to use the font you should remove it from the package and comment it out in the .css file
Comment: Thanks for the expansion on my answer. I wouldn't have thought to try including a master font list in each CSS file, so I didn't know it would error like that. For the mimetype for truetype, I've been using that declaration for at least 3 years, and haven't had it return invalid or cause any problems in any reading systems. Do you have a reference for why `application/x-font-ttf` is better?
Comment: From experience beyond iBooks I have found past issues with other platforms, devices and validation tools if it is not written with `media-type="application/x-font-ttf"`.
Comment: Interesting, thanks. I'll keep my eyes open for any errors that come back from any of the other retailers we deal with.


Why is kinetic energy denoted by the letter $T$ in quantum mechanics?

Question: Kinetic energy is often written as $K$, $KE$ or $E_k$. Where does $T$ come from in quantum mechanics? Why and how did it come to be different?
Comment: I thought that it might be because it was close to $U$ and $V$ in the alphabetical order. It was just a speculation of mine, anyway.
Answer: Gaspard-Gustave Coriolis and Jean-Victor Poncelet used the name "quantité de travail" (quantity of work) and "travail mécanique" (mechanical work) to denote the kinetic energy. I guess that this (the term "travail") could be the origin of the symbol $T$. See at the end of https://en.wikipedia.org/wiki/Vis_viva
(Be patient for my superficiality, I am very far from being a historian of science, I am just a mathematical physicist and this post migrated here from PSE.)
Comment: Sorry it seems I took I mistake! I am correcting my answer.
Comment: D. Bernoulli is mentioned in Wikipedia text, but not for using $T$.
Comment: I forgive your mistake. I have to admit mathematical physics is an intriguing distraction. If you happen to be in Trieste, there's a beer and some company waiting :-)
Answer: I'm pretty sure Lagrange started this, in Méchanique analitique (1788, p. 224; 1809, p. 263; 1811, p. 311; 1815, p. 2): his predecessors mostly worked with the vis viva ($=2T$) instead.
(As to why he chose the letter  $T$, no idea. Words like kinetic, energy, work, appeared only later.)


Is there a preferred sequence for buying upgrades in the armory?

Question: When playing campaign in StarCraft II there is not so much credits you can spend for the upgrades in the armory. Is there any preferred sequence in buying them?
Are there any more that other worth getting?
Answer: Okay, lame answer time.
The point of the campaign upgrade system is that the choice is yours to make. The developers have stated that they want you to play through the game multiple times, playing the missions in different orders, trying different research, getting different armory upgrades. So there is no correct answer for the "preferred sequence" - making different choices will give you a different experience, and a part of the fun is talking with your friends about how you did it differently and trying something else on a later playthrough.
Myself, I picked up enhanced Medics healing speed, increased armor, upped Bunker health, and Perdition Turrets to keep my base defenses alive and well without much assistance. Then I  upgraded my Reapers to go on entertaining adventures with my super-mobile special ops reaper brigade, and am having a blast with it.
None of the upgrades I have seen so far look useless. The only wrong decisions you can make are buying upgrades and then never using them - don't enhance your Marauders but then build only Marines - and not buying anything at all.
Comment: I can't agree with this strongly enough.  I like marine/marauder so I upgraded those, but I've already started a second play through around more factory based units.  I can't wait to try rushing ghosts with perma cloak.
Answer: Warning: This is clearly subjective.
For most early missions of the game, even on brutal, all you really need are the marine and medic upgrades. Marines and medics cost very little and have a lot of staying power against most enemy units.
Answer: Use what you upgrade, keep in mind you're going to end up with a basic base in later missions looking something like Barracks, Factory, Starport, Mercenary Building, then some sort of CC and pool of scvs.  It's basically the 2 Barracks straight tech build for player vs player but with a merc building in augment to the 2nd barracks.
The only difficult mission in Brutal Campaign is going to be the last one.  Generally, you'll want to upgrade mostly all of your buildings then think of what units you'd want to dedicate to for that mission.  A healthy unit composition is great, but keep in mind if you're strengthening certain units, you'll obviously want to make more of them.
In your final mission, you get to choose to shut off the air harass or the nydus harass.  The air harass will require you to create anti air units to stave off, while the nydus harass will have you leaving your base to kill worms.  Whichever you prefer to do, you'd be best probably in upgrading those units.  IE if you want to send reapers or bc's after the nydus or something, be nice to have those guys beastly.  OR let's say you want to stop the air, and you beast up some marines and medics and toss in some better rockets


Is it ok to sit on the top tube of my Cannondale CAAD12 (2017 model)?

Question: I just picked up my new Cannondale CAAD12 bike the other day. During my first ride I occasionally was sitting on the top tube (waiting for traffic lights etc). After the ride was over I actually realised this might be bad for the frame and I started to worry, especially because the CAAD12 frames are so light considering they are made from aluminium. Any ideas if sitting on the top tube of a lightweight alu bike like the CAAD12 is a no-no? I inspected the frame and it seems ok though, but I am not an expert. My weight is around 170 lbs.
Comment: @Criggie Presumably sitting [like this](https://www.youtube.com/watch?v=Uv4Li18UILM). A matter of comfort, rather than necessity. The pros do this all the time. They're rather lighter than the asker but I still wouldn't guess it'd do any harm.
Comment: @Criggie The first one doesn't seem implausible if the lights have a long cycle (no pun intended).
Comment: It doesn't seem any slower to get going from sitting on the toptube (as at the beginning of the video @DavidRicherby linked) than from standing over it.  If I'm expecting a long wait (just seen the lights change) I have to do that or risk getting cramp in my calf, especially at the end of a long ride -- or lean the bike further than I'd like to in the presence of cars
Comment: Related https://bicycles.stackexchange.com/questions/28758/would-a-person-standing-on-the-top-tube-damage-a-carbon-fibre-road-bike/28764#28764
Comment: I'll note that a bike designed for routine use should be able to take, without damage, a "hit" to the top tube when the cyclist falls.
Comment: Probably best not to, mostly because I can't picture how you sit on the top tube when stopped at the lights.  Is the standover height such that you have to sit on it?  Consider leaning the bike a big more, keeping the backside on the saddle and the prime foot on the pedal ready to go.
Comment: A bike frame must take your weight and more.
Comment: @DavidRicherby maybe - but I can't see how anyone would do any of that at a red light.
Comment: https://www.youtube.com/watch?v=BM0oQIRIyrs road bikes are a lot tougher than you think,  you are fine.
Answer: If sitting on your top tube causes damage to your bike, the bike wasn't very safe to begin with. I imagine while sitting you have at least one foot on the ground, so your entire mass is not resting on the tube.
The amount of force that a frame must withstand is much greater than the weight of the rider. If simply placing 170 lbs as a static load was enough to damage your frame, there would be no way to safely ride the bike.
Update: While not the most scientific test, I decided to try a worst case top tube sitting scenario. I went for a 5 mile ride last night on my alloy road bike. While not as high speed, low drag as the OP's rig, it is all I have. I, a rather large 6'2" 210lbs rider, sat on my top tube while riding for as much of my ride as possible. I do not recommend this, as it turned out to be very painful, and I got some looks form other riders for some reason. Most of the ride was on roads that are not in the best condition (holes, cracks, gravel). Again, I do not recommend this. After completing my 5 miles on the top tube, I brought my frame to work for some x-rays. I x-rayed the entire main triangle, as well as the welds to the rear triangle. I can report that there was no signs of any damage (cracking or deformation of the alloy). There was a bit of damage to me however, as apparently it is a bad idea to sit on the top tube while moving.
Update 2: the mechanical engineers I work with and I conducted an experiment. We got some aluminum alloy tubes (approx 1.25mm wall thickness). It as standard 6061, which is not exactly what is used in the OP's bike. We welded up a main triangle as close to the geometry of the OP's bike as we could. We have learned a few things from this:

None of us should quit our day jobs to build bike frames. Man is that hard.
The strength of the main triangle is insane, assuming the forces are perpendicular to any side. We were about to load a static load of over 350lbs on the "top tube" before it buckled.
Bike frames are not designed to handle rotational forces. The frame will twist up with relatively low force (a torque of about 200lbft.) This force was exerted at what would be the head tube.
Science is great.

In conclusion, go ahead and sit on your top tube at lights. Since I read this post I have started doing it and it can be quite comfortable.
Comment: Sitting on top tube is also a load the bike is necessarily designed for. Think about a soda can: strong enough to stand on, but still easily crushed from the sides.
Comment: So we have a crunched frame that we will be taking measurements from. We will then build a simulated frame and play around. We will be using 6061 alloy of a know thickness and will then try to take our findings and relate then to this exact model. We could do this through math, but things are slow right now and this should be very fun.
Comment: But the yield strength of the side it's still significantly higher than the maxim force that soda will exert. I spoke to some of the mechanical engineers I work with about this (also cyclists). They don't see am issue with this, but we will be conducting some experiments on this in the next few days.
Comment: I'll be waiting for updates. I wouldn't expect the tube to fail either, but I'm not trying it on my own bike.
Comment: That's quite an experiment. Nice. I hope you recover quickly.
Answer: It is a force the bike is designed for
Consider this  below.
Force 1 and 3 are the axles and they carry your weight.
Bending the ends of the top tube mimic force 2.
Is that force 2 equal to your weight - no.
But it is a force the top tube is designed to handle.
This is a force from the axle in a different direction that resulted in a bend failure of the top tube.  The top tube must be designed to take some very strong bending forces.
Think about if you want to bend the top tube. You could apply bending on both ends or you could stack it on two blocks and stand in the middle. At the molecular level it is resolved shear stress. I have stood on all my bikes - steel, aluminum, titanium, and carbon fiber.
Jump bikes but you see you see riders come down hard on the top tube.
Racers sitting on the top tube

Seat on top tube
Comment: I'm quite sure force 2 in first picture is not how physics works. Also, the question is not about bending the top tube but denting it.
Comment: Your second photo looks like a total bodge job and intended for a child to sit on. I don't think it demonstrates anything, other than that somebody has put a small weight on a top-tube and it hasn't broken yet. (Sure, I agree that top tubes should be strong enough to cope with people sitting on them; I'm just saying that this one photo is very poor eveidence that does nothing to strengthen your case.)
Comment: @DavidRicherby Think what you might
Comment: @ojs Be sure.  If you think the question about denting then feel free to present evidence a butt will dent a top tube.
Comment: I'll note that, in better-quality steel frames (I don't know what the practice is for Al or fiber), the tubes are "butted", meaning the walls are thicker at the ends than in the middle.  So your torque on the top tube is being applied to a thick portion of the tube -- probably 5-10 times stronger than the tube in the middle.