Lucky Ten Thousand (TIL)

Things that don't belong anywhere else. (Check first).

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Zohar
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Re: Today I Learned

Postby Zohar » Fri Mar 03, 2017 1:37 pm UTC

We use an octagonal stop sign in Israel as well. Not sure how common it is in the rest of the world.
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HES
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Re: Today I Learned

Postby HES » Fri Mar 03, 2017 2:15 pm UTC

Very. It's probably the most widely recognisable sign in the world.
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Liri
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Re: Today I Learned

Postby Liri » Fri Mar 03, 2017 2:20 pm UTC

Zohar wrote:We use an octagonal stop sign in Israel as well. Not sure how common it is in the rest of the world.

I'm going to guess it's an export of the US. Like how planes and ships have to pass by each other on the right-hand side.
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Flumble
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Re: Today I Learned

Postby Flumble » Fri Mar 03, 2017 4:18 pm UTC

Liri wrote:
Zohar wrote:We use an octagonal stop sign in Israel as well. Not sure how common it is in the rest of the world.

I'm going to guess it's an export of the US. Like how planes and ships have to pass by each other on the right-hand side.

Yes, it's described in the History section of the wikipedia page linked by ObsessoMom.

US standardizes it -> other countries (kind of) copy it -> UN conventionizes it -> at least half the world uses it.

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Snake
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Re: Today I Learned

Postby Snake » Mon Mar 06, 2017 4:19 pm UTC

Today I learned that in Japanese culture there is a word Ikigai, meaning "the reason of my life", or even "the reason i wake up every morning". According to Japanese, everyone has an Ikigai in life but it takes time to find it (still thinking: what is my Ikigai)...Also the Japanese have this white-eyed doll Daruma bringing luck and also helping to set up goals: when you have one in mind you paint one eye, once the goal is achieved, you can paint another eye.

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Re: Today I Learned

Postby Felstaff » Sun Mar 12, 2017 10:16 pm UTC

TIL what jizz means.
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addams
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Re: Today I Learned

Postby addams » Mon Mar 13, 2017 2:00 am UTC

I learned something.
Deer Stink.

I live where there are a great many wild deer.
I have habituated the ones in my little garden.

They are not afraid of me.
They eat from my hands.

I can scratch some of them behind their ears
and on their withers and even their little antler buds.

Being that close to Deer,
I've learned they have a smell.

It's kind of a Goat like smell.
I don't like it. (gack)

I like the smell of horses.
I don't like the smell of Deer nor Goat

Now that I know the smell,
I can smell them when I can't see them.

I'm a little surprised how often they are close to me and I didn't see them.
I would be playing in the garden and think I was smelling foliage.

Now, I know that smell is Deer.
I still like being able to feed them by hand.

We Can't make them go away.
I may as well enjoy them.

Did I tell you?
I learned Deer will climb stairs to get to my peaches?

They will eat stuff they don't eat, if I put it in a pot and nurture it.
Basically Deer are Stinky Jerks. But; They are pretty stinky jerks.
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Re: Today I Learned

Postby Magnanimous » Mon Mar 13, 2017 1:59 pm UTC

TIL about sundial cannons. At noon, sunlight focuses through the lens and ignites the tiny cannon, so it's a mechanical alarm clock.
Spoiler:
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Re: Today I Learned

Postby addams » Sun Mar 19, 2017 7:34 am UTC

The funniest thing I learned today:
"The Twelve Apostles shared a Honda Accord,"

I also learned Water Gardens thrive with Clay Soil.
That is terrible soil for any other kind of gardening.

My cup runneth over with clay soil.
Life is, just, an exchange of electrons; It is up to us to give it meaning.

We are all in The Gutter.
Some of us see The Gutter.
Some of us see The Stars.
by mr. Oscar Wilde.

Those that want to Know; Know.
Those that do not Know; Don't tell them.
They do terrible things to people that Tell Them.

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Re: Today I Learned

Postby ObsessoMom » Fri Mar 31, 2017 4:37 pm UTC

addams, you might enjoy this compilation (should I say "pile-up"?) of Biblical cars.

Today I Learned that the bird known as the roadrunner, like all members of the cuckoo family, exhibits zygodactyly (feet with toes facing both forward and backward). I would have thought this a significant disadvantage for running. Huh.

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Re: Today I Learned

Postby addams » Sat Apr 01, 2017 1:29 am UTC

Yes.
I do enjoy a little 'light-hearted' auto-religious humor.

Thank you, ObsessoMom.
Life is, just, an exchange of electrons; It is up to us to give it meaning.

We are all in The Gutter.
Some of us see The Gutter.
Some of us see The Stars.
by mr. Oscar Wilde.

Those that want to Know; Know.
Those that do not Know; Don't tell them.
They do terrible things to people that Tell Them.

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Flumble
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Re: Today I Learned

Postby Flumble » Sun Apr 02, 2017 1:34 am UTC

TIL about the Runge-Kutta methods. Hopefully it will fix my discrete-time physics simulation going completely bonkers* when the time step is a bit large.

*the critically-damped struts currently act like they're anti-damped

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Re: Today I Learned

Postby addams » Sun Apr 02, 2017 4:04 am UTC

I sleep easy knowing there are Posters Out There SomeWhere;
That are able to not only understand but also use that information.
Life is, just, an exchange of electrons; It is up to us to give it meaning.

We are all in The Gutter.
Some of us see The Gutter.
Some of us see The Stars.
by mr. Oscar Wilde.

Those that want to Know; Know.
Those that do not Know; Don't tell them.
They do terrible things to people that Tell Them.

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Flumble
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Re: Lucky Ten Thousand (TIL)

Postby Flumble » Sun Apr 02, 2017 5:27 pm UTC

Let's make it more concrete. :)

I'm trying to simulate an airplane in javascript. When it's touching the ground, it should rest on its landing gear —which has springs to avoid an infinite acceleration while landing or other weird hacks. The springs are damped by the way so they don't bounce for all eternity. Unfortunately, because the simulation does steps of as much as 1/30th of a second, it turns out that the easiest way to simulate it* is unstable**. And since I'd rather not do 10 times (or whatever it takes) as many simulation steps, I was looking for alternative methods to make the springs behave and learned about RK4.

[edit]good news everyone, with RK4 implemented it is wonderfully stable. Even if I drop the plane from 1km, it just sinks through the ground (the spring really is more like a rubber sheet at 1m height and the ground is just a hologram) and comes back up.[/edit]

*calculate acceleration (per step duration), then add acceleration to speed, then add speed to position
**At least at this scale. It works fine if you make the springs 100 times as long or gravity 10 times as small or time 10 times as slow.
Last edited by Flumble on Mon Apr 03, 2017 2:42 am UTC, edited 1 time in total.

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rivulatus
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Re: Lucky Ten Thousand (TIL)

Postby rivulatus » Sun Apr 02, 2017 10:47 pm UTC

TIL: That you are slower at reading sentences that are written in all UPPER CASE then reading a sentence of Mixed/lower case words.

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Re: Lucky Ten Thousand (TIL)

Postby PM 2Ring » Mon Apr 03, 2017 12:47 pm UTC

Flumble wrote:[edit]good news everyone, with RK4 implemented it is wonderfully stable. Even if I drop the plane from 1km, it just sinks through the ground (the spring really is more like a rubber sheet at 1m height and the ground is just a hologram) and comes back up.[/edit]


Yay! You might also like to take a look at a symplectic integrator, like Leapfrog or Verlet. They aren't as well-known as R-K, but their big advantage is that (unlike Euler or R-K) they conserve energy, which is kinda useful in sims of physical systems. ;)

As I said in this old post
PM 2Ring wrote:Lots of sites promote RK4 (and its variants), especially sites oriented to game writers, and it's certainly a good general-purpose integrator, but it's not really the best choice for gravity sims. I recommend Leapfrog integration: it's quite easy to code, but it's main feature is that it conserves energy, which RK4 does not, so over the long term Leapfrog will be more accurate than RK4. Verlet might be slightly more accurate than Leapfrog over time scales of the order of your time step, but over longer time scales they are equivalent, IIRC.

Another feature of Leapfrog is its reversability. As Wikipedia says, "One can integrate forward n steps, and then reverse the direction of integration and integrate backwards n steps to arrive at the same starting position".


But maybe this is a topic for the Coding. Mathematics, or Science fora...
Last edited by PM 2Ring on Mon Apr 03, 2017 4:07 pm UTC, edited 1 time in total.

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Re: Lucky Ten Thousand (TIL)

Postby WibblyWobbly » Mon Apr 03, 2017 2:46 pm UTC

PM 2Ring wrote:
Flumble wrote:[edit]good news everyone, with RK4 implemented it is wonderfully stable. Even if I drop the plane from 1km, it just sinks through the ground (the spring really is more like a rubber sheet at 1m height and the ground is just a hologram) and comes back up.[/edit]


Yay! You might also like to take a look at a symplectic integrator, like Leapfrog or Vervet. They aren't as well-known as R-K, but their big advantage is that (unlike Euler or R-K) they conserve energy, which is kinda useful in sims of physical systems. ;)

As I said in this old post
PM 2Ring wrote:Lots of sites promote RK4 (and its variants), especially sites oriented to game writers, and it's certainly a good general-purpose integrator, but it's not really the best choice for gravity sims. I recommend Leapfrog integration: it's quite easy to code, but it's main feature is that it conserves energy, which RK4 does not, so over the long term Leapfrog will be more accurate than RK4. Vervet might be slightly more accurate than Leapfrog over time scales of the order of your time step, but over longer time scales they are equivalent, IIRC.

Another feature of Leapfrog is its reversability. As Wikipedia says, "One can integrate forward n steps, and then reverse the direction of integration and integrate backwards n steps to arrive at the same starting position".


But maybe this is a topic for the Coding. Mathematics, or Science fora...

Sorry, don't mean to harp on typos, but this one might cause confusion - it's "Verlet", isn't it?

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Re: Lucky Ten Thousand (TIL)

Postby PM 2Ring » Mon Apr 03, 2017 4:13 pm UTC

WibblyWobbly wrote:Sorry, don't mean to harp on typos, but this one might cause confusion - it's "Verlet", isn't it?

Oops! Thanks for that!

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Flumble
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Re: Lucky Ten Thousand (TIL)

Postby Flumble » Mon Apr 03, 2017 7:19 pm UTC

PM 2Ring wrote:Yay! You might also like to take a look at a symplectic integrator, like Leapfrog or Verlet. They aren't as well-known as R-K, but their big advantage is that (unlike Euler or R-K) they conserve energy, which is kinda useful in sims of physical systems. ;)

Thanks for the suggestion. Though for this (game-level) simulation I'm using quite a bit of "poetic freedom" already, so I'll stick with RK4.

Speaking of conserving energy: I was perplexed to read that, if you apply a force F at a point p on an object, the object gets a linear force of F and a torque of F×p.
Thinking about it more, the torque part makes sense, because, if you consider some second force at the object's center that cancels the linear force, there should be a net torque of F×p, to which only the first force can contribute. And the linear part makes sense too, because, if you consider a second force F at -p, the torque should cancel out (and not produce a linear force, because) and the total linear force should be 2F, so the linear force from either one of the forces must be F.


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