I bet you didn't know that this thread would have a part 2

Overheal

BaZmO* wrote: »

Yes. It’s one of the the reasons why you have lobbies in hotels, because the lobby breaks the air from rushing in once a door is open to the outside. Or using revolving doors also stops this from happening.

The venturi effect is essentially the velocity increasing due to constriction. The pressure drops but the velocity increases. It also applies to fluid dynamics.

Air IS a fluid.

valoren

Fourier wrote: »

So gravity must be from spacetime curving and not a force.

I've always liked Richard Feynman's elegant analogy for describing how spacetime is curved.

If you took an intelligent bug on a plane one dimensional surface it can start at point A, crawl 100 inches north, turn right (90 degrees), crawl another 100 inches, turn right again, crawl 100 inches, turn right again, crawl another 100 inches and it ends up at point A. This geometrically clever bug has created a square.



However on a Sphere (i.e. a curved surface) doing the exact same thing results in this;


If the surface is curved then it could never complete a square. It would always be off by some amount. The conclusion being that the bug accepts that it must be on a curved surface.

In three dimensions, a straight line (a geodesic) is defined as uniform velocity in a straight line, if we try the same sort of experiment in spacetime, we are unable to complete a rectangle



In the above diagram, H is the height in space between two clocks. We start both clocks for 100 elapsed seconds and see that we can't complete the rectangle as the clock at the higher space runs slower than the bottom clock. Like the intelligent bug whose square is all askew due to curvature, Einstein concluded that spacetime itself must be curved.

More in depth detail below.

http://www.feynmanlectures.caltech.edu/II_42.html

Chancer3001

You lost me with the bug on the sphere bit.

Why wouldn't it end up back at the starting point while walking a "square"?

New Home

Think about latitude and longitude lines on a globe: they don't form perfect squares. I think it's something like that, anyway.

Chancer3001

New Home wrote: »

Think about latitude and longitude lines on a globe: they don't form perfect squares. I think it's something like that, anyway.

But that's cos the sphere gets "fatter" towards the equator.

Surely if you walked 1000 miles in each direction youd 100% end up back where you started?

BaZmO*

Chancer3001 wrote: »

But that's cos the sphere gets "fatter" towards the equator.

Surely if you walked 1000 miles in each direction youd 100% end up back where you started?

The distances between latitude lines decrease the closer you get to the poles.



Back in secondary school a teacher put the following as a random question in one one of our commerce exams one year.

"You start at a point, you travel 10 miles south, then 10 miles east, then 10 miles north. You're back where you started. Where are you on earth?"

Fourier

Chancer3001 wrote: »

But that's cos the sphere gets "fatter" towards the equator.

Surely if you walked 1000 miles in each direction youd 100% end up back where you started?

It's actually because the Earth gets fatter toward the equator that walking 100 miles in each direction doesn't have you end up back where you started.

I'll put up a bit more of an explanation later today.

Chancer3001

Ah jaysus. This is confusing stuff.

Apparently a triangle on a sphere can have three 90degree angles.

I was always taught the angles in a trianhle add up to 180degrees.

ohnonotgmail

Chancer3001 wrote: »

Ah jaysus. This is confusing stuff.

Apparently a triangle on a sphere can have three 90degree angles.

I was always taught the angles in a trianhle add up to 180degrees.

On a flat surface they do.

Fourier

Chancer3001 wrote: »

Ah jaysus. This is confusing stuff.

Apparently a triangle on a sphere can have three 90degree angles.

I was always taught the angles in a trianhle add up to 180degrees.

You're not wrong, they add up to 180 degrees, provided the triangle is in a flat space. Like a sheet say.

However if it lies on a curved space like a sphere they won't add up to 180 degrees.

You can actually use how much they fail to add to 180 as a way to check how curved something is.

More to come soon.

seagull

Is it still a triangle if it's not on a flat surface?

ohnonotgmail

seagull wrote: »

Is it still a triangle if it's not on a flat surface?

it is still a triangle but the properties of a triangle that you are familair with from school dont apply. those properties, like the angles adding up to 180 and the Pythagorean theorem, only apply to the euclidean plane i.e. a flat surface.

Fourier

So here you can see both a square and a triangle on a sphere:



You can see on the square that two of the sides overshoot the point of closure and continue on past where they meet. Triangles have angles that sum to over 180 and also circles will have circumferences greater than pi times their diameter. This is called positive curvature.

On other shapes, like a horse saddle, two lines will undershoot the point of closure and fail to seal off the area, triangles will have angles that add to less than 180 and circles will have circumferences less than pi times their diameter. This is called negative curvature.



Another way of measuring it is how two lines move away from each other as they extend forward. Note how for positive curvature they flow toward each other. I'll mention this again below.



Of course spheres and saddles are special cases where they're negatively or positively curved everywhere. In other shapes it varies.

Like a kettle:


Or a wine glass:


Light blue is very negative curvature, Yellow is very positive curvature. If you draw a little square near a yellow point it will overshoot closing off. Or going back to my third picture, little lines drawn near it will flow together.

Now that's enough for 2D surfaces.

For 3D surfaces you can't use how a square closes off to check curvature, you need a cube. Instead of checking the angles of triangles, you need to check the angles of pyramids. And instead of circles, you need spheres.

A human can't imagine a curved 3D space, but mathematically it's basically as easy to handle as the 2D case.

And this is in fact leads to what Einstein's Field Equation says:


The left hand side measures how much cubes close off and the right hand side measures the density of mass.

What it's basically saying is that cubes will overshoot their closure more the more dense the matter is inside them. However using the way of thinking of curvature given in the third picture it means lines will flow together near points with high mass, i.e. things will move toward high mass => gravity.

Owl's eyes are not spherical like ours but are actually tubular.
As a result of this they cannot change the direction of their eyeballs (or should that be eyetubes?), hence the necessity that they have such an extensive range of motion in their head and neck.

Capt'n Midnight

Fourier wrote: »

It's actually because the Earth gets fatter toward the equator that walking 100 miles in each direction doesn't have you end up back where you started.

I'll put up a bit more of an explanation later today.

It's hotter at the equator , so things expand ?

The number of people to graduate from university/college in this country in 1952 was around 1,400, while in 2017 it was around 70,000. That is a 50 fold increase over a time period when the population as a whole didn't even double (increased by 60%).

In the late 60s the figure was about 3,500, so even since then it has increased 20-fold (population was then similar to the early 50s).

Edit: I just remembered that the numbers of women graduating in the 50s and 60s would have been very low and that the high numbers of today reflect their equal (probably higher?) participation in third level, but even taking that into account the numbers are interesting I think.

Also, source is page 3 of this ESRI paper from the late 60s: https://www.esri.ie/system/files/media/file-uploads/2012-09/GRS43.pdf

Another interesting note: "Irish industry at the present time (November 1968) only recruits about 100 graduates a year." :O. Different times!

Sephiroth_dude

Sections of the Autobahn in Germany have no speed limit.

There is a big building just off the M1 at Donabate, you can see one elevation of it as you're driving into the village, it even has its own wind turbine for power generation


This is the Tesco distribution centre for Ireland. All goods are shipped to Dublin port, brought there and then loaded up for distribution to the 150-odd Tescos around the country, including all the NI stores.


https://www.google.ie/maps/@53.4781389,-6.199248,3a,24.5y,39.42h,90.1t/data=!3m6!1e1!3m4!1sEq3Rwv6uO5nio6LnaeBTPg!2e0!7i13312!8i6656


It looks big, even from that point which is about 1.5km away. And it IS big.


Huge, in fact.


It has a footprint of over 80,000 sq. metres, which is impressive on it's own. But the fact that it is essentially one big rectangular cube means it has a volume that is even more impressive: over 1.5 million cubic metres. To put that in context, it is the 10th biggest building, by volume, in the world. In. The. World.

It was number 9 until the Finnish entry at number 8 was completed last year.

Edit: forgot the Wikipedia link

https://en.wikipedia.org/wiki/List_of_largest_buildings

Quazzie

Sephiroth_dude wrote: »

Sections of the Autobahn in Germany have no speed limit.

I know as I was just driving 250km/h on it two weeks ago :P

b318isp

Capt'n Midnight wrote: »

It's hotter at the equator , so things expand ?

At the equator, the effective gravitational acceleration is 9.7805 m/s2. This means that the true gravitational acceleration at the equator must be 9.8144 m/s2 (9.7805 + 0.0339 = 9.8144).

At the poles, the gravitational acceleration is 9.8322 m/s2. The difference of 0.0178 m/s2 between the gravitational acceleration at the poles and the true gravitational acceleration at the equator is because objects located on the equator are about 21 kilometers further away from the center of mass of the Earth than at the poles, which corresponds to a smaller gravitational acceleration.

https://en.wikipedia.org/wiki/Equatorial_bulge

Sephiroth_dude

Quazzie wrote: »

I know as I was just driving 250km/h on it two weeks ago :P

:eek::eek:

Esel

The mountain whose summit is furthest from the centre of the earth is Chimborazo in Ecuador.

https://en.m.wikipedia.org/wiki/Chimborazo

Skylinehead

Esel wrote: »

The mountain whose summit is furthest from the centre of the earth is Chimborazo in Mexico.

https://en.m.wikipedia.org/wiki/Chimborazo

Ecuador is not Mexico :pac:

The main reason for this is that the Earth bulges at the equator, giving it an extra 20km or so of distance to the centre compared to the poles.

Esel

Nixonbot wrote: »

Ecuador is not Mexico :pac:

The main reason for this is that the Earth bulges at the equator, giving it an extra 20km or so of distance to the centre compared to the poles.

It is now. I edited the Wiki page to make it so.

Everyone on this thread knows about the Earth's spare tyre already.

lapua20grain

Deleted User wrote: »

There is a big building just off the M1 at Donabate, you can see one elevation of it as you're driving into the village, it even has its own wind turbine for power generation


This is the Tesco distribution centre for Ireland. All goods are shipped to Dublin port, brought there and then loaded up for distribution to the 150-odd Tescos around the country, including all the NI stores.


https://www.google.ie/maps/@53.4781389,-6.199248,3a,24.5y,39.42h,90.1t/data=!3m6!1e1!3m4!1sEq3Rwv6uO5nio6LnaeBTPg!2e0!7i13312!8i6656


It looks big, even from that point which is about 1.5km away. And it IS big.


Huge, in fact.


It has a footprint of over 80,000 sq. metres, which is impressive on it's own. But the fact that it is essentially one big rectangular cube means it has a volume that is even more impressive: over 1.5 million cubic metres. To put that in context, it is the 10th biggest building, by volume, in the world. In. The. World.

It was number 9 until the Finnish entry at number 8 was completed last year.

Edit: forgot the Wikipedia link

https://en.wikipedia.org/wiki/List_of_largest_buildings

There are 700,000 pallets spaces with on average of 1.8million cases of stock picked and dispatched from there with a further approx 8k roll cages of slow moving stock trunked to there from other Tesco depots in the UK which is then distributed to ROI stores. The majority of stock is sourced in Ireland and not through Dublin port as you have stated, I know this as I worked in their supply chain and distribution.

Deleted User

Deleted User wrote: »

There is a big building just off the M1 at Donabate, you can see one elevation of it as you're driving into the village, it even has its own wind turbine for power generation


This is the Tesco distribution centre for Ireland. All goods are shipped to Dublin port, brought there and then loaded up for distribution to the 150-odd Tescos around the country, including all the NI stores.


https://www.google.ie/maps/@53.4781389,-6.199248,3a,24.5y,39.42h,90.1t/data=!3m6!1e1!3m4!1sEq3Rwv6uO5nio6LnaeBTPg!2e0!7i13312!8i6656


It looks big, even from that point which is about 1.5km away. And it IS big.


Huge, in fact.


It has a footprint of over 80,000 sq. metres, which is impressive on it's own. But the fact that it is essentially one big rectangular cube means it has a volume that is even more impressive: over 1.5 million cubic metres. To put that in context, it is the 10th biggest building, by volume, in the world. In. The. World.

It was number 9 until the Finnish entry at number 8 was completed last year.

Edit: forgot the Wikipedia link

https://en.wikipedia.org/wiki/List_of_largest_buildings

Now I know where to head when the zombie apocalypse hits.

Sleepy

I pass the Tesco Distribution Centre every day. Our Zombie apocalypse plan involves raiding it before heading out to Lambay

Esel

Sleepy wrote: »

I pass the Tesco Distribution Centre every day. Our Zombie apocalypse plan involves raiding it before heading out to Lambay

You can eat the wallabies there too.

Capt'n Midnight

https://xkcd.com/2129/

mzungu

The Eiffel Tower was originally supposed to be in Barcelona. But thinking the thing would end up looking like an eyesore, the city rejected Gustave Eiffel's plans, and he was forced to repitch the project elsewhere. Luckily, Eiffel found a home for his idea in Paris. The rest is history.