Do planets become globular by rotating about their own axis or by rotating about another bodies axis like the sun?

Peak Warming Man said:

Do planets become globular by rotating about their own axis or by rotating about another bodies axis like the sun?

neither

Peak Warming Man said:

Do planets become globular by rotating about their own axis or by rotating about another bodies axis like the sun?

Planets become globular because of gravity.

Even in the absence of a sun, even if they don’t rotate, a planet sized hunk of material will become roughly spherical because of gravity: the spherical shape minimised gravitational potential energy.

To further elucidate: Suppose tomorrow I create a non-rotating solid cube of cold granite, 10000 km on the side. Immediately (though slowly) the shear stresses in the cube will cause it to fracture and crumble into something fairly similar to a sphere.

Peak Warming Man said:

Do planets become globular by rotating about their own axis or by rotating about another bodies axis like the sun?

Isn’t it due to mass above a certain limit forming spheres because of gravity

dv said:

Peak Warming Man said:

Do planets become globular by rotating about their own axis or by rotating about another bodies axis like the sun?

Planets become globular because of gravity.

Even in the absence of a sun, even if they don’t rotate, a planet sized hunk of material will become roughly spherical because of gravity: the spherical shape minimised gravitational potential energy.

To further elucidate: Suppose tomorrow I create a non-rotating solid cube of cold granite, 10000 km on the side. Immediately (though slowly) the shear stresses in the cube will cause it to fracture and crumble into something fairly similar to a sphere.

So Bizaro world could never exist

OK, what is the force or the flatulence that makes galaxies become planar?

dv said:

Immediately (though slowly) the shear stresses in the cube will cause it to fracture and crumble into something fairly similar to a sphere.

You sure about the slowly bit?

Peak Warming Man said:

OK, what is the force or the flatulence that makes galaxies become planar?

Gravity again

cant get away from it

Peak Warming Man said:

OK, what is the force or the flatulence that makes galaxies become planar?

Gravity.

(+ inertia, but that is neither a force nor a flatulence).

The Rev Dodgson said:

dv said:

Immediately (though slowly) the shear stresses in the cube will cause it to fracture and crumble into something fairly similar to a sphere.

You sure about the slowly bit?

I suppose “slowly” is a bit vague. I was concerned that by saying “immediately” I would give an impression that this it will snap to a spheroid in a matter of seconds. By “immediately” I mean that it will immediately fracture and start to change.

Also big black holes are usually right in the middle of a galaxy.

they influence things too

dv said:

The Rev Dodgson said:

dv said:

Immediately (though slowly) the shear stresses in the cube will cause it to fracture and crumble into something fairly similar to a sphere.

You sure about the slowly bit?

I suppose “slowly” is a bit vague. I was concerned that by saying “immediately” I would give an impression that this it will snap to a spheroid in a matter of seconds. By “immediately” I mean that it will immediately fracture and start to change.

Furry nuff then.

I suppose it would take quite some time before it reached a state of equilibrium. (on a structural engineers timescale anyway).

I wonder if anyone has done an FEA of the process.

Would the same apply to artificial materials and/or structures would they also form a globe over time if they were originally constructed as a cube

Cymek said:

Would the same apply to artificial materials and/or structures would they also form a globe over time if they were originally constructed as a cube

Well, what artificial materials? Any material will fracture under sufficient shear stress.

The Rev Dodgson said:

dv said:

The Rev Dodgson said:

You sure about the slowly bit?

I suppose “slowly” is a bit vague. I was concerned that by saying “immediately” I would give an impression that this it will snap to a spheroid in a matter of seconds. By “immediately” I mean that it will immediately fracture and start to change.

Furry nuff then.

I suppose it would take quite some time before it reached a state of equilibrium. (on a structural engineers timescale anyway).

I wonder if anyone has done an FEA of the process.

you may want to pick a slightly more homogeneous and less structured engineering material for that particular calculation

Formation of disk galaxies

Why are the galaxies planar?

The Formation of Galaxies

How Galaxies Work

diddly-squat said:

The Rev Dodgson said:

dv said:

I suppose “slowly” is a bit vague. I was concerned that by saying “immediately” I would give an impression that this it will snap to a spheroid in a matter of seconds. By “immediately” I mean that it will immediately fracture and start to change.

Furry nuff then.

I suppose it would take quite some time before it reached a state of equilibrium. (on a structural engineers timescale anyway).

I wonder if anyone has done an FEA of the process.

you may want to pick a slightly more homogeneous and less structured engineering material for that particular calculation

Than “furry nuff” you mean? I can’t actually find properties for that. I’d probably use concrete anyway.

I vaguely remember analysing a square Earth myself for SSSF, years ago. It was only static though.

Obviously, over a long time such a block would also undergo physical changes … the energy from the collapse would lead to local melting, metamorphism would take place, compression would cause chemical changes, there’d be internal heating from all of this plus the fact that granite is slightly radioactive etc.
Over thousands and millions of years, this granite block would become even more spherical.

But it should collapse into something pretty ball-like within a month.

The Rev Dodgson said:

diddly-squat said:

The Rev Dodgson said:

Furry nuff then.

I suppose it would take quite some time before it reached a state of equilibrium. (on a structural engineers timescale anyway).

I wonder if anyone has done an FEA of the process.

you may want to pick a slightly more homogeneous and less structured engineering material for that particular calculation

Than “furry nuff” you mean? I can’t actually find properties for that. I’d probably use concrete anyway.

I vaguely remember analysing a square Earth myself for SSSF, years ago. It was only static though.

How long did it take to knock the rough edges off?

The Rev Dodgson said:

diddly-squat said:

The Rev Dodgson said:

Furry nuff then.

I suppose it would take quite some time before it reached a state of equilibrium. (on a structural engineers timescale anyway).

I wonder if anyone has done an FEA of the process.

you may want to pick a slightly more homogeneous and less structured engineering material for that particular calculation

Than “furry nuff” you mean? I can’t actually find properties for that. I’d probably use concrete anyway.

I vaguely remember analysing a square Earth myself for SSSF, years ago. It was only static though.

no, dv postulated a cube made of granite; I think your concrete cube would make life a little easier on your FEA analysis

dv said:

Obviously, over a long time such a block would also undergo physical changes … the energy from the collapse would lead to local melting, metamorphism would take place, compression would cause chemical changes, there’d be internal heating from all of this plus the fact that granite is slightly radioactive etc.
Over thousands and millions of years, this granite block would become even more spherical.

But it should collapse into something pretty ball-like within a month.

Based on the collapse speed of some large structures in NY about 16 years ago, I’d say the initial collapse would take about 25 minutes.

diddly-squat said:

The Rev Dodgson said:

diddly-squat said:

you may want to pick a slightly more homogeneous and less structured engineering material for that particular calculation

Than “furry nuff” you mean? I can’t actually find properties for that. I’d probably use concrete anyway.

I vaguely remember analysing a square Earth myself for SSSF, years ago. It was only static though.

no, dv postulated a cube made of granite; I think your concrete cube would make life a little easier on your FEA analysis

Even if I went for granite, I’d just treat it as weak concrete.

Density also plays a part in gravitational collapse

Densities can be distributed differently throughout the cloud of gas

Peak Warming Man said:

OK, what is the force or the flatulence that makes galaxies become planar?

Do planets become globular by rotating about their own axis or by rotating about another bodies axis like the sun?

OK, there is a standard model of planet formation. But it is also well known that the standard model doesn’t work, for a couple of different reasons.

The standard model is called “cold accretion”.

A solar system starts off as a cloud of cold hydrogen with impurities. The impurities include methane, water and silica compounds. This cold cloud has probably accumulated as a collection of cast-off matter from stars approaching the end of their life.

Then comes the nearby supernova. The shock wave compresses the cloud until it starts to collapse under the force of gravity. The matter from the supernova also injects more silicon and iron into the mix, as well as radioactive isotopes of which the most vital from the point of planet formation is aluminium-26.

The cloud of cold matter contracts and a star, or several stars, form in the centre. The number of stars, and the distribution of planets, depends on the angular momentum. As the stars light up, the radiation pressure starts pushing hydrogen gas out of the solar system, leaving solids behind that will eventually coalesce into planets.

At the same time, there is ionisation, electric charges bring together the solid components that go to make up the first dust grains, up to a maximum diameter of a millimeter or so.

The dust grains then grow by impact. Initially extremely hot, the first materials to crystallize out are diamonds, then calcium aluminate, then Forster’s. At this time what we have is called “chondrules”, spherical blobs of matter from 1 mm to 1 cm across. The chondrules coalesce further by impact into chondrites, where the chondrules are glued together by a low melting point amorphous material called “matrix”.

Gravity plays no part in the formation of small asteroids, only first electrostatic attraction and later inelastic impact, helped along by a modicum of heating.

Only when asteroids get to 2 to 10 metre across …

To be continued.

Peak Warming Man said:

OK, what is the force or the flatulence that makes galaxies become planar?

http://www.askamathematician.com/2014/04/q-why-are-many-galaxies-our-solar-system-and-saturns-rings-all-flat/

gravity and collisions.

My BOTE calcs indicate that the maximum shear in a homogeneous cube will be about

12 * G * rho^2 * s^2

where G is universal gravitational constant
rho is density
s is the length of the side

Density of granite is about 2800 kg/m^3.
Length of the side I’ve given as 10000000 m.
G is 6.67 × 10-11 m^3/kg/s^2

So that works about to be 52000 MPa.

Ultimate shear strength is usually about 0.5 to 0.9 times the ultimate tensile strength. So for granite it is going to be something in the vicinity of 25 MPa.

Cold-tempered steel has at best a USS of more like 1000 MPa, and I think that’s the highest for any material that can be manufactured in bulk. The shear stresses in a 10000 km cube of cold-tempered steel will be 400000 MPa, so it is still 400 times higher than the material can stand.

There is so much to learn.

dv said:

Ultimate shear strength is usually about 0.5 to 0.9 times the ultimate tensile strength. So for granite it is going to be something in the vicinity of 25 MPa.

Sounds high.

Tensile strength of concrete is about 3 MPa.

The Rev Dodgson said:

dv said:

Ultimate shear strength is usually about 0.5 to 0.9 times the ultimate tensile strength. So for granite it is going to be something in the vicinity of 25 MPa.

Sounds high.

Tensile strength of concrete is about 3 MPa.

Concrete, somewhat famously, is pissweak in tension.

dv said:

The Rev Dodgson said:

dv said:

Ultimate shear strength is usually about 0.5 to 0.9 times the ultimate tensile strength. So for granite it is going to be something in the vicinity of 25 MPa.

Sounds high.

Tensile strength of concrete is about 3 MPa.

Concrete, somewhat famously, is pissweak in tension.

That’s why they infest it with concrete cancer.

To be continued … Very briefly.

This is where aluminium-26 comes in. As the size increases beyond a few metres the heat from the 26Al starts to melt the asteroid.

Grows further into a rocky or icy planet whose eliipsoidal shape comes from the balance between rotation and gravity.

Rocky or icy core then grabs an atmosphere to become a gas giant like Jupiter.

Three problems with that scenario, but I don’t want to bore you with them.

To be continued … Very briefly.

This is where aluminium-26 comes in. As the size increases beyond a few metres the heat from the 26Al starts to melt the asteroid.

Grows further into a rocky or icy planet whose eliipsoidal shape comes from the balance between rotation and gravity.

Rocky or icy core then grabs an atmosphere to become a gas giant like Jupiter.

Three problems with that scenario, but I don’t want to bore you with them.