Why aren't less massive objects also spherical like planets and moons?

[Ethan Singer]

given that gravity pulls things together into spheres, how much mass is needed to do so? Smaller objects such as asteroids, meteors, and that bench in the park don't just turn into spheres, because they lack enough gravitational force to dominate the shape. So at approximately how massive do objects need to be to condense into a sphere?

you would think that the gravity needed to overcome the force of it's constituent particles shaping themselves into a park bench wouldn't be that large... Wouldn't denser objects condense into a sphere, because there's more gravitational influence per cubic meter? In fact, wouldn't the size of objects such as park benches floating in space give us a clue into their composition, because larger non-spheracle objects might be less dense and therefore exert less gravity?

 

[phinds]

...you would think that the gravity needed to overcome the force of it's constituent particles shaping themselves into a park bench wouldn't be that large...

Well, YOU might think that but anyone who has studied physics wouldn't.

Wouldn't denser objects condense into a sphere, because there's more gravitational influence per cubic meter?

, by that logic, any large metal object should condense into a sphere. You may have noticed that they don't.

In fact, wouldn't the size of objects such as park benches floating in space give us a clue into their composition

No

because larger non-spheracle objects might be less dense and therefore exert less gravity?

No clear what you mean by that.

Furthermore, If the Earth weren't a sphere, suppose it's gravity wasn't enough to condense it into it, how would we know?

We're clever. We'd figure it out

Could we measure such a difference?

yes

What if it's a lot bumpy-er than Nasa let's us know?

OK, if you are going start a conspiracy theory that NASA is lying to us, you need to find another forum to do it on.

Why aren't mountains shown on the "globe"?

Because they don't stick out far enough

What if it's not even flat, but a tetrahedron? Tetrahedrons would have giant pointy things going out to space you know... Now there's your conspiracy.

Again, that's not a discussion for this forum

 

[jim mcnamara]

PS: the Earth has a tummy bulge problem at the equator, so it is not a true sphere anyway. It is best represented by an oblate spheroid.
https://en.wikipedia.org/wiki/Figure_of_the_Earth.

FWIW the shape of the Earth was correctly known long before we had NASA or satellites. Isaac Newton found it.
https://www.scientificamerican.com/article/earth-is-not-round/

@Ethan Singer Please stop alluding to nonsense, it is the fast track to thread closure.

 

[Ethan Singer]

"by that logic, any large metal object should condense into a sphere. You may have noticed that they don't"

By large I meant larger than a park bench, but smaller than the moon. When I said denser objects would most likely become spheres, I should've clarified:

Suppose you're in space and have a magic machine that can produce uranium out of nothing. No, you have TWO machines that produce Uranium and Lead out of nothing (and at the same rate). So you start producing a clump of uranium here, and a clump of Lead there. I say that the clump of uranium will even out and become a sphere before that lead clump does, because the uranium has more mass, and would exert more force per cubic area. Now obviously the clump would have to be really large, and the pump would have to be on for a loooong time, but you get my point.

"OK, if you are going start a conspiracy theory that NASA is lying to us"

Poe's law. That entire third paragraph was a joke... I figured it'd be obvious. Deleting it.

 

[phinds]

Suppose you're in space and have a magic machine that can produce uranium out of nothing. No, you have TWO machines that produce Uranium and Lead out of nothing (and at the same rate). So you start producing a clump of uranium here, and a clump of Lead there. I say that the clump of uranium will even out and become a sphere before that lead clump does, because the uranium has more mass, and would exert more force per cubic area. Now obviously the clump would have to be really large, and the pump would have to be on for a loooong time, but you get my point.

Yes, I think (aside from the impossibility of actually doing what you state) that you are correct assuming the clumps have the same size and shape but different masses

Poe's law. That entire third paragraph was a joke... I figured it'd be obvious. Deleting it.

Once a post has been replied to, modifying it is bad form. You can add a comment at the end (if you get to it soon enough, since edit ability lapses after some number of hours) or just make a note in a subsequent post.

 

[davenn]

Smaller objects such as asteroids, meteors, and that bench in the park don't just turn into spheres, because they lack enough gravitational force to dominate the shape. So at approximately how massive do objects need to be to condense into a sphere?

No, that isn't the reason

Suppose you're in space and have a magic machine that can produce uranium out of nothing. No, you have TWO machines that produce Uranium and Lead out of nothing (and at the same rate). So you start producing a clump of uranium here, and a clump of Lead there. I say that the clump of uranium will even out and become a sphere before that lead clump does, because the uranium has more mass, and would exert more force per cubic area. Now obviously the clump would have to be really large, and the pump would have to be on for a loooong time,

if it ( the masses of lead and uranium) is already solid or near solid, then it won't produce a sphere

but you get my point.

no, because you haven't thought this throughdo you know or understand WHEN and WHY/HOW the earth, moon, other planets etc became spheres ?Dave

 

[phinds]

f it ( the masses of lead and uranium) is already solid or near solid, then it won't produce a sphere

Dammit, you're right. I hate it when you do that

 

[mfb]

A sufficiently large clump of uranium or lead will form a sphere. “Solid” just means there is some resistance, but solids are not magical objects. At some point gravity wins and makes a sphere out of it (or a spheroid, with rotation). For the same reason Earth cannot have 20 km tall mountains (but Mars with its lower surface gravity can).
For typical rocks in the solar system the critical size is somewhere around 1000 km diameter. Uranium has a much higher density but it is also a stronger material, I don’t know what wins. Lead is quite weak.

 

[davenn]

A sufficiently large clump of uranium or lead will form a sphere. “Solid” just means there is some resistance, but solids are not magical objects. At some point gravity wins and makes a sphere out of it (or a spheroid, with rotation). For the same reason Earth cannot have 20 km tall mountains (but Mars with its lower surface gravity can).
For typical rocks in the solar system the critical size is somewhere around 1000 km diameter. Uranium has a much higher density but it is also a stronger material, I don’t know what wins. Lead is quite weak.

But you understand where I am trying to get the OP to consider

the Earth and other planets etc, forms their spherical shape when they were, for the most part, still molten. When, under the influence of gravity, forming spheres is very easy.

Dave

 

[mfb]

The planets form molten for the same reason they get spherical. The gravitational energy is larger than the binding energy between atoms in the solids (apart from some prefactors) - the impacts release enough energy to melt the material. Smaller asteroids don't melt from impacts.

 

[Bandersnatch]

approximately how massive do objects need to be to condense into a sphere?

For typical rocks in the solar system the critical size is somewhere around 1000 km diameter.

It's actually closer to 400-600 km diameter, depending on composition (icy-rocky respectively). This paper goes through the derivation in an entertaining fashion:
The Potato Radius: a Lower Minimum Size for Dwarf Planets, Charles H. Lineweaver, Marc Norman

 

[mfb]

Well, it was a very rough estimate. In terms of experimental results: Vesta with 520 km mean diameter is not very round, Ceres with 900 km is round. All other asteroids with a measured shape are either smaller than Vesta or larger than Ceres.
Iapetus at 1500 km is not in hydrostatic equilibrium, while Enceladus at just 250 km is - but with an ocean and an ice crust that is not surprising.