What in gravity makes water on Earth into a ball?

[doglover9754]

So, from my last thread “Why doesn’t the water fall out of the Earth?” I asked if the reason why water doesn’t fall out of the Earth even on its tilted axis. Then the conclusion was gravity. To be more precise, gravity is the one that pulled the water on Earth into a ball form. What in gravity forms the ball? Is there something in the formula for gravity that is similar or close enough to the formula of a sphere? Like why did gravity pull all of the earth’s materials to form what we have today; a giant sphere that supports life? Why a sphere? I mean, it’s could've been any other shape like a cube or something, but why a sphere? There is something in common with the formula for gravity and the formula for a sphere. If anyone could clear this up for me, that would be awesome! I’m sorry if this is a troublesome/stupid question again. As I said before, I’m just your everyday weird and analytical kid who’s constantly thinking like this.

 

[doglover9754]

Honestly, I’m not even sure if this is in the right forum category... if it’s in the wrong place, I’m very sorry! I’m still trying to get used to most of this

 

[haruspex]

any other shape like a cube

Least potential energy.
Or consider the overall direction of gravity at some point on a face of a cube made of water or sand, but not the centre of the face. Near one corner perhaps. Being attracted to every other part of the cube, the net pull would be (roughly) towards the centre of the cube... not exactly because of the stronger pull towards the nearer parts, etc., but certainly not perpendicular to the face. So there would be a tendency to roll or run towards the centre of the face. That would continue until a sphere is formed.

 

[doglover9754]

Least potential energy.
Or consider the overall direction of gravity at some point on a face of a cube made of water or sand, but not the centre of the face. Near one corner perhaps. Being attracted to every other part of the cube, the net pull would be (roughly) towards the centre of the cube... not exactly because of the stronger pull towards the nearer parts, etc., but certainly not perpendicular to the face. So there would be a tendency to roll or run towards the centre of the face. That would continue until a sphere is formed.

Ahh... that makes sense... thanks!

 

[Drakkith]

It's basically the same reason that you yourself can't hold up a mountain. The weight of the mountain is far too large and you'd just be squished into a pancake. Well, the same is true for water, gasses, and even rocks. The immense weight of the material above squishes the lower material, spreading it out into a roughly spherical shape. This is also one reason that mountains are typically pointy, with a narrow peak and a wide base. A straight cylinder the height of Mount Everest would not be able to support its own weight. The bottom would be squished and would spread out until the weight is more evenly spread out.

Now, it's not quite intuitive why this spreading out results in a spherical shape instead of just a flat, mountain shape. Well, imagine that instead of having a cylinder on the Earth's surface, we have a hundred large cylinders pointing outwards from a central point with their bases all resting on each other, held in place by their own gravity. In other words, cylinder A is held in place by the gravity of cylinder B, C, D, etc, and the same is true for the others. When each of these cylinders begins to get squished, the shape formed by their pancaking bases is roughly spherical.

 

[doglover9754]

It's basically the same reason that you yourself can't hold up a mountain. The weight of the mountain is far too large and you'd just be squished into a pancake. Well, the same is true for water, gasses, and even rocks. The immense weight of the material above squishes the lower material, spreading it out into a roughly spherical shape. This is also one reason that mountains are typically pointy, with a narrow peak and a wide base. A straight cylinder the height of Mount Everest would not be able to support its own weight. The bottom would be squished and would spread out until the weight is more evenly spread out.

Now, it's not quite intuitive why this spreading out results in a spherical shape instead of just a flat, mountain shape. Well, imagine that instead of having a cylinder on the Earth's surface, we have a hundred large cylinders pointing outwards from a central point with their bases all resting on each other, held in place by their own gravity. In other words, cylinder A is held in place by the gravity of cylinder B, C, D, etc, and the same is true for the others. When each of these cylinders begins to get squished, the shape formed by their pancaking bases is roughly spherical.

Ok... I get it... but I’m still confused. What in gravity causes this balance (I honestly don’t know what to call it)? Just gravity in general or just a part of gravity?

 

[Drakkith]

Ok... I get it... but I’m still confused. What in gravity causes this balance (I honestly don’t know what to call it)? Just gravity in general or just a part of gravity?

It's not that this is part of gravity, it's that this is the result of gravity's interaction with matter under a certain set of conditions. The most important condition is that the amount of matter needs to be large enough to give the resulting body enough mass. Otherwise gravity isn't strong enough to compress and flatten this matter into a spherical shape. This is why most asteroids are oddly shaped. They simply don't have enough mass for gravity to compress them into a sphere.

Put another way, you won't find any gravitational rule or law that says that gravity will make an object spherical. Instead you have to look at the conditions of a system (a "system" being a collection of matter in this case) and then apply the rules of gravitation, mechanics, and several other areas to determine what will happen. Because we've already done this for many different types of objects, we know that if an object is above a certain mass limit then gravity will always turn it into a spherical shape. There's just no way that regular matter can withstand the immense weight without being compressed into a sphere if an object is above a certain mass limit, regardless of the exact type of matter (rock, ice, liquid water, hydrogen gas, etc).

 

[doglover9754]

It's not that this is part of gravity, it's that this is the result of gravity's interaction with matter under a certain set of conditions. The most important condition is that the amount of matter needs to be large enough to give the resulting body enough mass. Otherwise gravity isn't strong enough to compress and flatten this matter into a spherical shape. This is why most asteroids are oddly shaped. They simply don't have enough mass for gravity to compress them into a sphere.

Put another way, you won't find any gravitational rule or law that says that gravity will make an object spherical. Instead you have to look at the conditions of a system (a "system" being a collection of matter in this case) and then apply the rules of gravitation, mechanics, and several other areas to determine what will happen. Because we've already done this for many different types of objects, we know that if an object is above a certain mass limit then gravity will always turn it into a spherical shape. There's just no way that regular matter can withstand the immense weight without being compressed into a sphere if an object is above a certain mass limit, regardless of the exact type of matter (rock, ice, liquid water, hydrogen gas, etc).

Ohhhhh! Ok... that makes more sense

 

[sophiecentaur]

Why a Sphere?
If you take a sphere of an obviously soft material for convenience. Water is hard to grab hold of and move it but it's the same principle. Put the sphere out in space, well away from anything else of significant mass. A good explanation to justify the sphere is based on Energy, rather than Forces.
If you want to change its shape - say into a sausage shape - you will need to do some work on it to push the sides in together and pull out the ends away from the centre of mass. That means you will be increasing the overall Potential Energy. Any system will tend to return eventually to a more stable shape with less potential energy. The most stable shape will have minimal Potential Energy and the symmetry of a sphere means that it has the lowest potential energy (if the material can flow). A rubber sausage has a stable sausage shape because the electric forces between molecules keeping it in shape will balance out the gravitational forces. (Electric plus Gravitational Potentials will be a minimum as a sausage shape) Release the Electric forces (i.e. melt it a bit) and the stable shape will then be a sphere. Rocky planets, when they're big enough, have molten cores for long enough for the gravity to overcome the electric forces and they become spherical - except that they all spin to some extent, in which case the forces required to keep them together are too weak to maintain a spherical shape and they are fatter around the equator. Even stars also tend to spin and they are not perfect spheres, either. (Difficult to spot, except in the case of the Sun, I think)

 

[doglover9754]

Why a Sphere?
If you take a sphere of an obviously soft material for convenience. Water is hard to grab hold of and move it but it's the same principle. Put the sphere out in space, well away from anything else of significant mass. A good explanation to justify the sphere is based on Energy, rather than Forces.
If you want to change its shape - say into a sausage shape - you will need to do some work on it to push the sides in together and pull out the ends away from the centre of mass. That means you will be increasing the overall Potential Energy. Any system will tend to return eventually to a more stable shape with less potential energy. The most stable shape will have minimal Potential Energy and the symmetry of a sphere means that it has the lowest potential energy (if the material can flow). A rubber sausage has a stable sausage shape because the electric forces between molecules keeping it in shape will balance out the gravitational forces. (Electric plus Gravitational Potentials will be a minimum as a sausage shape) Release the Electric forces (i.e. melt it a bit) and the stable shape will then be a sphere. Rocky planets, when they're big enough, have molten cores for long enough for the gravity to overcome the electric forces and they become spherical - except that they all spin to some extent, in which case the forces required to keep them together are too weak to maintain a spherical shape and they are fatter around the equator. Even stars also tend to spin and they are not perfect spheres, either. (Difficult to spot, except in the case of the Sun, I think)

Ok... I think I got it. Thank you guys! It’s so much clearer and my mind finally feels at ease (especially on my first day back to school. I guess that means no more coming on too often for me )

 

[paulo84]

I would love more insight on 'There is something in common with the formula for gravity and the formula for a sphere'?

 

[anorlunda]

It is not just gravity. Water droplets also tend to be spherical even without gravity. That is true with water from your garden hose, or a weightless drop of water on the space station. The reason is the same as with planets, because it minimizes the potential energy.

The picture above is from a very entertaining paper, The Potato Radius: a Lower Minimum Size for Dwarf Planets you might have fun reading it.

 

[dkotschessaa]

If I may say so, I wish we had more questions like this on PF, and more question askers like @doglover9754

People may think the content is basic, but explaining it in simple terms is not easy. The responses are great and enjoyable to read.

 

[russ_watters]

Ok... I get it... but I’m still confused. What in gravity causes this balance (I honestly don’t know what to call it)? Just gravity in general or just a part of gravity?

The force that brings things together, but the balance/result of the force isn't gravity, it's geometry: if you have an object to your left and an object to your right and you pull them together, the meet in the middle. That's 1D symmetry. A symmetrical 3D object is a sphere.

 

[Chestermiller]

It is not just gravity. Water droplets also tend to be spherical even without gravity. That is true with water from your garden hose, or a weightless drop of water on the space station. The reason is the same as with planets, because it minimizes the potential energy.

View attachment 218192
The picture above is from a very entertaining paper, The Potato Radius: a Lower Minimum Size for Dwarf Planets you might have fun reading it.

For a droplet from a garden hose, isn't it surface tension that does it, rather than gravity.

 

[anorlunda]

For a droplet from a garden hose, isn't it surface tension that does it, rather than gravity.

Yes. That's exactly my point. Many forces, not just gravity, pull things into spherical shapes.

The OP was asking about what is special about gravity that makes spheres. See below.

What in gravity forms the ball? Is there something in the formula for gravity that is similar or close enough to the formula of a sphere?

 

[doglover9754]

I would love more insight on 'There is something in common with the formula for gravity and the formula for a sphere'?

What I’m getting from all the info is it’s pretty much a cause and effect kind of reaction. Because there is gravity pulling all kinds of ways, the shape of the Earth is a sphere.

 

[doglover9754]

If I may say so, I wish we had more questions like this on PF, and more question askers like @doglover9754

Aw... thanks! I’ve never heard anyone say that before! All my friends call me weird, too smart, and all thoes names that makes me feel like I don’t belong... you cheered up my day! If I could, I’d give that like 100 likes! (And that’s not me being sweet... it’s my honest opinion)

 

[sophiecentaur]

Many forces, not just gravity, pull things into spherical shapes.

Yes. In particular, it's the Electric forces of attraction between adjacent molecules. The forces between the molecules on the surface are asymmetrical and they do not 'pull outwards' but approximately over a hemisphere, facing inwards. The equilibrium position of all the molecules will tend to bring them together to form a sphere. That's the minimum PE condition.

What I’m getting from all the info is it’s pretty much a cause and effect kind of reaction. Because there is gravity pulling all kinds of ways, the shape of the Earth is a sphere.

As has already been stated, it isn't just gravity. Gravity dominates with large objects because gravitational forces from each molecule add up (no negative g forces). In large objects, the equal numbers of protons and electrons tends to cancel the effect of Electric forces (attractions and repulsions almost balance out) in large objects so G dominates.
But remember, the individual E forces are massive compared with the individual weak gravitational forces. The life of an Elephant is dominated by gravity whilst the life of a mosquito is dominated by electric forces of nearby molecules. For a mozzie the air is more like a very light oil, which slow everything up. If it stops flying, it drifts very slowly downwards. On the other hand, an Elephant . . . . . .

 

[doglover9754]

Yes. In particular, it's the Electric forces of attraction between adjacent molecules. The forces between the molecules on the surface are asymmetrical and they do not 'pull outwards' but approximately over a hemisphere, facing inwards. The equilibrium position of all the molecules will tend to bring them together to form a sphere. That's the minimum PE condition.

As has already been stated, it isn't just gravity. Gravity dominates with large objects because gravitational forces from each molecule add up (no negative g forces). In large objects, the equal numbers of protons and electrons tends to cancel the effect of Electric forces (attractions and repulsions almost balance out) in large objects so G dominates.
But remember, the individual E forces are massive compared with the individual weak gravitational forces. The life of an Elephant is dominated by gravity whilst the life of a mosquito is dominated by electric forces of nearby molecules. For a mozzie the air is more like a very light oil, which slow everything up. If it stops flying, it drifts very slowly downwards. On the other hand, an Elephant . . . . . .

Um... ok... that was kinda confusing. Could you maybe restate that so I can understand better?

 

[russ_watters]

Um... ok... that was kinda confusing. Could you maybe restate that so I can understand better?

He's just saying that other forces besides gravity can produce the same effect when pulling objects together; the effect being geometric symetry.

 

[sophiecentaur]

Um... ok... that was kinda confusing. Could you maybe restate that so I can understand better?

Sorry, that was a lot of stuff to absorb in one go. Bottom line is that Electric Forces are much stronger than gravitational forces BUT they mostly cancel out. Gravity is never canceled out or screened by anything so it goes on operating over all space. E forces dominate in the small scale and G forces dominate in the large scale.
In the simple cases, the most stable shape, whether held by E or G forces, is symmetrical in all respects - a sphere.

 

[anorlunda]

@doglover9754 , You have been told again and again that many types of forces create spheres. There is nothing special about gravity and spheres. Does that answer your question?

 

[doglover9754]

He's just saying that other forces besides gravity can produce the same effect when pulling objects together; the effect being geometric symetry.

That makes sense... thanks!

 

[doglover9754]

@doglover9754 , You have been told again and again that many types of forces create spheres. There is nothing special about gravity and spheres. Does that answer your question?

Mhm! Thanks everyone who helped! Sorry if I made people repeat it. It’s a tad bit hard for me to understand stuff that I’m not learning in school. Most of the questions that I ask are just random things that pop up in my head even if it isn’t at my lerninb level. Sorry if I stressed anyone out . I am weird. Aren’t I?

 

[anorlunda]

The question has been answered. Thanks all. It is time to close the thread.