CaptKaspar

Gravity Decreases Closer to Core?

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Why does gravity become less the closer you get to the core? 

I’m down for the wacky gravitational effects at the alien structures in the core, but a good distance outside the core? 

Essentially gravity becomes higher the further you get from the center of mass, this makes no sense and is bass ackwards.

 

 

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Newton's law states: The gravitational attraction force between two point masses is directly proportional to the product of their masses and inversely proportional to the square of their separation distance.

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Gravity is zero at the core because equal amounts of mass are pulling you from all directions and they cancel each other out.

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8 hours ago, Flashman said:

Gravity is zero at the core because equal amounts of mass are pulling you from all directions and they cancel each other out.

🤦‍♂️

You’d have to ignore all other forms of mass in the universe to even begin to entertain this line of thinking. 

(Un)fortunately, there are other planets, moons, and a star in this game ‘universe’.

Did you know that the mass center of Earth is not at the center of the planet? Moon really screws that up (among other things). 

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4 hours ago, CaptKaspar said:

🤦‍♂️

You’d have to ignore all other forms of mass in the universe to even begin to entertain this line of thinking. 

(Un)fortunately, there are other planets, moons, and a star in this game ‘universe’.

Did you know that the mass center of Earth is not at the center of the planet? Moon really screws that up (among other things). 

Its a close enough approximation, no?

As for other objects, they are very far away, perhaps except the moons. Lets for the sake of argument say the moons would have a significant impact on gravitational forces at the core of a planet(i dont know that they would, do you?). Consider their rate of rotation, or orbit, whichever fits, would you really want to turn planets cores into gravitational roller coasters?

Edited by Hermann

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1 hour ago, Hermann said:

Its a close enough approximation, no?

As for other objects, they are very far away, perhaps except the moons. Lets for the sake of argument say the moons would have a significant impact on gravitational forces at the core of a planet(i dont know that they would, do you?). Consider their rate of rotation, or orbit, whichever fits, would you really want to turn planets cores into gravitational roller coasters?

Yes, I do know the mass of the moon has a significant impact on the center of mass of Earth. Tides are the most obvious manifestation of that. Look up how you can have two high tides in the same day. Wouldn’t there just always be one high tide whenever the moon was highest overhead? Yet, that is not the case here on Earth.

Mathmatically: The mass of the Moon is 7.35*1022 kg and the mass of the Earth is 6.00*1024 kg. The distance between the Moon and the Earth is 3.80*105 km.

xcm = (0m*6.00*1024 kg + 3.80*108 m*7.35*1022 kg)/(6.00*1024 kg + 7.35*1022 kg) = 4598666 m = 4600 km.
Since the radius of the Earth is 6378 km, the center of mass of the Earth-Moon system is inside the Earth.

So the mass of the moon offsets the center of mass by 4600km. I’d say that’s significant. 

The effect of gravity is so intense towards the core of Earth that it builds up immense pressures from more dense objects trying to force their way down. These pressures greatly increase the temperatures in the core. This leads to a molten outer core and an extremely dense solid innner core. 

The core of Earth is resoundingly not hollow. 

Gravity doesn’t just go away in space. There is still gravity. It is the reason why our galaxy doesn’t spiral out of control, why the planets stay in orbit around the sun, and why our moon stays in orbit around our planet. There is a perceived lack of gravity on the ISS and spaceships for instance because the ISS is in a constant freefall towards Earth. (Professionally I am a pilot and I can tell you from aerobatic flying that you can experience ‘zero’ gravity here on Earth by having your acceleration rate towards the Earth match that of gravity’s.) The reason it doesn’t crash into Earth is because of its horizontal velocity that keeps it in orbit. The effects of that gravity are lessened by the above stated Newtonian Law. 

I am fine with the core of the planets/moons in Astroneer being alien structures that somehow warp gravity. Btw the cores are already ‘gravitational rollercoasters’ because of this.

What I want to know is why gravity is higher on the surface than say 1/2 way to the core. At what point does gravity begin to decrease again on planets? Its lowest at the core and highest at the surface. This is bass ackwards. 

 

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11 minutes ago, CaptKaspar said:

 

What I want to know is why gravity is higher on the surface than say 1/2 way to the core. At what point does gravity begin to decrease again on planets? Its lowest at the core and highest at the surface. This is bass ackwards. 

 

Because this is a game and by creating the surface gravity of a planet naturally dictates the gravity as you get to the center. I doubt SES has interest in doing the math for the mass and distances of all the other interstellar bodies and piles of code it would require to work out realistic gravity in the planets inner layers.

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3 minutes ago, nobillyno said:

Because this is a game and by creating the surface gravity of a planet naturally dictates the gravity as you get to the center. I doubt SES has interest in doing the math for the mass and distances of all the other interstellar bodies and piles of code it would require to work out realistic gravity in the planets inner layers.

It doesnt have to be mathematically correct, it should just go up and not down as you approach the core. Heck, leaving it constant and not change at all would make a lot more sense. 

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My suspicion is that SES coded gravity to be X amount at Y distance from a core regardless of the size/mass of the object. This allows a planet with a smaller radius to have lower gravity and a planet with a longer radius would have increased gravity. So the smaller planet with less mass has a perceived lower gravity at the surface than a larger planet would. This would work great if we stayed on the surface. However, a major component of this game is digging to the core. 

 

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1 minute ago, CaptKaspar said:

It doesnt have to be mathematically correct, it should just go up and not down as you approach the core. Heck, leaving it constant and not change at all would make a lot more sense. 

I agree but it would be less interesting. Higher gravity would make falling from even the shortest of heights deadly and even kill you for jumping in the mantel, making it less fun.

 There are a lot of things that make little sense in this physics rewritten universe. Such as how you can get 5 extenders that are kind of large placed on the ground from the same amount of copper as it takes to make one tiny work light and one resource makes something small and only 4 make something very large. Realistically A large shuttle should take a few hundred resource nuggets to build. They are just keeping it simple and fun for the masses and mot making Space Explorer Simulator 2019.

All of the quirks and physics rewrites are to me what helps make it fun and alien. If it were too close to reality I'd not feel like I was in such a preposterous alien environment and it would be less interesting.  

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If we are going to ignore basic physics, lets have more fun with it.

Planets are procedurally generated. Have it so you dont know if the gravity is going to go up, down, or remain constant as you approach the core. That each planet and each play through would be a different experience. 

Through a tiny bit of lore in there to explain why this could be. Something about the arrangement of the alien structures. Something to go on. Just brainstorming.

The first time I was playing with a friend and we dug sufficiently down far enough to notice the gravity decreasing we were both like WTF is happening? This makes no sense. 

Edited by CaptKaspar

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11 hours ago, CaptKaspar said:

🤦‍♂️

You’d have to ignore all other forms of mass in the universe to even begin to entertain this line of thinking. 

(Un)fortunately, there are other planets, moons, and a star in this game ‘universe’.

Did you know that the mass center of Earth is not at the center of the planet? Moon really screws that up (among other things). 

Damn it! Just when I was liking Flashman's simple description :(

LOL

A.T.

Edited by Astroneer Tips

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12 hours ago, CaptKaspar said:

🤦‍♂️

You’d have to ignore all other forms of mass in the universe to even begin to entertain this line of thinking. 

(Un)fortunately, there are other planets, moons, and a star in this game ‘universe’.

Did you know that the mass center of Earth is not at the center of the planet? Moon really screws that up (among other things). 

Not really.  The closest object to us is the moon and the effect of gravity from the moon doesn't affect gravity on us as individuals at all.  It does affect the tides, but that is a different thing, meaning that you don't feel gravity differently according to the phases of the moon.   Other planets, much less all objects in the universe, have even less of an effect on us as a person.  

https://www.popsci.com/science/article/2011-02/fyi-if-i-fell-through-earth-what-would-happen-center Take a look at the link, perhaps it will explain it to you better than I can (or care to :) ).

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BTW, before I get a reply about "at all", I should say that I mean in a noticeable way.  One doesn't notice differences in gravity from the moon as the distance from you personally changes. And again, it is really close.  There is a difference, but it is a rounding error.

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On 2/25/2019 at 10:45 PM, CaptKaspar said:

Why does gravity become less the closer you get to the core? 

I’m down for the wacky gravitational effects at the alien structures in the core, but a good distance outside the core? 

Essentially gravity becomes higher the further you get from the center of mass, this makes no sense and is bass ackwards.

Because this is how gravity works. In the case of earth, and probably most other planets, you will see a slight increase before it goes down. But it does go down to zero eventually. There is a curve to it depending on size and variations in density.

1592624740_earthgrav.png.ce6b4f5cf02f411d0c139dda2d77cd92.png

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

6 hours ago, CaptKaspar said:

Yes, I do know the mass of the moon has a significant impact on the center of mass of Earth. Tides are the most obvious manifestation of that. Look up how you can have two high tides in the same day. Wouldn’t there just always be one high tide whenever the moon was highest overhead? Yet, that is not the case here on Earth.

Mathmatically: The mass of the Moon is 7.35*1022 kg and the mass of the Earth is 6.00*1024 kg. The distance between the Moon and the Earth is 3.80*105 km.

xcm = (0m*6.00*1024 kg + 3.80*108 m*7.35*1022 kg)/(6.00*1024 kg + 7.35*1022 kg) = 4598666 m = 4600 km.
Since the radius of the Earth is 6378 km, the center of mass of the Earth-Moon system is inside the Earth.

So the mass of the moon offsets the center of mass by 4600km. I’d say that’s significant. 

The effect of gravity is so intense towards the core of Earth that it builds up immense pressures from more dense objects trying to force their way down. These pressures greatly increase the temperatures in the core. This leads to a molten outer core and an extremely dense solid innner core. 

Well no, earths center of mass and center of gravity(while center of mass does not equal center of gravity) only shifts ever so slightly with the tidal forces caused by the moon and the sun, but these effects are minuscule. And scaled down to astroneer-size planets, imperceivable. 

The mass of the moon offsets the earth-moon barycenter by a significant amount relative to earths center. But this is irrelevant to perceived gravity on, or in the earth, as any gravity acting on you, also acts on Earth. Therefore it cancels out and you dont feel it. There is a reason Earth is round(well almost...). If center of gravity was 4600km from the core.. well. that just would not happen.

Extreme pressure does not equate extreme gravity. Pressure is the result of the collective force of gravity on the entire planet. As you go deeper the pressure column gets taller, adding pressure, even though gravity gets weaker.

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3 hours ago, Hermann said:
  • Because this is how gravity works. In the case of earth, and probably most other planets, you will see a slight increase before it goes down

i don't think it matters what planet you are on.  Whether you are on Earth, or some gas giant, it shouldn't matter.  The density of the planet or the core of the planet shouldn't matter. Mass is mass, no matter how dense.

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9 hours ago, CaptKaspar said:

The first time I was playing with a friend and we dug sufficiently down far enough to notice the gravity decreasing we were both like WTF is happening? This makes no sense. 

I can imagine your surprise!

I actually REQUESTED this as a feature during Alpha - but it turns out it was already in the game anyway. And as per the discussion, this is a legitimate physics property of any gravitational body like a planet.

Flashman nailed it:

On 2/26/2019 at 4:50 PM, Flashman said:

Gravity is zero at the core because equal amounts of mass are pulling you from all directions and they cancel each other out.

You know how you can simulate gravity in 2D on a trampoline, using balls that will 'orbit' around each other? This example also extends to other uses as well. 

Consider a ball on a trampoline - the more level it is, the less 'gravity' it has. The surface curves 'downward' as it get closer to the ball. A 'heavy ball has a 'steeper' curve which indicates the gravitational strength.

But UNDERNEATH the ball, the surface curves to 'flat' or 'level' right in the middle - and it will always have one spot (regardless of the size of the ball) that is flat. You can't avoid that flat spot.

It's also applies if you put a metal ring on a trampoline as well - i.e. the 2D equivalent/cross section of a hollow sphere e.g. a Dyson Sphere. The surface of your trampoline will always be 'flat' across the inside of the ring - which means that if you're inside a hollow sphere, regardless of distance from the interior surface or proximity to the centre, there will always be net zero gravity anywhere inside. But I digress...

 

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5 hours ago, Ben Sando said:

I can imagine your surprise!

I actually REQUESTED this as a feature during Alpha - but it turns out it was already in the game anyway. And as per the discussion, this is a legitimate physics property of any gravitational body like a planet.

Flashman nailed it:

You know how you can simulate gravity in 2D on a trampoline, using balls that will 'orbit' around each other? This example also extends to other uses as well. 

Consider a ball on a trampoline - the more level it is, the less 'gravity' it has. The surface curves 'downward' as it get closer to the ball. A 'heavy ball has a 'steeper' curve which indicates the gravitational strength.

But UNDERNEATH the ball, the surface curves to 'flat' or 'level' right in the middle - and it will always have one spot (regardless of the size of the ball) that is flat. You can't avoid that flat spot.

It's also applies if you put a metal ring on a trampoline as well - i.e. the 2D equivalent/cross section of a hollow sphere e.g. a Dyson Sphere. The surface of your trampoline will always be 'flat' across the inside of the ring - which means that if you're inside a hollow sphere, regardless of distance from the interior surface or proximity to the centre, there will always be net zero gravity anywhere inside. But I digress...

 

If you wanted to use a simplified patched conics approach to understanding gravity wells, thats fine. It’ll get you to the moon (a little off course but close enough for government work)! 

 

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9 hours ago, Hermann said:

Because this is how gravity works. In the case of earth, and probably most other planets, you will see a slight increase before it goes down. But it does go down to zero eventually. There is a curve to it depending on size and variations in density.

1592624740_earthgrav.png.ce6b4f5cf02f411d0c139dda2d77cd92.png

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

Well no, earths center of mass and center of gravity(while center of mass does not equal center of gravity) only shifts ever so slightly with the tidal forces caused by the moon and the sun, but these effects are minuscule. And scaled down to astroneer-size planets, imperceivable. 

The mass of the moon offsets the earth-moon barycenter by a significant amount relative to earths center. But this is irrelevant to perceived gravity on, or in the earth, as any gravity acting on you, also acts on Earth. Therefore it cancels out and you dont feel it. There is a reason Earth is round(well almost...). If center of gravity was 4600km from the core.. well. that just would not happen.

Extreme pressure does not equate extreme gravity. Pressure is the result of the collective force of gravity on the entire planet. As you go deeper the pressure column gets taller, adding pressure, even though gravity gets weaker.

Perfect graph thank you! 

So as we can see acceleration rate of gravity increases as we approach the core. 

As stated in the original post I wanted to know why gravity increases as we approach the core, not when in the core. As originally stated im fine with the gravity in the core, not with the effects of gravity as we approach the core. 

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As someone who had to run all these equations as homework problems while getting his physics degree, I find this conversation quite entertaining.

As a note, the solution—shown quite clearly in the graph from the wikipedia article—summarizes as the force of gravity overall *decreasing* the closer to the center you get.

The back-of-the-envelope calculation for this is easiest to understand if you buy a specific result: namely that if you consider a spherical shell of uniform mass, the sum of the gravitational forces anywhere in that shell cancel. The reason this is interesting is that, for a radially symmetric body, the is then trivial to show that when you sum the gravitational effect of all the spherical shells, the only parts that contribute to the downward pull of gravity are the parts closer to the center than you are. In that model, an ever-shrinking amount of mass is pulling you inward.

Obviously, that is a simplified model, but it turns out that the general idea holds in the more complex scenarios, because when the spherical shells are non-uniform, their deviations from spherical uniformity only introduce higher order perturbations to the result, with the overall effect still being the same: the acceleration due to gravity goes down, the.closer you get to the center.

It turns out Astroneer has it—at least to a first order approximation—physically correct.

Edited by Paploo

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36 minutes ago, Paploo said:

As someone who had to run all these equations as homework problems while getting his physics degree, I find this conversation quite entertaining.

As a note, the solution—shown quite clearly in the graph from the wikipedia article—summarizes as the force of gravity overall *decreasing* the closer to the center you get.

The back-of-the-envelope calculation for this is easiest to understand if you buy a specific result: namely that if you consider a spherical shell of uniform mass, the sum of the gravitational forces anywhere in that shell cancel. The reason this is interesting is that, for a radially symmetric body, the is then trivial to show that when you sum the gravitational effect of all the spherical shells, the only parts that contribute to the downward pull of gravity are the parts closer to the center than you are. In that model, an ever-shrinking amount of mass is pulling you inward.

Obviously, that is a simplified model, but it turns out that the general idea holds in the more complex scenarios, because when the spherical shells are non-uniform, their deviations from spherical uniformity only introduce higher order perturbations to the result, with the overall effect still being the same: the acceleration due to gravity goes down, the.closer you get to the center.

It turns out Astroneer has it—at least to a first order approximation—physically correct.

Right. The total summation of mass that is ‘below’ you is what is pulling you down. Your proximity to that mass determines your acceleration rate of gravity. The relationship of mass summation to your proximity of that mass peaks prior to reaching the core. Not on the surface. The graph shared shows this. Yes at the center of the core the acceleration rate of gravity is effectively zero in a simplified explanation.

This conversation derailed with people trying to explain why gravity would be near zero at the center of the core. Never a point in the discussion. The core is much bigger than the dead center.The topic title is meant to discuss why gravity decreases approaching the core, not the dead center of the core. 

The above graph illustrates this very simply.

BTW for everyone who is focused on the wrong discussion. At the very center there is an alien structure that does not have zero or even near zero gravity. So the only thing that is accurate here is that upon entering the core the gravity decreases. It is wrong prior to entering the core and it is wrong again at the core center. Again I am fine with the wacky physics at the core center because I can suspend disbelief because of wacky alien stuff. Gravity though should be increasing while digging through the mantle. 

For simplicity sake I suggested that gravity should just remain constant prior to reaching the core (read the core and not the center of the core). 

Gravity does not have a linear decrease as you appoach the core. Even upon entering the core it is not a linear decrease.

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CaptKaspar I'm curious what makes you sure that the core of a planet in Astroneer is composed of material with significantly higher density than the rest of the planet?

Because as you see in the graph Hermann posted, gravity only increases SLIGHTLY within a rather short distance of the core according to the PREM model. A planet with linear density or constant density would have no increase near the core.

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2 hours ago, Syro said:

CaptKaspar I'm curious what makes you sure that the core of a planet in Astroneer is composed of material with significantly higher density than the rest of the planet?

Because as you see in the graph Hermann posted, gravity only increases SLIGHTLY within a rather short distance of the core according to the PREM model. A planet with linear density or constant density would have no increase near the core.

Good point.

If Astroneer's planets are homogenous - which they certainly were, and even with 'harder' terrain the density is the same as surface dirt (i.e. your soil containers fill volumetrically the same, regardless of the terrain type) then the natural assumption is that there is no iron core like Earth's, and therefore there is no comparison to the PREM (being the Preliminary Reference EARTH Model, not Astroneers!).

Certainly a constant gravitational force would be easy - I dare say it may have existed before we even had the pre-Alpha - but System Era chose to implement gravity as a function of location/altitude. Was it a gameplay choice? Maybe. Was it a reflection of actual homogenous-planet gravitational curves? Definitely. Would it inspire we players and fans to discuss, research, muse? Absolutely - And I think it's great because of this!

 

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