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I was thinking that the reason behind the gravitational force is the magma, but after studying Newtonian model of gravity I am confused. It says that there can be gravitational force of attraction between any two masses. Then won't the gravitational force act only downwards? What actually causes the gravitational force of attraction?

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  • $\begingroup$ I was thinkinking that the reason behind the gravitational force is the magma. The Sun doesn’t have any magma but it attracts the Earth. $\endgroup$
    – G. Smith
    Nov 30, 2020 at 1:41
  • $\begingroup$ Are you confusing the gravitational force with the magnetic field of the earth? The gravitational force acts downwards, while the magnetic field looks like a dipole field. $\endgroup$
    – Andrew
    Jun 7, 2021 at 20:15

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In Newtonian mechanics, mass is what causes gravitational "pull". It is a pull,and only a pull, between any two masses and all masses. The magma is not itself a source of gravitational force, but the reason why it is so hot is a cause of the gravitational force. Planets form from relatively small space rocks, called asteroids, and smaller particles sometimes referred to as space dust. It is the gravitational pull that they create on each other that brings them together, and the collisions between them as they are "getting stuck" that heats them up. The reason why magma and the rest of the Earth's core are so hot is all the heat that was generated as the planet formed.

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Then wont the gravitational force act only downwards?

Newton's law of gravitation states that gravity pulls together. It is a force which pulls one object towards the center of another. It just so happens that, on the surface of the Earth, something like 99.999% of the gravitational pull you feel is that of the Earth, and the center of the earth is so far away that it simply feels like "down." However, if you were to visit the opposite side of the globe, obviously it would have to pull "down" from that direction too -- it always pulls towards the center of the planet.

Likewise, Jupiter is actually pulling on you right now. However, if you did the calculations, it is quite weak, so we typically ignore its effects for every purpose outside of space flight.

What actually causes the gravitational force of attraction?

This kind of question is surprisingly tricky. In Newtonian physics, we merely observe that there is a force which pulls any two masses together. It doesn't say anything about why or how, just that it happens. While we sometimes teach physics as the fundamental rationale behind the laws of the universe, in reality it's just a collection of patterns that we have observed. We have observed that 100% of massive objects have a pull on each other (if you look at them with sufficiently exacting instruments).

Gravity is a particularly troublesome thing in this respect, due to general relativity. In general relativity, we change the way we think about gravitation completely. Completely. In GR, gravity is not a force, but rather a consequence of the curvature of space. This is a very tricky idea, so its okay if it doesn't make sense. I bring it up simply because "gravity" itself is an especially strange case in that we have two different models of how the world works, and they treat it in two completely unrelated ways. So "what causes it" is going to be mired in the muck dealing with these different models. Both predict the effects of gravity, they just differ on why gravity does what it does.

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I'm guessing you're mentioning magma because you are considering that the earth must have a mass due to something so that it will attract stuff above/on its surface, I will answer extensively thinking you're confused about how the physics work --

Think of the earth as a whole, with its incredible amounts of water, magma, rocks, whatever you can imagine, even air (sometimes we forget how much air actually would weight if we could somehow put it on a scale), all that as a whole will give you an enormous mass M, you could think now that earth is not the massive not-so-spherical thing it is but rather a single point with mass M and a location somewhere nearby the geometrical centre of the earth, (here you're allowed to think the earth is almost-spherical and think about where the geometrical centre would be and it wouldn't be so bad of an apporximation of where the centre of mass will be), anyways, now you would be another point with mass m [your mass] and a location r, which I'm guessing is different from the earth's centre or it would be damn hot there.

With those funny abstractions you could now figure out with old Newtonian physics that each one of them attract each other with a force proportional to their masses (that somehow "emanates" from those point particles) and inversely proportional to their distance. Do you notice something wrong here? Using this logic as you went closer and closer to the centre of mass the gravitational pull would basically become infinite,the accepted laws of physics would break down there but it looks pretty normal, right?

Ok now let's consider the following mental experiment

You're standing on the surface of the earth, your mass is m and there your weight is w_s, If you were able to move toward the centre of the earth what would happen to your weigh?

"Your weight would decrease as you moved towards the center of the earth. Only the part of the Earth that's closer to the center than you are contributes to your weight. As you move towards the center, there is less and less of the Earth closer to the center than you are, so the gravitational field gets weaker and your weight decreases. (Your mass, however, stays the same.) In general, gravity also gets stronger as you get closer to the center of a massive object, but it turns out that the effect of having less mass closer to the center than you are is more important. At the exact center of the Earth, the gravitational field is actually zero. " [1]

So yes, there is a trick there, in real life planets are far from being point masses, right? if you are within earth (at a distance less than the distance from the centre to the surface) then as you approach earth's centre of mass less and less mass is "pulling" you towards the centre of mass, however since we (people) really can't be that close to the earth's centre of mass we use that the force we feel due to the earth's surface is f = ma where m is your mass and a = -g (the acceleration due to gravity) which we do calculate safely with the "gravitational force of attraction between any two masses" you can see how here.

And there it is natural to notice that g is calculated as an average "The acceleration due to gravity varies slightly from g at different places on Earth, due to the Earth's rotation, variances in the distribution of the Earth's mass, and the location's elevation. Consequently, a standard value of g (...) is often adopted when high precision is not necessary. " [3]

If all you wanted to know is what makes the earth so massive you can check this and this

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Think of a very simple argument some scientists believe its due to gravitons but the funny thing is they have failed to detect even one and the other is the known knowledge of general theory of relativity and Newtons law of gravity... But if you merge the two you would get a new answer that surprises you... In general theory of relativity, gravity is just simple worpes and curves in the fabric of space leading the earth rotates about its axis well it leads to an amazing phenomena which we call in classical mechanics centripetal force that pulls a body towards the center of axis of earths rotation giving meaning to newtons law of gravity. A body on earth would then experience the attraction towards the center. Imagine this impossible scenario when one day the earth stops its rotation just at once. What would happen? .. basically that's why nothing on earth flies off into space.

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    $\begingroup$ I don't follow... in classical mechanics something spinning would indeed fly off into space without gravity. $\endgroup$ Jun 7, 2021 at 20:16

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