# Thread: A question on gravity and earth

1. we know that gravity is the attracting force acting between two objects.. so one question struck me:
Although hypothetically, if we manage to go deep into the Earth such that the mass of Earth below me is equal to the mass of Earth above me(that means the 'upper earth' and 'lower earth' is pulling me by the same magnitude of force), then does it mean that I will float on the spot if not get ripped apart?

2.

3. You won't get ripped apart, but at the gravitational "center" of the earth you would experience a zero net force. You would just be "floating" there as you put it.

4. Originally Posted by Markus Hanke
You won't get ripped apart, but at the gravitational "center" of the earth you would experience a zero net force. You would just be "floating" there as you put it.
yes I know about the gravitational center.. but Im asking about the place where the mass is equal, up and down.....
will we float?

5. The place where the "mass is equal" IS the gravitational centre. Yes, you'll float. They all float down there...

For an explanation, check out Newton's Shell theorem. Basically, on the inside of a giant ping-pong ball, you would also not experience gravity at any point within it, no matter how thick the walls are. You would also not get ripped apart, as the force vectors act on each point in the ping-pong ball, exactly cancelling out to a net of zero.

6. well.. thank u Kalster.. I was picturing it wrong.. Although I didnt understand the shell theorem

7. Originally Posted by Ani
well.. thank u Kalster.. I was picturing it wrong.. Although I didnt understand the shell theorem
Basically it turns out that if you work out the gravitational forces acting on a body inside a hollow sphere, that the forces always cancel out, no matter where inside the hollow sphere the body is. When you translate that to the earth, then it means that if the earth was a perfect sphere made up of the same stuff everywhere, that if you go down 1km say, then all the matter 1km down from the surface of the whole planet stops acting on you gravitationally. So if you descend all the way to the middle, then there would be zero gravitational force acting on you.

Obviously the earth is not a perfect sphere and made of the same stuff everywhere, but you'd still get the same approximate effect.

8. Originally Posted by KALSTER
Originally Posted by Ani
well.. thank u Kalster.. I was picturing it wrong.. Although I didnt understand the shell theorem
Basically it turns out that if you work out the gravitational forces acting on a body inside a hollow sphere, that the forces always cancel out, no matter where inside the hollow sphere the body is. When you translate that to the earth, then it means that if the earth was a perfect sphere made up of the same stuff everywhere, that if you go down 1km say, then all the matter 1km down from the surface of the whole planet stops acting on you gravitationally. So if you descend all the way to the middle, then there would be zero gravitational force acting on you.

Obviously the earth is not a perfect sphere and made of the same stuff everywhere, but you'd still get the same approximate effect.

Find it a lil bit confusing but get the general idea!

9. Think of yourself inside a hollowed out sphere. It doesn't really matter how thick the surface of that sphere is, it can be a thin shell, or you could be in a little hole at the centre of the Earth.

If you are in the centre of the hole, there is the same amount of gravity coming from whichever direction you choose, so it seems obvious that it would all cancel out and you would be weightless, in free-fall.

But what is less obvious is that, wherever you are in that hollowed out spherical area, and however large that area is, you would be weightless. You wouldn't start accelerating towards one side of that shell if you weren't in the centre of it. The reason for this is explained by the shell theorem, but I find an easy way to imagine it is to consider the side of the shell you are closest to. Now consider the exact point on the inside of that shell that you are closest to. The gravity coming from that direction is exerting the most influence on you, whilst the gravity coming from the opposite side is exerting the least influence on you, so it is easy to think that you would start moving towards the closest side.

But what you have to consider is the whole of that inside surface. As you consider positions closer and closer to one side, the area of that side that influences you "the most" gets smaller, whilst the opposite area that influences you "the least" gets larger, so the gravity always cancels out wherever you are!

10. Originally Posted by SpeedFreek
Think of yourself inside a hollowed out sphere. It doesn't really matter how thick the surface of that sphere is, it can be a thin shell, or you could be in a little hole at the centre of the Earth.

If you are in the centre of the hole, there is the same amount of gravity coming from whichever direction you choose, so it seems obvious that it would all cancel out and you would be weightless, in free-fall.

But what is less obvious is that, wherever you are in that hollowed out spherical area, and however large that area is, you would be weightless. You wouldn't start accelerating towards one side of that shell if you weren't in the centre of it. The reason for this is explained by the shell theorem, but I find an easy way to imagine it is to consider the side of the shell you are closest to. Now consider the exact point on the inside of that shell that you are closest to. The gravity coming from that direction is exerting the most influence on you, whilst the gravity coming from the opposite side is exerting the least influence on you, so it is easy to think that you would start moving towards the closest side.

But what you have to consider is the whole of that inside surface. As you consider positions closer and closer to one side, the area of that side that influences you "the most" gets smaller, whilst the opposite area that influences you "the least" gets larger, so the gravity always cancels out wherever you are!
thanks... i kinda get it..

11. Hi Any,

in case you were in this center of the planet, which was described by yourself, you were pretty much shaken therefore earth was being in a constant rolling.

Steve

12. Originally Posted by Steve Miller
Hi Any,

in case you were in this center of the planet, which was described by yourself, you were pretty much shaken therefore earth was being in a constant rolling.

Steve
How very true!

13. Even today, perhaps especially today.

14. Originally Posted by Steve Miller
Hi Any,

in case you were in this center of the planet, which was described by yourself, you were pretty much shaken therefore earth was being in a constant rolling.

Steve
Well you would be floating in 0 gravity so all you would observe is that the Earth is moving around you.. Why would you be rotating along?

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