- #1

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But if the gravitional field is so low down there how can the pressure be so high while "P=ρgh" ?

PS: ρ stands for the Earth's average density

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- Thread starter Mohammad Hunter
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- #1

- 13

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But if the gravitional field is so low down there how can the pressure be so high while "P=ρgh" ?

PS: ρ stands for the Earth's average density

- #2

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I think what you're missing is that yes, the gravity gets to zero at the center, and is small near the center, BUT ... it's not small or zero for most of the volume and ALL of that volume contributes to the pressure on the center so the fact that the gravity is zero/small there is irrelevant to the pressure. If there were ONLY a small ball there then it would have low pressure because nothing would be pressing on it, but that's not the case.

But if the gravitional field is so low down there how can the pressure be so high while "P=ρgh" ?

PS: ρ stands for the Earth's average density

- #3

Drakkith

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- #4

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I don't understand, if you draw where the gravitional forces are facing in the core you get that there are mass on the sides pulling everything in the very center out but once they're all the same size, the forces get canceled. Now about one mm away from the center we have the same thing but there's as big as 1mm worth of material more on one side and as much less on the other causing the force to grow larger with the function XI think what you're missing is that yes, the gravity gets to zero at the center, and is small near the center, BUT ... it's not small or zero for most of the volume and ALL of that volume contributes to the pressure on the center so the fact that the gravity is zero/small there is irrelevant to the pressure. If there were ONLY a small ball there then it would have low pressure because nothing would be pressing on it, but that's not the case.

In that case the highest pressure should be somewhere in the middle where there's big enough g and big enough mass...

- #5

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Mass doesn't effect pressure on its own, it needs gravitional field and while g is at zero (g=M

I hope I made sense

- #6

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You make sense but you are applying a concept where it is not applicable. You are treating the core as though there is no mass outside the core. It just doesn't work. Reread what both Drakkith and I have said and do it from the point of view that we are right (since we are) and you need to figure out WHY we are right instead of continuing to argue that we are wrong.Mass doesn't effect pressure on its own, it needs gravitional field and while g is at zero (g=M_{e}×G÷r_{e}^{2}since M_{e}=0 then g=0) the weight equals zero therefore the pressure equals zero

I hope I made sense

- #7

Drakkith

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The gravitational pull on a mountain is not zero.Mass doesn't effect pressure on its own, it needs gravitional field and while g is at zero (g=M_{e}×G÷r_{e}^{2}since M_{e}=0 then g=0) the weight equals zero therefore the pressure equals zero

I hope I made sense

- #8

Drakkith

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I don't understand, if you draw where the gravitional forces are facing in the core you get that there are mass on the sides pulling everything in the very center out but once they're all the same size, the forces get canceled. Now about one mm away from the center we have the same thing but there's as big as 1mm worth of material more on one side and as much less on the other causing the force to grow larger with the function X^{2}.

In that case the highest pressure should be somewhere in the middle where there's big enough g and big enough mass...

That's not how pressure works. Pressure is the force applied perpendicular to a surface divided by the surface area of that surface. A 1,000 pound block with a bottom surface area of 100 square feet puts a pressure of 10 lbs per square foot on the surface of the Earth. Since the Earth isn't a flat object, but spherical, the surface area

You're correct in that the weight of some parcel of material approaches zero as you reach the center of the Earth, but that's only at the center. Everywhere else the material making up the Earth has non-zero weight and presses down.

- #9

A.T.

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When your finger is in a vise, the forces on it cancel as well, but it still hurts.if you draw where the gravitional forces are facing in the core you get that there are mass on the sides pulling everything in the very center out but once they're all the same size, the forces get canceled.

- #10

Spinnor

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Note the change in pressure with the change in depth near the middle of the Earth goes to zero, that is because the gravitational force is nearly zero? Pressure at the center of the Earth is additive?

- #11

A.T.

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Yes, the mass near the center doesn't weight much itself, so it doesn't add much to the pressure.

Note the change in pressure with the change in depth near the middle of the Earth goes to zero, that is because the gravitational force is nearly zero?

Stacked weight is additive. When I put a light box on you, you will be fine. When I put an elephant on top of that box you will be flat.Pressure at the center of the Earth is additive?

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- #12

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I get it now, I just had to read everything a few timesThat's not how pressure works. Pressure is the force applied perpendicular to a surface divided by the surface area of that surface. A 1,000 pound block with a bottom surface area of 100 square feet puts a pressure of 10 lbs per square foot on the surface of the Earth. Since the Earth isn't a flat object, but spherical, the surface areadecreasesas you go down towards the core while the weight pressing downincreases(Weight of mountain + weight of underlying crust + weight of underlying mantle). This leads to an enormous amount of pressure on the core since it is holding up the weight of the rest of the Earth on its relatively small surface area.

You're correct in that the weight of some parcel of material approaches zero as you reach the center of the Earth, but that's only at the center. Everywhere else the material making up the Earth has non-zero weight and presses down.

Thanks for the answer :)

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