# B Pressure in the core of the earth

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1. Mar 26, 2017

I'm an empirical sciences student who loves physics. While studying geology I learned that the core of the earth is considered to be solid and the reason for that is because the pressure is too high. Although according to my calculations g( gravitional field) equals zero and when you go out the gravitional field slowly starts to grow bigger.
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. Mar 26, 2017

### phinds

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.

3. Mar 26, 2017

### Staff: Mentor

The pressure is so high because of the weight of all the material pressing down on it. Just imagine the force that the weight of a mountain puts on the Earth, not to mention entire continents and all of the underlying material in the mantle. Added up, all that leads to an enormous amount of pressure on the core.

4. Mar 26, 2017

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 X2 .
In that case the highest pressure should be somewhere in the middle where there's big enough g and big enough mass...

5. Mar 26, 2017

Mass doesn't effect pressure on its own, it needs gravitional field and while g is at zero (g=Me×G÷re2 since Me=0 then g=0) the weight equals zero therefore the pressure equals zero

6. Mar 26, 2017

### phinds

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.

7. Mar 26, 2017

### Staff: Mentor

The gravitational pull on a mountain is not zero.

8. Mar 26, 2017

### Staff: Mentor

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 decreases as you go down towards the core while the weight pressing down increases (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.

9. Mar 26, 2017

### A.T.

When your finger is in a vise, the forces on it cancel as well, but it still hurts.

10. Mar 26, 2017

### Spinnor

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. Mar 27, 2017

### A.T.

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

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.

Last edited: Mar 27, 2017
12. Mar 27, 2017