Re: Clarification of reason for pressure at Earth's Core

The question: I used the search Engine and read David Scarboro's explanation that Earth's core is under pressure because "The presure (sic) is of course due to the weight of the earth above it". Intuitively, that doesn't seem to make sense. Why isn't the Earth's weight distributed evenly across it's mass, like say, an Orange? David's answer implies the planet is trying to collapse upon itself, doesn't it? I understand that weight is a force, so is the force due to the Earth's spin? Or is the force due only to gravity? (in which case I assume no one really has an answer to explain the force itself, only it's mechanical properties)

Hello Jonathan,

Concerning the statement that does not make sense to you: “The pressure is of course due to the weight of the earth above it": I think that you might be confusing ‘pressure’ with ‘gravity’. Maybe this will help:

Imagine that we have drilled a hole to the center of the earth (ignoring the difficulties of doing that!). When this magic hole is one mile deep, you are lowered to the bottom. At this depth, we replace all of the dirt back into the hole (on top of you). You would feel the weight, and pressure, of the entire one mile column of dirt pressing down on you. If we repeated this experiment with a ten mile deep hole, you would feel (about) 10 times the weight of a one mile column of dirt. The reason for the “nearly” is because gravity would be slightly different ten miles below the surface of the earth! If you put a scale at the bottom of our 10 mile deep hole and stood on it, you weight would indicate slightly differently. [note added by MadSci Admin: There is a previous answer in our archives which discusses how the gravity changes with depth in the Earth:
http://www.madsci.org/posts/archives/2000-06/962116625.Ph.r.html]

In fact, your indicated weight would continue to get smaller as the hole gets deeper. As we reach a depth equal to the dead center of the earth, the scale would indicate zero weight!

Why does this happen? Remember that gravity is an attracting force between all masses. When we stand on the surface of he earth, all (or virtually all) of the earth’s mass is below us. Each particle in the earth is attracting us and the net result that we feel is the gravity from the entire earth. But now, when we are deep in the earth, some of the mass is below us, pulling us down, and some if the mass is above us, pulling us up. In the case of being at the center of the earth- equal amounts of mass are above and below so that the net force of gravity is zero. [note added by MadSci Admin: There is a previous answer in our archives which discusses the details of the mathematics of this:
http://www.madsci.org/posts/archives/2000-06/962116209.Ph.r.html]

Getting back to that column of dirt poured on top of you: The local gravity pulling on the column gets progressively smaller the deeper you get, but the total force, or weight of the column is the sum of the mass of each particle in the column, times the local gravity that that particle feels. So the deeper you go, the greater the weight or pressure of the column will be, even though the ‘rate’ of increase decreases with depth.

So David’s answer is correct that the earth IS trying to collapse upon itself, but now you can see that the force of gravity on an object would decrease to zero as the object approaches the center of mass of the earth.

You last question relating to the effect from the rotation of the earth:

The rotation of the earth does reduce the effect of gravity, and reduces the weight that would show on a scale. However, this effect is very small. The maximum reduction of weight would be at the equator which has the greatest spin velocity. Even at the equator, the reduction of weight is less that 1/10 of 1% of the total weight from gravity!

I hope that this is helpful to you.

Regards,

Jay Shapiro