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Name: Jonathan
Status: Student
Grade: Other
Location: TX
Country: United States
Date: Spring 2010


Question:
I am trying to understand what causes the pressure at the Earth's core. If it is weight (a force), then what is the source of that force? Is it caused by the Earth's rotation? Or gravity? And why is the force so much greater the deeper you go inside the Earth? It seems as though the Earth would be more like an Orange, which sustains its own weight, else why does it not collapse within itself?



Replies:
Jonathan,

Gravity as we understand it is a property of objects that have mass. This means that just by having mass, matter attract each other.

Imagine that you were in zero - g space (the point in space where the gravity of the Moon and the Earth cancel out). If you could then release two dust particles (in such a way that you did not give either particle any inertia), then those two dust particles would eventually attract each other, move toward each other, and eventually connect.

Now imagine you could measure the force between these two particles (by maybe inserting a very sensitive probe in between them), you would measure a force due to the gravitational attraction between these two particles. Now suppose you were to add another dust particle (imagine them forming a triangle), this would mean that p1 attracts p2 and p3, p2 attracts p1 and p3, and so on. These attractions tend to pull all the particles inward, so the strongest force measurement will still be in the middle of the triangle. Adding more particles will just continue to increase the force felt in the middle.

So it is with the Earth (or any body of matter). The highest intensity of the force due to gravitational attraction will be in the middle of the mass.

As to why an orange does not collapse onto itself (or for that matter, why the Earth does not collapse onto itself), gravitational force is very weak. Think of the fact that you can lift metal objects with a magnet - the magnet's attractive force is able to combat the force of gravity from the entire Earth! Because gravitational forces are very weak, matter tends not to collapse under that force. The only time we note that there is so much matter that the combined gravitational forces collapses the object is in incredibly massive objects like black holes and stars that have burned out.

Greg (Roberto Gregorius)
Canisius College


Hi Jonathan,

Gravity is the main force that holds the earth together, and explains much of the pressure underground. The earth is massive enough to stay fairly tightly bound by gravity, but the forces are not sufficient to cause the rock and metal that make up the earth to collapse into another form of matter. Unlike gaseous stars or planets, Earth's materials are relatively strong and stable, preventing any kind of overwhelming collapse.

Hope this helps,
Burr Zimmerman


Pressure at any point within the Earth is caused by the weight of Earth materials above that point. The Earth materials all attract each other by gravity. Anything between these interacting bodies will experience a pressure, because the materials on all sides will press in as they attract each other.

The pressure is greater closer to the center of Earth because there is more stuff on all sides pressing inward. A the surface of Earth, most of Earth's mass is below you, and only the atmosphere is above you. The mutual attraction is fairly small: only about 1 bar. (That is "atmospheric pressure.") Deeper down, more mass on both sides presses inward more firmly. The greatest pressure will be at Earth's center of mass, into which the greatest amount of mass presses in all directions.

An orange does not hold itself together by gravity. The mutual attraction of the different parts of an orange is ridiculously weak. It does not collapse in on itself because no force acting on the orange is strong enough to squish it very far. Earth has much more mass, and much more mutual gravitational attraction, than an orange.

Richard E. Barrans Jr., Ph.D., M.Ed.
Department of Physics and Astronomy
University of Wyoming


Pressure at the Earth's core is caused by the weight of all of the stuff on top of it. The source of the force of weight is gravity. Earth's rotation would cause a centrifugal force which is directed outward from the center of the earth. See:

http://en.wikipedia.org/wiki/Centrifugal_force

So Earth's rotation causing centrifugal force would diminish the weight force. Pressure is greater the deeper you go because the deeper you go the more stuff is on top to weight down. If the earth were mushy inside like an orange then it would collapse in on itself until the matter is compacted enough to bear the weight of the stuff weight in on top of it. Instead, the Earth's core is layered as described at this URL.

http://en.wikipedia.org/wiki/Structure_of_the_Earth

Here is a site that has diagrams:

http://www.google.com/images?hl=en&q=Earth%27s+core&um=1&ie=UTF-8&source=uni v&ei=vAHiS7-oGIaBlAf12cCRAg&sa=X&oi=image_result_group&ct=title&resnum=1&ved =0CDEQsAQwAA

Sincere regards,
Mike Stewart



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