# The Naked Scientists Forum

### Author Topic: Pressure at the centre of the Earth  (Read 3095 times)

#### derek_mcc

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##### Pressure at the centre of the Earth
« on: 04/01/2015 10:56:58 »
Hi folks.  I am aware that the pressure is very high at the centre of the Earth.  However at a drinks party last night someone was elaborating the following idea: the point at the centre has as much mass above it as below it, therefore there is no "weight" pushing down on it from any direction - hence the pressure should be negligible or zero.  By the same logic - pressure should decrease all the way down from the surface.

I get the feeling that there is a flaw in this argument - but if not how are the high pressures down there created?

#### RD

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##### Re: Pressure at the centre of the Earth
« Reply #1 on: 04/01/2015 12:03:51 »
... the point at the centre has as much mass above it as below it, therefore there is no "weight" pushing down on it from any direction - hence the pressure should be negligible or zero ...

If the Earth was hollow , then yes the gravitational forces cancel out, and an object in that hollow would be weightless  ...

Quote from: wikipedia.org/wiki/Shell_theorem
If the body is a spherically symmetric shell (i.e., a hollow ball), no net gravitational force is exerted by the shell on any object inside, regardless of the object's location within the shell.
http://en.wikipedia.org/wiki/Shell_theorem

However Earth is not hollow.
« Last Edit: 04/01/2015 12:05:46 by RD »

#### CliffordK

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##### Re: Pressure at the centre of the Earth
« Reply #2 on: 04/01/2015 23:30:32 »
If, say you had a bubble in the middle of the earth, then there would be no gravity in the bubble (i.e.  one would simply float in the bubble like in space).  That may not be exactly true because the gravitational center of the Earth shifts somewhat because of the moon, sun, and other celestial bodies, but it is close enough.

However, say you measured the pressure/weight contribution of all the rock, say 1 mile from the center, it would also have a negligible contribution to the pressure at the center.  But, if you add up the mass of the rocks for 3,959 miles (6,371 km) in any direction from the center of the Earth, the total contribution of weight of the entire column (or wedge) of rock would be extreme.  Thus you get pressed from all directions by the high pressure.

Now, there is a bit of a static vs dynamic pressure paradigm, but not real exciting.  Say you hold a walnut in your hand.  Now place a 5 pound anvil on top of the walnut.  You press up with 5 pounds, the anvil presses down with 5 pounds.  It is the same if you place it on the table.  The anvil presses down with 5 lbs, and the table presses up with 5 lbs.  So, how many Pounds of pressure does the walnut experience?  Well, it is 5 lbs, not 10.  Perhaps it would be best to consider the pressure per square inch on the walnut.  If each half of the shell is about 1 square inch, then assuming even distribution of weight, it gets 5 psi on top, and 5 psi on the bottom.  Sum it, and one gets 10 lbs over 2 sq in, or 5 psi overall.

#### derek_mcc

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##### Re: Pressure at the centre of the Earth
« Reply #3 on: 06/01/2015 10:43:13 »
Ahhhhh - I think I'm beginning to see .....
If I took a square mile of rock from the surface of the earth - it is being pulled down with the full gravitational effect of the planet.  If I took the same amount of rock from just above the centre - if that rock were isolated then it would have less pull towards the centre than the rock at the surface.  *However* it ISN'T isolated.  The square mile of rock at the surface pushes on the one below that, which pushes on the one below that, ..... which means there IS a huge weight pushing down on the centre from all directions.
If that's correct - then problem solved.  Thanks very much.

#### The Naked Scientists Forum

##### Re: Pressure at the centre of the Earth
« Reply #3 on: 06/01/2015 10:43:13 »