Gravity at the center of the Earth?

<BLOCKQUOTE id=quote><font size=2 face="Verdana, Arial, Helvetica" id=quote>quote:<hr height=1 noshade id=quote>[Jim]: The pressure at the center of a sphere is no greater than the pressure at the surface. This is a very important fact that is now not known in the field.<hr height=1 noshade id=quote></BLOCKQUOTE id=quote></font id=quote><font face="Verdana, Arial, Helvetica" size=2 id=quote>

Indeed. To my knowledge, it is not known anywhere outside of your mind.

If a fluid planet has uniform density, then the force of gravity gets weaker as one goes deeper into the planet. It goes to zero at the center.

In the same planet, the pressure continually rises as one goes deeper. (We measure this in Earth's oceans.) Exactly how and why does it behave on the way down so that it reverses and ends up at zero pressure at the center? This is contrary to all experimental evidence that I am familiar with.

<BLOCKQUOTE id=quote><font size=2 face="Verdana, Arial, Helvetica" id=quote>quote:<hr height=1 noshade id=quote>[MV]: If some fusion reactions were occuring at a planet's core, what would the gravity be at the core assuming that the fusion was under extreme high pressures? Would there be exponential shielding effects as the gradient of pressure increased? MV<hr height=1 noshade id=quote></BLOCKQUOTE id=quote></font id=quote><font face="Verdana, Arial, Helvetica" size=2 id=quote>

Gravitational force would still be zero at the core. Pressures are high, but forces come equally from all directions and cancel. It might be possible for densities to rise to the point where shielding became major, but probably not without a total core collapse first. -|Tom|-

You have refered to experimental evidence that shows pressure continues to increase with depth without any reguard to gravity. Can this proof be accessed and where? I guess a new idea has to begin somewhere so I suppose as you said at this time this one only exists in my mind. Look at the units for pressure and see that it is simply mass per unit of area. What you are saying is quite impossible. The pressure at the center would reach infinity. The mass at the center of a sphere is all around and resting at the gravity center and that is near the surface not near the center.

<BLOCKQUOTE id=quote><font size=2 face="Verdana, Arial, Helvetica" id=quote>quote:<hr height=1 noshade id=quote>You have refered to experimental evidence that shows pressure continues to increase with depth without any reguard to gravity. Can this proof be accessed and where?<hr height=1 noshade id=quote></BLOCKQUOTE id=quote></font id=quote><font face="Verdana, Arial, Helvetica" size=2 id=quote>

Are you really unfamiliar with the knowledge that water pressure increases with depth? Every diver knows this. I don't keep citations in that field. Jim. Please read a book.

<BLOCKQUOTE id=quote><font size=2 face="Verdana, Arial, Helvetica" id=quote>quote:<hr height=1 noshade id=quote>What you are saying is quite impossible. The pressure at the center would reach infinity. The mass at the center of a sphere is all around and resting at the gravity center and that is near the surface not near the center.<hr height=1 noshade id=quote></BLOCKQUOTE id=quote></font id=quote><font face="Verdana, Arial, Helvetica" size=2 id=quote>

I don't understand a word of this. Earth's center of gravity is at its center. The pressure there is just the weight of all the higher layers pressing down. That is hardly infinite.

Unless someone else has the patience to carry on, I give up. Jim, you know too little, but assume that your knowledge is nearing completion. With a little more experience you will come to realize that none of us knows how much we don't yet know until we have learned nearly all there is to learn in a given field. That is why it typically takes 20+ years of education to reach the doctorate level -- the level that nominally qualifies someone as a researcher. -|Tom|-

Hum, Would not the pressure "weight" toward the core of a planet decrease somewhat, Say once you pass to within less then 25 percent of the plant radius (I guess this assumes a cold core). This as more of the planets mass would be above you which would tend to decrease gravity a tad and corresponding drop in pressure.

I think Jim's comment about the pressure at the core of a sphere being the same as the surface would only apply if the sphere was composed of gas inside say a steel shell. I suspect that that would only apply for gas filled sphere's of say less then 10-20 kM in diameter.

For much larger gas spheres you would see an increase in pressure toward the center ( Up to a point inward that is) once there was enough mass for gravity to have some effect. Eventually if you had enough gas the pressures would build up enough to generate heat which would generate more pressure, which generates more heat, etc. etc till you had yourself a star. Takes a fair amount of gas though. Even though the pressure would be high due to heating the gravity at or near the center would be quite low.

I'm probably still an idiot. But hey I've just a practical engineer. I am hopping to learn though.



Ben

<BLOCKQUOTE id=quote><font size=2 face="Verdana, Arial, Helvetica" id=quote>quote:<hr height=1 noshade id=quote>Hum, Would not the pressure "weight" toward the core of a planet decrease somewhat, Say once you pass to within less then 25 percent of the plant radius (I guess this assumes a cold core). This as more of the planets mass would be above you which would tend to decrease gravity a tad and corresponding drop in pressure.<hr height=1 noshade id=quote></BLOCKQUOTE id=quote></font id=quote><font face="Verdana, Arial, Helvetica" size=2 id=quote>

No, this mixes force and pressure. No matter how deep the atmosphere or the ocean was, there would never come a point where pressure started to decrease. Pressure, in a manner of speaking, is the accumulated weight of all the layers directly above. For example, 14 psi means that a column of air one square inch in area from the ground to the top of the atmosphere would weigh 14 pounds.

At the center, force (gravity) goes to zero. But the pressure at the center is still coming in from all sides because each layer squeezes on the one below it. That lower layer relays the pressure it feels from above, and adds its own weight to it. And so on to the center.

<BLOCKQUOTE id=quote><font size=2 face="Verdana, Arial, Helvetica" id=quote>quote:<hr height=1 noshade id=quote>I think Jim's comment about the pressure at the core of a sphere being the same as the surface would only apply if the sphere was composed of gas inside say a steel shell. I suspect that that would only apply for gas filled sphere's of say less then 10-20 kM in diameter.<hr height=1 noshade id=quote></BLOCKQUOTE id=quote></font id=quote><font face="Verdana, Arial, Helvetica" size=2 id=quote>

A steel shell is a rigid structure that resists pressure (up to some crushing point) and does not relay exterior pressure into its interior. But everything inside the Earth below the crust is already getting pressures way above the crushing point, and so has no significant ability to resist relaying those pressures on down to the center.

<BLOCKQUOTE id=quote><font size=2 face="Verdana, Arial, Helvetica" id=quote>quote:<hr height=1 noshade id=quote>I'm probably still an idiot. But hey I've just a practical engineer. I am hoping to learn though.<hr height=1 noshade id=quote></BLOCKQUOTE id=quote></font id=quote><font face="Verdana, Arial, Helvetica" size=2 id=quote>

You ask good questions. No one is an idiot for not knowing something, and the exercise of asking and answering questions is how we all probe the limits of our knowledge and our understanding. I ask plenty of questions every day -- every time I pick up a journal. <img src=icon_smile_wink.gif border=0 align=middle> -|Tom|-

Ok, Thought about it and your right. There would be quite a bit of pressure at the center but zero or near zero gravity too. I wonder how far from the center of a planet the zero G extends and does the gravity increase linearly with distance or increase steeply only a short distance from the center.

Thanks again,

Ben