center of the earth

No, it simply depends on our observations of gravity. Why? If you can observe the volume of the Earth, which you can, and you can observe the mass of the Earth, which you can, then you can derive its density, just like you can for anything else.No assumption required. It's not "assumed." It's derived from direct observation. How would magentism attract bodies that are not magnetic? I mean you could posit that when you get on a bathroom scale, it's not your weight being measured, but rather the force of angels pushing down on your shoulders. But why would we believe you? And if the "angels" you propose appear to act just like gravity does for everyone else, why should we believe that gravity doesn't act on you too?The things you think of as assumptions aren't assumption at all; they're just observations that you don't understand. You don't understand how looking at a pair of metal balls can tell you the mass of the Earth. That doesn't mean that they can't.

It's very easy to derive when satellites are passing over mountains instead of valleys from their orbital behavior, and our biggest mountains are scarcly pimples on the Earth's surface - not even noticeable against your 1/4 sextillion tons.Go read a book. Maybe a ninth-grade science book.

Absolutely. And it tells us how the mass is distributed.A satellite's orbit is affected very slightly by weensy little variations in the amount and distribution of material in the body it's orbiting. Sophisticated instrumentation can detect those effects and translate them into a map of density and mass distribution. Previous measurements ahve been made on the orbits of satellites put up for other purposes. Recently, satellites dedicated to this kind of measurement have been put in orbit, and more are coming.. Mapping with GRACE: Twin Satellites Chart Changes in Earth's Gravitational Field, Combined Gravity Field Model EIGEN-CG01C, GOCE — Surfer of the gravitational field.{Added in edit: Everyone should look at the second link, especially the very bottom of the page. Cool images}This message has been edited by JonF, 01-24-2005 17:27 AM

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No, it simply depends on our observations of gravity

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Meaning no other forces, or factors could affect the observation of earth's affect on other planets. Only a precise density. [/quote]Why? If you can observe the volume of the Earth, which you can, and you can observe the mass of the Earth[/quote] Thats why I brought up the shoe boxes, how can we "observe" the matter inside, or mass?

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You don't understand how looking at a pair of metal balls can tell you the mass of the Earth. That doesn't mean that they can't.

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I guess if the earth is a lot like a metal ball, or a lump of lead, they could tell us something. But is it?

There's no such thing as an undetectable factor that could disturb a measurment, because we would be able to detect it through it disturbing our measurements. So, yes, meaning no other forces or factors are involved, because the word "gravity" includes all factors in play with regards to what we're measuring. Mass we detect by its effects on other objects. We don't have to actually see it to know that it's there because all matter has mass. No, it doesn't matter if the Earth is like a metal ball or not; the balls detect the mass of the Earth, not the metal-ball-ness of the Earth. They would detect the mass of a solid Earth the same way they would detect the mass of a hollow Earth.The fact that they're metal balls has absolutely nothing to do with what they measure.

Perhaps Cosmo had intended the 19th century view of the aether and/or Mach's "principle" that Einstein once founded a discussion of cosmology on, and generates so much polemic that a modern physics journal will not publish material on that basis? And no, I dont begrude you.

"because it is impossible to recognize whether a "dent" in the gravitational field has its origin in the interior of the Earth or on the surface. Only in conjunction with other methods, like seismology, can the causes be separated. " [!!!! back to the waves again!]
". In other places they drift apart and this is where material from the deep inside the Earth rises to the surface. The re-searchers are interested in what is hidden beneath these fault zones. " [so am I, maybe one day they will know.]
"It rides over a slightly wavy washboard. In the region of stronger gravitational force, it speeds up and climbs, whereas in a region of weaker gravitational force it slows down and drops. Following the path of the satellite exactly, the terrestrial gravitational field can be recon-structed from the orbital deviations. " [from your link]
So, a sattelite rides over a wavy washboard, so to speak. Then, we get a little variation we can map out. We can see dips and bulges in the field. Fine.

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Sophisticated instrumentation can detect those effects and translate them into a map of density and mass distribution..

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In other words then, a bump would tell us an area is more dense. We see a bulge on the surface, or rather, can detect one. Briefly, why is this? I wonder, because it hasn't hit me yet, as to how the bulge adds up to a required density inside earth. Is it, then, because this is the only thing that could attract?

Perhaps. Like a sunny day on a beach, warmth engulfs me.

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There's no such thing as an undetectable factor that could disturb a measurment, because we would be able to detect it through it disturbing our measurements

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True, long as you knew your measurements were disturbed!

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So, yes, meaning no other forces or factors are involved, because the word "gravity" includes all factors in play with regards to what we're measuring.

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So all forces = gravity, and in all their play, they are totally detected. Fine.

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Mass we detect by its effects on other objects. We don't have to actually see it to know that it's there because all matter has mass

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Yes, I think matter and mass are most often detectable. But you know, they say God is in the details!

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No, it doesn't matter if the Earth is like a metal ball or not; the balls detect the mass of the Earth, not the metal-ball-ness of the Earth.

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Ah. OK. So these balls were attracted to earth in a way that indicates the earth has a certain density. Would, by chance, that happen to be how much gravity was at work on them? So, something like, 'if they fall real fast, we know it is very very dense overall in the earth'? Hey, did they do sixty or seventy of these experiments, some with things non magnetic, in different parts of the earth? You know, we wouldn't want to do it on a dip or a bulge!

Yup.

Yup. Literally tens of thousands, maybe hundreds of thousands, of times. With all kinds of different materials, metal and non-metal, magnetic and non-magnetic, in all sorts of different places, with increasing precision over time, and with different types of apparatus. Cavendish's version is a standard experiment in many college courses. People have done it with different materials hoping to find some exception to Newton's laws. Nobody has yet. Instructions for doing the experiment at the University of Chicago as part of Physical Sciences PS 11900: Stellar Astronomy and Astrophysics. Google turns up lots of similar things at other universities.One of many papers including work on different materials is A new precise determination of Newton's gravitational constant. Se also Experimental Gravity Program, Department of Physics and Astronomy, University of California, Irvine.Have you noticed that every possiblity you raise that might be a problem with the theory has been anticipated by many scientists, often hundreds of years ago, and has been tested far more thoroughly than you could have imagined? Reflect on that and your state of knowledge.

No. They indicate that the Earth has a certain MASS, and that the balls were a certain (measured) distance from the center of that mass. The density is a calculated side-dish - we can get it from mass divided by volume, but it's unimportant in Cavendish's experiment.

My first reaction was that it was "trolling" behavior.To think that one of the first experiments, and one devised in 1783 sets the limits to measurements is extraordinarily obtuse.http://www.physics.sfsu.edu/~ggrist/490/Cavrpt/cavrpt.html

OK. Now, it seems your links have proved garavity works, and that it works on everything. I suspected as much. Otherwise I suppose it would be called the theory of gravity. Thanks for digging that up. I couldn't see though in that, what was the thing that slowed us down from moving to the next point, which is as follows. It is certain gravity works on everything, and thank heaven for it. But is it certain it is only a result of the density of a planet? If so, fine, no need to linger on it. But, we know earth is very dense, yes, but basically, is gravity solely based on density? If so, how do we know?

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They indicate that the Earth has a certain MASS, and that the balls were a certain (measured) distance from the center of that mass

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So then, it is only mass that makes gravity?