Atmosphere - Page 5

dmm · #41 February 1st, 2001, 09:32 PM

Krakenup and Apache,
You're both confused. Fortunately for you, I'm a physicist, and I'm here to set you straight.

Both of your equations are correct, but you are both making bad assumptions.

If you use the equation that has only mass and radius, then you have to keep in mind that you only use the mass INSIDE the radius. Jupiter's solid surface, if it has one, is obviously way below the gaseous surface, so you would not use the total mass of Jupiter for your M. (It would be way too high.)

On the other hand, if you use the equation that has only density and radius, then you have to keep in mind that you have to use the AVERAGE density INSIDE the radius. Since the density increases as you get closer to Jupiter's core, you clearly should not simply use Jupiter's overall average density. (It would be way too low.)

But, of course, all this talk is silly anyway, because the heat and pressure at Jupiter's solid surface (if it has one) would be enormous. Nothing could live there. Any life would have to be in the atmosphere. At what radius? Who can say?

apache · #42 February 1st, 2001, 09:38 PM

First off, yes, the 3520 was a typo, one I noticed and thought I fixed, but apparently did not.
Now, no no no, density has nothing to do with gravity. It is only distance and mass, and density is a function of mass and volume. At a distance of R from some mass M, the gravitational acceleration at that distance is constant, no matter how dense the mass is.
If you have a black hole with the mass of the sun, it will definitely be a few million times denser than the sun. However, the gravitational force on the earth from the black hole will be exactly the same as the force the sun has on the earth, if the earth orbited a black hole at the same distance it orbits the sun.
Now, if you were at the center of Jupiter, theoretically you would either experience infinite gravity or gravitational force based on the mass of the planet. Logic says its no gravity, but then again, there cannot be zero gravity, in fact there would at least have to be a force completely pulling you apart because you are completely surrounded by the mass of Jupiter.
But then again, we really do not know since we have never been to the center of any planet or moon or anything to see what the gravitational forces would be.

dmm · #43 February 1st, 2001, 10:17 PM

quote:
Originally posted by apache:
If you have a black hole with the mass of the sun, it will definitely be a few million times denser than the sun. However, the gravitational force on the earth from the black hole will be exactly the same as the force the sun has on the earth, if the earth orbited a black hole at the same distance it orbits the sun.

Your statement is correct but you are missing the point. The AVERAGE density of the mass INSIDE the earth's orbit is the SAME for the sun and the black hole.

quote:
Originally posted by apache:
Now, if you were at the center of Jupiter, theoretically you would either experience infinite gravity or gravitational force based on the mass of the planet. Logic says its no gravity, but then again, there cannot be zero gravity, in fact there would at least have to be a force completely pulling you apart because you are completely surrounded by the mass of Jupiter.

I'm sorry, but I cannot give you even partial credit for this answer. You could have either used integral calculus or else used Green's Theorem and symmetry arguments. Either way, the correct answer is that the gravitational force at the center of a planet is zero.

quote:
Originally posted by apache:
But then again, we really do not know since we have never been to the center of any planet or moon or anything to see what the gravitational forces would be.

We do not need to go there and measure it. We have known the answer from theory since Newton's Principia was published.

Krakenup · #44 February 1st, 2001, 10:25 PM

quote:
Originally posted by apache:
First off, yes, the 3520 was a typo, one I noticed and thought I fixed, but apparently did not.
Now, no no no, density has nothing to do with gravity. It is only distance and mass, and density is a function of mass and volume. At a distance of R from some mass M, the gravitational acceleration at that distance is constant, no matter how dense the mass is.
If you have a black hole with the mass of the sun, it will definitely be a few million times denser than the sun. However, the gravitational force on the earth from the black hole will be exactly the same as the force the sun has on the earth, if the earth orbited a black hole at the same distance it orbits the sun.

Nice try at changing the ground rules, but we're talking about the gravity at the surface of a celestial body. The gravity at the surface of the black hole (however you define that) is nearly infinitely greater than the gravity at the surface of the sun because the black hole is nearly infinitely denser than the sun and thus has an infinitesimal radius. In the equation using mass and radius, this shows up as a very small r resulting in a very large gravitational force.

I realize, of course, what you are trying to say, but describing the phenomenon using only mass and radius is awkward. You could say: "Well, Uranus is very big and has a very big mass, and gravity only depends on mass and size, but its gravity is less than Earth's because . . . uh . . . well, the ratio of the mass to the size is not all that big." Since the ratio of the mass to the size is the density, the explanation becomes "Uranus has low surface gravity because of its low density." Simple, understandable and absolutely accurate.

apache · #45 February 2nd, 2001, 12:50 AM

Krakenup, I have given up on you. I really don't have a clue why we keep saying the same things differently and keep thinking the other one is wrong. The original argument I was making was that the surface of Jupiter is not at its outer atmosphere, and we honestly do not know what it's interior is composed of. Hence, declaring 'surface gravity' of a gas giant is pretty useless since we don't know if there is a surface or even where it is.
Now, as for DMM, if you have read some of my other Posts lately, you would know that I am not going to take a theory lying down.
First of all, there is the massively disputing theory of general relativity, which essentially throws Newtonian theories out the window. I'm not gonna argue anything about it, though, since I have a limited understanding, and its all theory anyways.
Second, anything regarding the gravitational forces at the center of a massive object is completely theoretical, and there is absolutely no experimental proof to support any of it. Therefore, taking it as a fact is something best done with a grain of salt, rather, an entire salt shaker.
Third, getting a net zero gravitational force is vectorial and mathematically correct, but the fact is that the gravitational forces could not cancel each other, putting no force on the body, they just act to keep the object at the center of the body in equilibrium, or in otherwords, they will not induce a velocity or accleration on the body as a whole in any direction.
However, if the object at the center could not withstand the forces pulling on it, it would indeed be ripped appart. Think of it like a reverse pressure. Rather than keeping an object from expanding, like air pressure acting on a balloon, it would keep an object from imploding.
Anyways, categorically saying I am wrong is wrong.

LintMan · #46 February 2nd, 2001, 08:13 AM

quote:
Originally posted by Puke:
since we have completely destroyed this unsuspecting little thread, i will ask:

does anyone remeber what the gas giant hydrogen breathers were like in the David Brin books (uplift/startide/sundiver)? i dont, but i remember he had 'em.

Brin doesn't say too much about the sun creatures in Sundiver (and nothing at all in uplift/startide, IIRC). At the time of Sundiver, I don't think humanity knew much about the sun creatures and was just establishing contact.

In Brin's second series (Heaven's Reach), there is more discussion about gas breathers, but they aren't Sun-based, just gas giant-based. Those gas breathers were amophous globules with a very different, image-based (?) thought and communication processes.

LintMan · #47 February 2nd, 2001, 09:10 AM

quote:
Originally posted by apache:

Now, as for DMM, if you have read some of my other Posts lately, you would know that I am not going to take a theory lying down.
First of all, there is the massively disputing theory of general relativity, which essentially throws Newtonian theories out the window. I'm not gonna argue anything about it, though, since I have a limited understanding, and its all theory anyways.
Second, anything regarding the gravitational forces at the center of a massive object is completely theoretical, and there is absolutely no experimental proof to support any of it. Therefore, taking it as a fact is something best done with a grain of salt, rather, an entire salt shaker.

General Relativity doesn't dispute Newtonian physics, it encompasses them. Newtonian physics works, as we see proven every day with our satellites, rockets, and planetary astronimcal observations. The term "theory" here does not mean "completely unproven" - newtonian and GR physics have been borne out in countless scientific experiments. To say "it's all theory anyway" implies it is all completely untested and anybody's guess, which is totally untrue.

You can say we can't be sure about the gravity at the center of a planet just because we haven't measured it directly, but the world uses the math of physics to predict behavior/properties of things reliably, all the time.

quote:

Third, getting a net zero gravitational force is vectorial and mathematically correct, but the fact is that the gravitational forces could not cancel each other, putting no force on the body, they just act to keep the object at the center of the body in equilibrium, or in otherwords, they will not induce a velocity or accleration on the body as a whole in any direction.
However, if the object at the center could not withstand the forces pulling on it, it would indeed be ripped appart. Think of it like a reverse pressure. Rather than keeping an object from expanding, like air pressure acting on a balloon, it would keep an object from imploding.

Hmmm. I'd say that if the net gravitational force on an object is zero, that by definition, all the gravitational forces have canceled out. Gravity is not like pressure in the example you give. For a small object (relative to the size of the planet), say a person, if the net gravity is zero for that person, every point (ie: every cell, every molecule) on/in the person will have a net gravity of zero. The tidal differences between, say, points on the person's left and right arms would be too small to matter.

Cybes · #48 February 2nd, 2001, 11:03 AM

quote:
Originally posted by LintMan:
The term "theory" here does not mean "completely unproven"

yah - the term for that is "hypothesis". a theory is a hypothesis which has been shown to explain currently known facts, and predict new ones.

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"Just think of it as Evolution in action" - 'Oath of Fealty', by Larry Niven & Jerry Pournelle.

[This message has been edited by Cybes (edited 02 February 2001).]

jimbob55 · #49 February 2nd, 2001, 03:51 PM

*wanders about staring off to the horizon* Nope, can't see the topic anywhere....

If I took 2 small (say the size of a beachball) black holes and through cunning and artifice managed to get them to orbit their mutual center of gravity.....Then i stuck my head into that center of gravity, would it clear my sinuses?
Or would it just turn my head into a thin string of skull and brain matter linking the two?

I ask, because this cold cure stuff doesn't work and I happen to have 2 small black holes lying round the place.

I am just joking. Please understand that any form of medical treatment involving singularities should only be used on the advice of a doctor (possibly of physics)

Alpha Kodiak · #50 February 2nd, 2001, 06:06 PM

quote:
Originally posted by jimbob55:
*wanders about staring off to the horizon* Nope, can't see the topic anywhere....

If I took 2 small (say the size of a beachball) black holes and through cunning and artifice managed to get them to orbit their mutual center of gravity.....Then i stuck my head into that center of gravity, would it clear my sinuses?
Or would it just turn my head into a thin string of skull and brain matter linking the two?

I ask, because this cold cure stuff doesn't work and I happen to have 2 small black holes lying round the place.

I am just joking. Please understand that any form of medical treatment involving singularities should only be used on the advice of a doctor (possibly of physics)

I think the correct treatment, assuming a small enough singularity, would be to insert the singularity into the sinus cavity to promote drainage. Of course, I am not a doctor, I only play one on TV. Ok, ok, I only dream of playing a doctor on TV. Actually, I'm just a software engineer, but I am relativistically certain that if you place a black hole up your nose, you will no longer have any concerns about sinus congestion.