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John VanSickle <evi### [at] hotmail com> wrote:
> Rick Measham wrote:
> > Mike Williams wrote:
> >
> >> The strength of the force varies with cos(latitude) and the angle to the
> >> local vertical is proportional to latitude. So people standing near the
> >> poles get very little force, and what force there is would be almost
> >> horizontal.
> >
> >
> > Which is where the original argument came from (and I agree) .. the
> > poles would implode, thus degrading the structure and causing the
> > equator to explode.
> >
> > One poster (somewhere, not here) suggested a band rather than a sphere.
> > The 'band' would be the equatorial region and would (somehow) hold
> > together as it spun.
>
> And then some other killjoy did the math and realized that if the ring
> were to be moved so that the star was no longer in the center, the
> situation would not correct itself naturally; if there were no
> artificial corrective measures, the ring would eventually collide with
> the sun. Roll the credits.
>
> BTW, Dyson himself did not postulate a solid sphere, but a large number
> of small bodies which collectively capture all of the output from a
> given star.
>
> Regards,
> John
For practicality I prefer Banks' 'Culture' ring model which is to have a
smaller ring orbiting in a star's life zone, spinning once per day to give
one g of centripetal accelleration and with a small rotation plane offset
from the sun. If I got my calculation right (a = lw^2) the radius is about
1.85 million km, which still gives a huge surface area for a reasonable
ring width - and you can build several in one system :-)
(Wanders off, dreaming of how to reduce Jupiter to workable material ...)
Mike.
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Larry Hudson wrote:
> No, it's not a resonnance. The way I've heard the two tides explained
> is that the ocean is raised by the moon's gravity on that side of the
> earth, but it also pulls the _earth_ away from the water on the far
> side.
Um, not really.
Tides are caused when any large body orbits a point. Consider two rocks
on the moon, one on the ground very close to the Earth, one on the
ground on the side we never see. The one on the ground close to the
Earth is going slower than it would if it were all by itself in the same
orbit without the moon. A lower orbit is a faster orbit, so the rock
there is going too slow, so it should fall down towards the earth. A
higher orbit is a slower orbit, but the rock on the far side is actually
travelling faster than the rock on the near side instead of slower, so
it would normally be "flung away" from the center. The smaller the
radius of orbit compared to the size of the orbiting body, the more
evident the effect. The stronger the gravity, of course, the more
evident the effect.
It hasn't anything to do with pulling the centers of planets towards or
away from anything. It has to do with the fact that from outside a
system, gravity can be calculated as a point source, but inside a system
you have to account for distances.
When a body gets close enough that the orbital pull overcomes its
gravity, it breaks up. That limit (for a fluid, where only gravity is
holding it together) is called the Roche limit.
http://en.wikipedia.org/wiki/Roche_limit
See the pictures for an explanation of tides that is actually correct. :-)
--
Darren New / San Diego, CA, USA (PST)
Neither rocks nor slush nor salted rims
shall keep us from our appointed rounds.
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Rick Measham schrieb:
>
> (And the gravity on the outside would be horrendous, the gases from the
> sun would kill you, there's not enough matter to make it .. yada yada ..
> great concept for SciFi, and ray tracing, but not real life!)
>
> Cheers!
> Rick Measham
That's another reason why I prefer the idea of a "ringworld" of the
concept of the dysons sphere. Make a rim with walls on both side so that
the atmosphere is kept inside and spin it. Solves at least the problem
of atmosphere distribution and variable gravity which is an inherent
problem of the concept of a spinning hollow sphere. AND you don't need
that much materia to build it. But even Larry Niven (AFAIK a physician
writing SciFi) had to introduce some sort of hyperdense material so that
this small hull want fly apart.
... dave
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Rick Measham schrieb:
> Mike Williams wrote:
>
>> The strength of the force varies with cos(latitude) and the angle to the
>> local vertical is proportional to latitude. So people standing near the
>> poles get very little force, and what force there is would be almost
>> horizontal.
>
>
> Which is where the original argument came from (and I agree) .. the
> poles would implode, thus degrading the structure and causing the
> equator to explode.
>
> One poster (somewhere, not here) suggested a band rather than a sphere.
> The 'band' would be the equatorial region and would (somehow) hold
> together as it spun.
>
> ------------------------------------------------------------------------
>
this is exactly the idea of the "ringworld". Larry Niven wrote about
twenty novells or so around a fictive world like this.
... dave
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On Fri, 14 Oct 2005 22:01:07 +0200, David El Tom wrote:
> But even Larry Niven (AFAIK a physician writing
> SciFi)
Larry Niven holds a BA in Mathematics and a minor in Psychology.
http://en.wikipedia.org/wiki/Larry_Niven
Jim
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Nobody has yet mentioned the *quantitative* demonstration that the
gravity of an uniform hollow sphere is cancelled inside.
Consider a particle within the sphere and a pair of equal and opposite
narrow cones whose apex is that particle. Each cone meets the sphere in
an ellipse. (The ellipses have similar shape because the axis of the
cones meets the sphere at the same angle on both sides.) The area of
the ellipse, and thus the amount of mass pulling the particle in that
direction, is proportional to the square of the distance from that part
of the sphere to the particle. But to get the amount of force you must
then divide by, guess what, the square of that same distance. Thus the
forces on the particle from the two opposite cones are equal and opposite.
--
Anton Sherwood, http://www.ogre.nu/
"How'd ya like to climb this high *without* no mountain?" --Porky Pine
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> "John VanSickle" a escrit
>> BTW, Dyson himself did not postulate a solid sphere, but a large
>> number of small bodies which collectively capture all of the output
>> from a given star.
Marc Jacquier wrote:
> How to avoid collisions between these bodies then?
They orbit at slightly different distances.
And I'll bet Dyson qualified the conjecture with some such phrase as "in
the limit", i.e. he doesn't expect perfectly complete capture, but the
amount of light captured gets ever more complete over time.
--
Anton Sherwood, http://www.ogre.nu/
"How'd ya like to climb this high *without* no mountain?" --Porky Pine
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> this is exactly the idea of the "ringworld". Larry Niven wrote
> about twenty novells or so around a fictive world like this.
I forget whether "twenty" in this case means three or four.
--
Anton Sherwood, http://www.ogre.nu/
"How'd ya like to climb this high *without* no mountain?" --Porky Pine
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David El Tom wrote:
> . . . . But even Larry Niven had to introduce some sort of
> hyperdense material so that this small hull want fly apart.
Hyperstrong, not hyperdense; making it heavier only adds to the problem.
--
Anton Sherwood, http://www.ogre.nu/
"How'd ya like to climb this high *without* no mountain?" --Porky Pine
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Mike Williams wrote:
> Where does your hollow asteroid get its light from?
Perhaps the other end is glass.
--
Anton Sherwood, http://www.ogre.nu/
"How'd ya like to climb this high *without* no mountain?" --Porky Pine
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