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Linked are some HB discussions concerning hollow celestial
bodies. Relevant issue: there is no gravity inside a hollow
celestial body, but the gravity on the outside of the hollow
body is the same as for a nonhollow body of the same mass
and size.
The gravity engine is a seesaw perched on
the edge of a
hole to the interior of the shell planet. A weight on one
arm accelerates under gravity, pulling the arm down. The
weight goes thru into the inside - now it is weightless and
moving only under inertia, with no accelerating force.
The seesaw is symmetrical. The other arms weight is
inside the shell and so has no gravity pulling it. The pull of
gravity on the other side pulls that side down and the
nongravity side up. On emerging to the top side gravity
gets it, but it continues under its own momentum to the
top. Repeat.
The cycle repeats. It has one of those frictionless bearings
you bunch of buzzkills so no losses there. And no air
resistance because we got rid of all that.
Enlarge the moon
Enlarge_20the_20Moon
[bungston, Sep 13 2014]
Moon VLO device
Moon_20VLO_20Device
Relevant hollow sphere lessons halfway down annos. [bungston, Sep 13 2014]
[link]
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This is no more perpetual motion than an asteroid
whizzing through space. Yes it will go on until
stopped but there's no point. |
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Where are the robots with lasers? |
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Well, let me think: If you took out the seesaw part, and just used one
weight, you could drop it in the hole, let it drift clear through the
hollow, rise up through another hole on the other side, and
immediately it would slow, drop back, and return cycle. That's just a
more abrupt version of any tunnel through the middle of any planet.
So yeah, that's just a funny orbit. |
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But this seesaw, well, the energy of the lower weight was put into the
weight by gravity --- and that, momentum-wize, is going to be
balanced exactly by the energy being put into the other weight by
gravity. All the momentum that gravity puts in is going to be opposed
by the gravity acting on the other weight. No net gain, then. |
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If this was a wheel, it might keep coasting forever, from your starting
impulse, gravity being irrelevant. But a seesaw, I don't see it
oscillating, even from your starting push... wait, it's just an odd case
of those swings where two people face each other (so there are two
weights) and the pivot is up above. One person goes way up, the
other drops directly below the
pivot to where their weight isn't being lifted as much as being moved
side-to-side. (Build a 90° arm, put a weight at each end, and a pivot in
the bend. Set it up on a normal planet, and any time one weight is
directly below the pivot, it's weight doesn't count, just the
momentum/mass.) So the design is less a seesaw than a freak
swing. Okay, it will oscillate from your starting push. |
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But no free power. Clever, though. Dang clever. [+] |
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You can build the same device on earth, by hanging the weights on weightless chains from the ends of the seesaw beam. The weight on the uphill end will hang on its chain, but the weight on the downhill end will rest on the ground and therefore put no force onto the downhill end of the seesaw. |
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Build it big enough and hang earth off it. Then it could be
used to heal cancer. |
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This would, shirley, be the same as a seesaw built
over water, with the weights being neutrally bouyant
in water (ie, no net force on a weight when it's below
the water surface, but a net force on it when it's
above the water surface). |
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I would recommend building the over-water
prototype before you set about hollowing out the
earth. |
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I agree with [MaxB]. I thought of that, too, but couldn't really come up
with frictionless water, so I left it out. |
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I did think of a long weightless stick with weighted ends, running all
the way through a planet (bog-standard or imaginarium shell). |
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You could always use superfluid helium... |
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As a matter of fact, I can always use some superfluid helium. Do you
happen to have some extra? |
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[I should have said that I read the anno above in the voice of Obi-Wan
Kenobi, with the Jedi handwave: "You could always use
superfluid helium." "We could always use superfluid helium. Move
along."] |
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I have lots extra. But not of superfluid helium, alas. |
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How is the rod running through the hollow planet, significantly different from a pendulum suspended above the pre-hollowed planet? If the pendulum is pendulated in a vacuum chamber and suspended from contactless bearings then I theorise that it too will mobilate perpetualistically |
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If you're going on the erroneous assumption that there's no gravity within a sphere, you may as well test it using frictionless water. Objects within a sphere will "fall" towards the centre of mass. |
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Without going into all the geometry of it, it comes
down to this: |
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An object touching the inside wall of the sphere is
obviously closest to that part of it, so there is a
gravitational force pulling it against the wall. |
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However, the opposite hemisphere, although further
away, has a much greater mass than the adjacent
portion of the wall, and pulls the object away from
the wall. |
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(This is a simplification, but you can quantize the
sphere into smaller components and add up their
effects with the same result. At the limit, you're
obviously integrating a continuous function.) |
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The geometry is such that these effects cancel out
perfectly, and there is no net force on an object
anywhere inside the sphere. |
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[pocmloc], I meant that the rod running through the planet was the
same, another analogy. |
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I thought the same thing, FlyingToaster. Too
much Edgar Rice Burroughs. The good lurch does
go into the math behind why a hollow sphere is
gravity free inside in the annos of the Moon VLO
idea. |
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It just feels like if there were some region with an
accelerating force right next to a nonaccelerating
force one should be able to suck energy off of it.
But at the end of the day this is not much
different from the same thing powered by
magnetism. |
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The lake and the floats is nifty but not the same
because even if neutrally buoyant the float in the
lake will be slowed by the water. |
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ah so.... as you approached the sphere gravity would increase then, as you went through the shell it would decrease, reaching zero as you passed completely into the interior. |
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This is nuts you guys. If you eliminate friction a totally
mundane seesaw is already perpetual without the addition
of magical gravity shells. What are you all smoking? |
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If you wanted to do some practical testing, start out by
looking over Faraday's "ice pail experiment". I believe you
could build an analogous device, and use something less
than a planetary mass... |
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(of course, it would also allow you to cheat: electrical
charge can be turned on and off much more easily than
gravity can) |
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[WcW], I happen to be all smoking hawt, but that isn't the point. |
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A regular seesaw isn't perpetual unless you put a couple of perfect
springs under it. The original idea up there would be perpetual, as
described, no springs. |
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But the claim was that it would generate power. We figured out that it
would not, by ... let's see ... the first anno. |
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So we are just hanging out, talking about it, and stuff like it, that won't
generate free power, either. That's what we are smoking. What are
you smoking? |
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Bacon, barring friction a perfect seesaw keeps on
rocking forever in relational motion to the mass it is
rocking on because there is no way for that inertia to
become any other form of energy. Springs, just as
they do in a more frictional system, do not and can not
make the system more perfectly symmetric and thus
do not make it any more perpetual. A perfect
symmetrical counterbalance acting in a gravitational
field will oscillate perpetually once started just as an
object in motion will remain in motion unless acted on
by an outside force. |
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See, where I come from (Carver Elementary School's playground), a
seesaw is a long board with a child seated on each end and a pivot in
the middle (sometimes called a "teeter-totter"). It can't keep going
forever, because in real life, a kid pushes with the legs to make it
change direction (or it just bonks into the ground and a spine gets
compressed). So I added springs to my imaginary one to get it to
oscillate. |
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Now, you may be thinking of a balance beam from a scale or a swing
or something, that would rock back and forth from an initial push. But
to me, a seesaw doesn't do that --- it needs something to send it back
where it came from. |
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//But to me, a seesaw doesn't do that --- it needs something to send it back where it came from. |
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Wouldn't a bounce off the ground have that covered? In a frictionless, airless, soundless world, where else is bounce momentum going to go? |
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A good bounce off the ground would indeed work, but the board
hitting the ground is so painful that most places put cushioning
material under the board to absorb impact energy. Which is why I
mentioned putting in springs, to get that elastic return. |
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A seesaw is not naturally oscillating. A swing is. |
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(Why has this gotten so dang confusing? Have seesaws truly been
removed from all playgrounds?) |
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A hollow planet with an opening in the shell for a mass to pass through essentially creates an odd shaped gravitational field. But, as others have pointed out, it's still a field and you can't extract work from it by shuffling mass around. At best, you can get back to where you started. |
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Also, you've put holes in the shell, so now there's a gravity pulling both masses down. |
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Say you give a space-contractor some plans and he asks "Can I use the material removed from the holes to make the masses?" So, now what? You've got two round plugs that bob up and down in the holes, and the whole thing starts looking like an Earth-based see-saw that traverses a gravity gradient due to altitude. |
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What? No. You don't need holes for the gravity to get out. Putting
holes in a shell affects nothing as far as the gravity is concerned,
except trivially. |
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Nobody is saying the original idea will generate energy. It will keep
oscillating once started, that's all. |
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I know the idea as advertised oscillates forever.
I'm adding that it's not much different from a large
see-saw on a frictionless Earth. Or something. |
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The holes are not trivial to me. Try to sketch this
thing with any appreciable thickness to the shell
and you'll see what I mean. |
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Remove the see-saw and drop one mass through one
hole. Put a spring at the opposite inside wall of the
planet. Much easier to visualize. |
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A pendulum be it double armed or simple will oscillate
in a gravitational field slowed only by friction in one
form or another.Your ridiculous hollow globe is simply
a monumental red herring which has clearly
distracted you into the illusion of having discovered
something when you have not. |
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But we have already agreed that the hollow globe is frictionless. |
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Then the pendulum will be exactly as perpetual in both
scenarios; perfectly perpetual. It will contain exactly the
same quantity of potential energy also, that is, the same
energy as it took to set it in motion. |
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Perpetual motion machines make for interesting thought experiments and sometimes amusingly choleric prose. |
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The hollow sphere concept is interesting, not ridiculous. It seems counterintuitive, even after having been spoonfed the physics. Even more counterintuitive is that gravity, which I think of as a continuum, might be present in full planetary splendor at one point and completely absent 10 meters away. |
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One must have ideas on the HB. A perpetual motion machine is a fine device for consideration of an unusual circumstance of forces. |
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//Even more counterintuitive is that gravity, which I think of as a continuum, might be present in full planetary splendor at one point and completely absent 10 meters away. |
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Sure, it's counter intuitive if you imagine an extremely dense material that is somehow both vanishingly thin compared to its radius and yet immune to thin-shell collapse. Look, I bunned this idea because I think it's clever, but a planet-sized shell is going to be MUCH thicker than 10 m. So thick, I think, that you'll actually have a noticeable continuum as you pass through the thickness. |
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For example, a hollow Earth with a thickness of 10 km (a wild guess at something that would avoid collapse), would have about 0.5% the mass of this Earth. If you "weigh" 70 kg here (apologies to Newton), you'd "weigh" 350 grams there. As you fell through the 10 km (!) deep hole, your "weight" would decline from 350 grams to zero. Not exactly sudden. It's roughly the same gradient that you'd experience moving from the surface of a 60 km diameter asteroid to 60 km above its surface. |
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A gravity engine such as you imagine relies on a
discontinuity of the gravitational field. As you up through
the field, you gain potential energy; as you go down the
field, you lose potential energy (but since it changes into
and out of kinetic, it doesn't get away from you unless
you frict or do work). If you could jump from one level of
the field to another without moving through the field in
between, then you'd have the desired effect. But the
simple fact that the gradient is steeper in one place than
another doesn't help - it's like running up the steps and
jumping off the porch. |
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With a hole in a shell, you can make the gravity field have
a weird shape, but it's not discontinuous. Doesn't matter
if it's a inch-thick shell of heat-treated neutronium - the
gradients can be extremely steep, but they're still a
continuity... and as long as you're tracing a closed path in
a continuous static field, when you arrive back at the
same point, you have the same potential. |
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I'm still at a loss for why this is even a special phenomena.
This same phenomenon of forces in attraction canceling out
can be replicated with magnets without impossible
spheres. The results are as mundane as expected. When
applied to both ends of an oscillating beam the results
cancel out producing a system with a period and energy
storage capacity identical to a simple pendulum: by
producing equal and opposite forces on either arm of the
pendulum as it moves you eliminate the "interesting" effect
entirely. |
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[WcW], we already figured out that it is like several other things, okay?
The OP just presented it in a different way, and we had the fun of
solving the puzzle. Not one annotation has said that it would work.
(Except you, acting like we all think it will.) |
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And yes, even if the sphere is collapsium or something, there will be a
gradient as you go down through the hole. |
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He's just proving that confusion is also a field phenomenon;
if you keep coming back to the same point... |
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There might be a market for hollow-point planets, for species that really hold a grudge. |
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Forgive me for missing that there was something puzzling
about this, it seems about as captivating as a wet paper
bag. |
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Carrying a load of books in paper bags can be exciting, in heavy rain, as I found out in April. |
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