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.-- bungston, Sep 13 2014 Enlarge the moon Enlarge_20the_20Moon [bungston, Sep 13 2014] Moon VLO device Moon_20VLO_20DeviceRelevant hollow sphere lessons halfway down annos. [bungston, Sep 13 2014] This is no more perpetual motion than an asteroid whizzing through space. Yes it will go on until stopped but there's no point.-- Voice, Sep 13 2014 Where are the robots with lasers?
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.
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.
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.
But no free power. Clever, though. Dang clever. [+]-- baconbrain, Sep 14 2014 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.-- pocmloc, Sep 14 2014 Build it big enough and hang earth off it. Then it could be used to heal cancer.-- pashute, Sep 14 2014 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).
I would recommend building the over-water prototype before you set about hollowing out the earth.-- MaxwellBuchanan, Sep 14 2014 I agree with [MaxB]. I thought of that, too, but couldn't really come up with frictionless water, so I left it out.
I did think of a long weightless stick with weighted ends, running all the way through a planet (bog-standard or imaginarium shell).-- baconbrain, Sep 14 2014 You could always use superfluid helium...-- MaxwellBuchanan, Sep 14 2014 As a matter of fact, I can always use some superfluid helium. Do you happen to have some extra?
[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."]-- baconbrain, Sep 14 2014 I have lots extra. But not of superfluid helium, alas.-- MaxwellBuchanan, Sep 14 2014 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-- pocmloc, Sep 14 2014 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.-- FlyingToaster, Sep 14 2014 No they won't.
Without going into all the geometry of it, it comes down to this:
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.
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.
(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.)
The geometry is such that these effects cancel out perfectly, and there is no net force on an object anywhere inside the sphere.-- MaxwellBuchanan, Sep 14 2014 [pocmloc], I meant that the rod running through the planet was the same, another analogy.-- baconbrain, Sep 14 2014 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.
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.
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.-- bungston, Sep 14 2014 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.-- FlyingToaster, Sep 14 2014 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?-- WcW, Sep 15 2014 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...
(of course, it would also allow you to cheat: electrical charge can be turned on and off much more easily than gravity can)-- lurch, Sep 15 2014 [WcW], I happen to be all smoking hawt, but that isn't the point.
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.
But the claim was that it would generate power. We figured out that it would not, by ... let's see ... the first anno.
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?-- baconbrain, Sep 15 2014 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.-- WcW, Sep 15 2014 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.
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.-- baconbrain, Sep 15 2014 //But to me, a seesaw doesn't do that --- it needs something to send it back where it came from.
Wouldn't a bounce off the ground have that covered? In a frictionless, airless, soundless world, where else is bounce momentum going to go?-- the porpoise, Sep 15 2014 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.
A seesaw is not naturally oscillating. A swing is.
(Why has this gotten so dang confusing? Have seesaws truly been removed from all playgrounds?)-- baconbrain, Sep 15 2014 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.
Also, you've put holes in the shell, so now there's a gravity pulling both masses down.
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.-- the porpoise, Sep 15 2014 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.
Nobody is saying the original idea will generate energy. It will keep oscillating once started, that's all.
I can't even ...-- baconbrain, Sep 15 2014 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.
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.
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.-- the porpoise, Sep 15 2014 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.-- WcW, Sep 16 2014 But we have already agreed that the hollow globe is frictionless.-- pocmloc, Sep 16 2014 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.-- WcW, Sep 16 2014 Perpetual motion machines make for interesting thought experiments and sometimes amusingly choleric prose.
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.
One must have ideas on the HB. A perpetual motion machine is a fine device for consideration of an unusual circumstance of forces.-- bungston, Sep 16 2014 //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.
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.
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.-- the porpoise, Sep 16 2014 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.
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.-- lurch, Sep 16 2014 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.-- WcW, Sep 16 2014 [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.)
And yes, even if the sphere is collapsium or something, there will be a gradient as you go down through the hole.-- baconbrain, Sep 16 2014 He's just proving that confusion is also a field phenomenon; if you keep coming back to the same point...-- lurch, Sep 16 2014 There might be a market for hollow-point planets, for species that really hold a grudge.-- not_morrison_rm, Sep 17 2014 Forgive me for missing that there was something puzzling about this, it seems about as captivating as a wet paper bag.-- WcW, Sep 17 2014 Carrying a load of books in paper bags can be exciting, in heavy rain, as I found out in April.-- not_morrison_rm, Sep 17 2014 random, halfbakery