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Modelled on those transparent balls you can put hamsters in to let them run safely around your home, this is basically the same but built for your fish.
At some point on the sphere is a watertight, screw-on panel which can be removed in order to get the fish inside. To get the fish into the sphere,
you simply push it into the fish's normal aquarium, and the fish will be sucked in along with the the water. Then remove ball from aquarium, dry off with towel, and set the ball on the floor.
Now, I'm pretty sure this next bit won't work, but it almost seems to make a strange kind of sense. As the fish moves toward the edge of the ball, the weight distribution in the ball shifts, and the ball itself shifts with it. Thus your little fishy can swim through your house just as effectively as if you'd flooded the whole place knee-deep in water.
As I said, I'm fairly certain that things won't be that simple, given the fish's swim bladder and its bouyancy and probably lots of other things that I don't know anything about either. I did try to come up with another mechanism for making the ball move with the fish, but the best I could come up with was the equally improbable mechanism of having the water filled-ball surrounded by another slightly larger ball which contains a couple of weighted ball bearings. Attach a magnet to the fish (another potential stumbling block, I'll agree), and as the fish influences the ball-bearings, the ball moves.
Still, even if the fish can't get the Bowl moving itself, the whole setup would make an excellent cat toy.
EcoSphere
http://www.abundant...tore/ecosphere.html
[Shz, Oct 04 2004, last modified Oct 21 2004]
Related in the annotations...
http://www.halfbake...idea/Fish_20Mobiles
Mechanisms for goldfish bowl travel [RayfordSteele, Oct 04 2004, last modified Oct 05 2004]
(???) G-suit
http://www.beyond20...g_00/story_742.html
Using water tubes to counteract G-forces [8th of 7, Oct 04 2004, last modified Oct 21 2004]
(?) Fish Controlled Robotic bowl-movers
http://www.ylem.org...iated/mediated.html
Location of fish is done using IR [stormo, Oct 04 2004, last modified Oct 21 2004]
Propulsion kit for Goldfish Bowl
http://www.halfbake..._20Propulsion_20Kit
shameless plug [roby, Oct 04 2004, last modified Oct 05 2004]
Look what I just found...
http://www.toothpas...ou-have-to-stay.gif
[k_sra, Oct 04 2004, last modified Oct 05 2004]
Walking the goldfish
http://www.amazon.c...51?v=glance&s=books
With a leash and some wheels, you could take him out. I have a signed copy of this book too.... [choaderboy, Oct 05 2004]
(?) Maywa Denki Nonsense Machines
http://www.maywaden.../products/naki.html
the third last idea on the page. [benfrost, Dec 16 2005]
Seth Weiner's Terranaut
http://www.we-make-...archives/007286.php
A different take on freeing goldfish to explore the non-aquatic world. His battery-powered fishbowl-on-wheels moves around the art gallery in a path determined by the direction in which the solitary goldfish swims. Winner of the Bartlebooth award. [jurist, Jul 07 2006]
Kenneth Rinaldo: Augmented Fish Reality
http://www.we-make-...archives/003117.php
Bowl moves in the direction the fish swims in. [jutta, Jul 22 2006]
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Annotation:
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Excellent concept, just one problem: the fish will *not* be able to propel the ball by swimming about. Moreover, if it could, the water in the bowl would turn over and over, discombobulating the fish. |
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However, your second mechanism is not unlike those rolling eyeballs they sell to children: an inner sphere in an outer sphere filled with water. When you roll them, the eyeballs stay pretty much vertical, a desired effect here. So you could do that. |
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This still provides no motive power to get the goldfish moving. But there's always the cat. |
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It's a very funny image. I guess this one might fall under the "impossible but in an interesting way" heading. |
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Would be a neat way for the goldfish to torment the cat, if it could only remember why it should, or how, or that there was a cat... |
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Put a set of optical sensors on it. When the fish moves out of the central "null zone" to one side of the bowl, little motors rotate the ball in that direction. |
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In a kind of limited "Turing test", I wonder if you could distinguish the motion of the fish from that produced by a random number generator. |
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// Now, I'm pretty sure this next bit won't work, // |
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You're right, it won't. So no roaming, but the fish globe is interesting. |
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Strap fish globe to remote controlled car. Fish! Get the cat, fish! Go! Go! (whhhiiirrrrrrRRRRR!!!!) |
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//the fish will *not* be able to propel the ball by swimming about.//
Simple physics disagrees. This is a nonequilibrium situation. The ball WILL roll, given low enough friction. |
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No it won't. The weight and pressure inside the sphere is the same, no matter where inside the fish is located. |
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Only in static equilibrium, not during acceleration. She rolls! Or rocks and rolls. Well, mainly rocks. |
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Well you're going to have to explain how that will work, then. |
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If the fish swims to one side of the sphere, it has merely displaced water that was already there. There is no change in weight or pressure, nothing to cause motion. The motion of the fish swimming is offset by the movement of the water away from it as it swims. |
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Agreed. Back to the drawing board, lads. |
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Are we going to vote on the laws of physics, then? This fishbowl sphere works in the same way that a child's swing works. You start with your feet off the ground, in equilibrium. Dosen't seem like you could get it swinging, but in fact, you can. |
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No, that's not a good analogy at all. A swing is a pendulum. Once you move it off center, gravity pulls it back. If the fish swims off center, there is no force to recenter it. |
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If the swing was inside a sphere, the sphere would roll to offset the swinging motion. This doesn't apply to the fish, as the fish is not attached to the sphere. |
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I own something similar, though it doesn’t roll. <link> However, creative + amusing + fish = croissant in my book. |
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Maybe an Aibo, or similar robotic dealy with a sealed fishbowl built into it. You could put the tank on some king of swiveling mounts to keep it from moving too much when the robot is running around. If it's important for the fish to be controlling the movement, I suppose you could have sensors to see where the fish moves, and have the robot move in that direction. |
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can you make one for humans? (I think it's time to let my sister out of the cage). |
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Dave the Cat: "Open the fish bowl door, Hal."
Hal the Guppy: "... I'm sorry, Dave, I can't do that." |
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I think the fish will have a limited life expectancy since of you seal it in a sphere, there will be no gas diffusion to remove CO2 and replace oxygen. You'll need to incorporate a gas scrubber. |
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As to movement, i think that if the fish swims vigorously, the turbulence and friction between the water and the bowl might produce a small amount of movement but since the fish is neutrally buoyant (electively) the movement of its mass within the water sphere is not going to produce any movement (I agree with [waugs]). |
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I've seen ecospheres - very, very highly recommended. |
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It *will* work - all you need is two concentric glass spheres. In the inner one you have your mouse, happily running about. Between the inner and outer spheres you have your fish, swimming. The locomotive power comes from the mouse; The aesthetics come from the fish. |
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[UB] I've always wanted to master the menace behind Hal's dead-pan computer voice. It's all in the pause. |
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Regarding the idea, it's not at all easy, after all, not many domestic fish like swimming against current. Otherwise, they'd go mad in a tank. Also, machinery or even suction in a tank is a bad idea, unless you want a fish smoothie. |
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Let's say you had a bluefin tuna or a salmon in a tank. You want it to swim without circling the tank. You could generate a water treadmill (not that fish tread)by pumping water in at one end and allowing over-/ out-flow at the other. This set-up is sometimes used to train Olympic swimmers, but it's not fish friendly. If the fish gets tired, it'll get whammed against the back grille, like you were dragging a steel net through the ocean at 35kts. |
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Instead, I propose a vertical vortex. The very large tank is shaped like a hamster wheel, but with smooth periferal walls. One (or both) side is a viewing window. Water jets are injected into the water at the base and if necesary strategic points around the outside. This creates a circular flow. If the fish's food is attached at a fixed point upstream of the bottom, viewers can watch as the tuna powers up to the mark at high speed, snatching it away. |
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If the fish tires and slows, it theoretically could get washed back, and get sloshed around and around like a doll in a washing machine. More likely, it'll learn to cruise up towards the centre of the wheel, where the water flow is slowest. |
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Need to add plenty of slime coating to prevent injury to the fish.
How about a small wheelchair, so it can be the Steven Hawking of fish? |
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Or a sort of piscine Davros ? |
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[FloridaManatee]'s 'water treadmill' is baked. My wife once kept fancy goldfish, one of whom (Stuffy the oranda - the fish were all named after Harpo Marx characters) would position his elf facing into the outlet from the filter / aerator and swim frantically while remaining stationary. After a while, he would relax and let the flow propel him to the far end of the tank. Great to watch. |
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//is not going to produce any movement (I agree with [waugs]).//
<Slamming down her cup of coffee, Pluterday scavenges thru a closet to find a mortarboard. This is now on her head, perched unsteadily over curlers. Pulling a stick a chalk from her sleeve, she rapidly sketches on a blackboard. A sphere on a hard level floor surface filled to the top with water appears, with a neutrally buoyant fish making loops inside along an axis parallel to the floor. Above this she writes in bold letters “conservation of angular momentum”. Double underlined.>
Remember “conservation of angular momentum”? Picture Goldie swimming in a counterclockwise circle, not touching the glass. A funny circle with the axis of it parallel to the floor. To swim like that Goldie has to push water in the opposite direction. So the mass of water in the sphere is now rotating in the clockwise direction. More slowly than Goldie because it is more massive than Goldie. But the water touches the inside of the sphere and (because of viscous drag) starts the sphere rotating in the clockwise direction also. So, oddly enough, the sphere rotates in the opposite direction of Goldie’s motion. And when Goldie finally has enough of going backwards and stops, so does the sphere.
<Wiping chalk dust from her hands, Pluterday now retires, sure of victory...> |
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//Stuffy the oranda//
Used to have 4 oranda's - great fish. |
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// Goldfish bowling would be fun // |
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Not, I imagine, for the goldfish...... |
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<borrows [pluterday]'s mortar-board to use as a fish-sphere rest>
If the sphere were less than completely full, Goldie's exertions would theoretically (and imperceptibly) move the water; assume Goldie facing to your right - surface of water rotates clockwise so that right end is lower than left end. Effective centre of gravity of water is moved to the left. Sphere rotates anti-clockwise. However, said he, pointing significantly, if the sphere were full, no such change would occur in the centre of gravity, thus no rotation would occur. In fact, said he, having just thought of it, given that Goldie is neutrally bouyant, Goldie's mass moving to the right would counteract the water's mass moving to the left, no movement of C of G would occur and Goldie would swim round in circles, as usual.
<returns [pluterday]'s hat> |
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<taking yet more medication>
C of G! This is not statics, its dynamics! If you drop a spinning cue ball perpendicular on a pool table, it runs off, doesn't it? Even though the C of G is exactly over the point of contact? |
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How is dear little Goldie going to overcome the inertia of the system in which he/she is the only denizen? |
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I don't really see how Goldie's situation is any different to cartoon depictions of a fugitive trying to propel a sailcraft with a desk fan. <equal and opposite reactions> |
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Theoretically, in a perfect environment without gravity or rolling resistance, it might be possible, but gravity and inertia overcome his/her heroic efforts. Goldie cannot obtain any purchase against the world outside the sphere, therefore goes nowhere but round and round inside the sphere, until the oxygen runs out. |
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Though... a ceiling fan, moving at 9.8m/s might just give Goldie the extra push needed... or not. |
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I think that the motion would arise as a result of friction in the laminar flow region between the surface of the bowl and the moving fluid generated by the fish; the fish is expending energy to move, and momentum is conserved. But I don't believe any movement results as a result of moving the centre of gravity, since if the fish is neutrally buoyant in a competely filled sphere, the system is effectively isotropic. |
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I'm starting to wonder if the Coriolis force might have some bearing on this problem. And whether the globe can be considered truly hermetic, as gravity acts both inside and outside the phase boundary. |
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//How is dear little Goldie going to overcome the inertia of the system in which he/she is the only denizen?//
Friction, dear UnaBubba, friction. Picture a little man in the sphere. No water. The little man takes a step, the sphere rolls. That’s because C of G varies of course, and its a little different from the Goldie’s situation. But it is still friction with the ground that makes it possible to get going. Now take a gyro. Totally symmetric, with a C of G right in the volumetric center. Spin it up in your hand. The housing produces a torque on your hand, doesn’t it? If you were to lay it on the ground, it would roll away. The angular momentum is converted to linear momentum by fictional contact with the ground. In this case, Goldie is the rotor of the gyro, and the sphere is the housing.
<but she moves! mumbles Pluter as they take her away to the stake...> |
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What if the fish wanted to get across the floor so badly that it repeatedly swam at full force against the inside of the sphere, bashing its head against the glass again and again? |
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//This is not statics, its dynamics!//
Only after the bowl is moving. Until then, it's just a fish-sphere sitting on a desk (or in a hat). |
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I understand your argument, [pluterday], but I doubt the ability of a goldfish to exert sufficient force or friction. A 300lb Bluefin Tuna, perhaps, but the weight of the water in a sphere, large enough to comfortably house such a powerhouse, is going to stuff things up yet again. |
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Yes, we'll have friction, but the limitations are insurmountable, I think. |
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Another question: Where do you perceive the moment of force to be greatest? |
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Roll, roll, roll the bowl
Gently across the floors.
Merrily, merrily, merrily, merrily,
Now I'll explore the great outdoors. |
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[UnaBubba] Let’s say Goldie is in a bowl with 100 times her own mass in water. If she makes 1 circle per second (typical lazy goldfish is Goldie), the water must rotate on average once per hundred seconds in the opposite direction. This rotation is imparted to the bowl, which then takes a quite leisurely stroll around the house until it gets underfoot and is accidentally crushed. Goldie is dead, but the baker boys say no, no, impossible, Goldie cannot be dead, since she could never have moved to begin with. They are all quite clear on this. A vote is taken, and Goldie is resurrected. |
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Ah, so we have a highly disciplined Attack Goldfish, trained to swim in circles, unlike the usual, random bimbling of a normal goldfish? |
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Horizontal circles, or vertical loops? |
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Horizontal circles will simply cause the sphere to rotate on the spot. You need vertical circles to make it progress, but due to gyroscopic effects (as pointed out earlier) it will not move in a truly straight line. Not that there is such a thing as a truly straight line, due to the curvature of spacettime induced by gravity. Or rather, gravity is the name we apply to the observed effect of spacetime curvature caused by mass. Or something. Excuse me while I just go and bash my head agains the walls of this transparent sphere I seem to be trapped in. |
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<adjusts browser to report on first appearence of "Crash Test Goldfish" on 1/2B, starts taking bets on who will post it> |
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Good grief. If we can get road cones to move on their own, I don't see why a goldfish ball is such a problem! |
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It seems quite obvious. The ball won't move unless the displacement of water results in an imbalance. This won't happen as long as the fish is floating as it exactly counteracts the displacement of water. |
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If a male Betta can be specially trained to blow bubbles at a fast enough rate to exactly counteract the forward thrust of his tail, AND to sit dead centre of the sphere while he's performing these antics, then the resultant imbalance of water may just be enough to overcome the friction between the billiard-smooth sphere and the baize surface on which it rests. Perhaps. |
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Okay, here's the solution: |
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Fill the sphere to a level below one quarter full. Train (or genetically modify) the fish to jump out of the water in the desired direction of travel, landing on the dry inside surface of the sphere, beyond the edge of the water. Viola! A center of gravity imbalance has arisen, and the sphere will move to rebalance the center of gravity. The motion will re-submerge the fish, at which time the entire process can be repeated ad infinitum. |
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//It seems quite obvious. The ball won't move//
Try this then:
The Goldie
10 parts rum
1 part dear departed Goldie
(quick frozen in liquid nitrogen)
Now look. Does it spin? |
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[Bx] - no GM needed. Put one of those walking catfish in there. It keeps trying to get out of the water, but never quite succeeds. |
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Put them on a treadmill... battery fish. Unlike electric eels. |
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I think this would probably work with snails. Just really slowly. |
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Fill the house with one meter of water. Add seals. Watch the fish bowl-ball move. |
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I like [notme]'s solution: motorize the bowl and drive it off sensors. Fish swims to this side, the bowl moves that way. This can also solve the oxygen problem too, as the motorized platform can contain a mini aerator as well. |
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You can make it fun for the fish, by making the robotic platform look like one of those battle robots from Robotech or MechWarrior. That way the fish can get its long-awaited revenge on your cat by chasing it around and scaring it! (WTAGIPBAN) |
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// You can make it fun for the fish // |
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So all fo a sudden, [krelnik] is the world's expert on what goldfish consider to be "fun". That raises somne intriguing possibilites, especially of he's writing from personal experience; either (1) [krelnik] can link telepathically with goldfish, or (2) [krelnik] is a goldfish that can operate a PC, or at least a web-aware PDA. |
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//Put one of those walking catfish in there. It keeps trying to get out of the water, but never quite succeeds// |
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Good idea lurch. One of my fish is a Plecotomus that frequently ‘suctions’ itself above the waterline. That would make one side of the sphere heavier. |
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Back in the day, I was pretty certain I wanted to be an aerodynamicist. Then I took fluid mechanics. It made my head swim. |
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I'm pretty certain that the Reynolds number here is pretty high, which means that the viscous forces related to a fish swimming through water are pretty negligible in comparison to the streamline inertial forces of displacing the water out of the way. As to what that means anymore, I have no idea. |
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Fish have a miniscule drag coefficient, which means that they don't need to expel very much power at all to move forwards. Because water is nearly incompressible, there is little pressure differential between nose and tail, and the only wake created is due to the frictional losses associated with maintaining a constant speed in a fluid, which are roughly proportional to its speed as long as the fish hasn't gone off on a caffeine binge. The wake created is for all practical purposes nonexistent, and the fish is largely 'slithering' through the water, creating as little 'dust' as a sidewinder would while crawling through the desert. |
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pluter's argument makes no sense at all. If there were anything to it, we'd all be driving fish powered cars. |
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Short of the fish getting out and pushing, the bowl ain't gonna move. |
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Dang, now I want to try an experiment. Motorized submarine or something to create a real current inside a glass jar. It just might do the trick. |
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waugs, don't forget that all 'internal force' vs. 'external force' boundary diagrams don't include internal energy generation as a factor. |
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what we need is a hamster in a wet suit with a tank of oxygen on his back. time events carefully though. |
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po dear, I'm laughing so hard I can barely see the keyboard. That annotation needs to be framed. |
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I think it's pretty well agreed by this point, that a globe full of water with a fish in it won't roll of it's own volition... we'll need some sort of sensory system to detect the fish's movements and respond with mechanical movement - a negative feedback system, maybye with water jets. Like: Goldy moves off-center, and water jets push him back automatically. The force needed to push Goldy back to the center of the sphere is translated into external movement through traditional means like motors and wheels. This isn't anywhere nearly as simple as just a goldfish ball, unfortunately... |
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I still think the cat will make this whole thing redundant. In more ways than one. |
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In what way sweetie? <dribbles> |
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I've got it - Octo-Balls. Ditch the fish entirely (but not literally, of course) and replace it with an octopus. Not only could they use their suckered tentacles to "walk" the ball around the floor, but they're a lot more interesting and exotic than goldfish anyway. Plus they change colour depending on their surroundings and their mood. Which would really freak the cat out. |
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"Where are you off to, dear?"
"Just taking the octopus for a walk, love." |
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(Wordgineer enters room, notices pluterday babling to herself in a corner, clearly insane by now) Goldie must move! If she can move water in one direction (which she must to propell herself forward), conservation of momentum states that if this water was to stop moving something else must turn - the sphere. The only arguement that has been made against this that makes any sense is that goldfish, by nature, wouldn't swim in upside-down circles. I argue Goldie wouldn't try to swim in an upside-down circle, but instead would collide with the side of the sphere. Thus she would still be imparting motion to the water without herself moving. |
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I propose the following experiment for doubters. Fill a tennis ball can with water. Give this tube to a friend, and have them walk up a hill. Set up a flat surface at the bottom of the hill. Have your friend roll the tube down to you, and quickly place it on the flat surface. When you let go, I'll bet you all of your fish that it will roll. |
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No, not the sphere. The fish itself moves in the opposite direction. |
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Right, but if you conceed that the water in the globe moves, then you must see that friction between the water and the globe will impart a tangential force on the globe. With nothing to counteract that force, the globe itself will move. |
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No, I don't see that at all. |
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(Worldgineer grabs mortarboard, wipes it off, and draws a free-body-diagram of a small piece of the sphere. This looks like a curved line, which is drawn in blue chalk. He then draws a red chalk arrow following that curve.) "This is the tangential frictional force. Any time you have water flowing past a surface, there is a fricitonal force." (waugs starts to speak, but is cut off) "There just is, otherwise boats could go as fast as they wanted. Now, " (hands the red chalk to waugs) "show me where an opposing force is. If you can't, then since Force = Mass * Acceleration, the thing will start to move." |
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What I don't see is how this even applies to the stated problem. Tangential frictional force has nothing to do with this issue. |
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We're talking about a fish inside a sphere full of water. The issue in question is: if the fish swims from the center of the sphere to one side, will the sphere roll in that direction? |
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The fish can swim around all it wants inside the sphere, moving the water any way it pleases. But it is still all contained inside the sphere. The combined weight of the fish and the water, and the pressure it applies to the inside surface of the sphere, will not change. Therefore the sphere will not move. |
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If it were so, then why haven't goldfish been knocking their bowls over all these years? |
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I find it quite amazing how many of you aren't getting this. |
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1. You're avoiding the question. Tangential frictional force has everything to do with the question. Please draw your force arrow. |
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2. That is not the issue in question. The question is can the fish move the sphere? I agree that if all the fish does is swim from the center to the edge, that may not move it at all. However, if the fish keeps swimming after she hits the side, moving the water in a circular manner, she will move the sphere. |
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3. I agree with the third sentence of the third paragraph in your arguement. However, you jump to a conclusion in the fourth sentence that I do not agree with. |
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4. Because bowls aren't spheres. |
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5. My sentiments exactly. |
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I'm not avoiding any question. As I said, I do not agree that tangential friction has anything to do with this problem. |
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I've clearly made my point. Argue with someone else. |
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Why do you say that tangential friction doesn't have anything to do with this problem? You have not made your point. |
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[pluterday}'s right. Fish and water don't have the same density. Replace the goldfish with a fish of drastically lower density, say, a fully inflated blowfish that takes up about a third of the volume. This would be similar to having a 2/3-full sphere of water, but with the air pocket able to move from side to side or top to bottom. |
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If the fish manages to create a current and maintain it, then he will move the sphere. But one short burst forwards will not do it. |
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The fish will eventually have to swim in vertical circles in order to keep moving, but not as soon as you might think.
Given that he is either above or below a horizontal axis through the center of the sphere. If he's above it and facing to the left as seen from the side, then as he swims forward, he'll create a totally miniscule current clockwise, and vice-versa if below. |
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Now, he continues swimming to the left. Provided that the bowl is sufficiently large that he doesn't immediately hit the edge, he will eventually have to swim against part of the current that he himself has created. However, since some of that current has been tapped off due to friction at the water / bowl interface, (thus powering the rolling of the ball), he'll still move forward, relative to the ball, if he continues at the same level of effort. And so he must turn some other direction, or simply pause 'til the current carries him back to the opposite side. |
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I think it's about time for a swimming-fish clock. |
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Provided that the force is large enough, (which I seriously doubt), my view is that there are no fundamental engineering mechanics reasons why the bowl would not move. However, I doubt that there would ever be enough current to get the ball rolling, even with a hundred fish. |
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The pufferfish fills itself with water in order to puff, and so that has nothing to do with its density. Where would it suddenly acquire a large source of gas? |
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bliss - I think we're debating it purely because it is a cute idea and as halfbakers would like to find a way to get it to work. |
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waugs - I'm sorry if you thought I was arguing - I was just trying to convey my point and let you know that I don't understand yours. |
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Thank you Ray, I was going for a visual to hopefully demonstrate inner movement can cause outer movement. Perhaps my magic 8 ball would have been more effective.
(And I highly recommend not going to blowfish.com for verification.) |
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I can just imagine a blowfish, filling itself with gas from who knows where, and popping to the surface violently. |
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//Fish and water don't have the same density.//
Of course they do. If the fish were more dense, it would sink; if it were less dense, it would float to the surface. I know that fish can control their bouyancy by muscular control of the swim bladder, but when they are just swimming around, fish and water have the same density. |
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//fish and water have the same density.// |
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Net, they have. However, this density is not uniformly distributed. Spin a fishbowl at 75,000 RPM for two minutes and you'll observe: |
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Some fish at top (may be bubbles). Some fish at bottom. Water in the middle. |
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Shirley this depends on how fast the bowl is accelerated from rest to maximum - or are you considering the massive "tidal" forces due to the steep centripetal force gradient though the medium ? |
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//not uniformly distributed//
Do you mean that the density of the fish is not uniformly distributed within the fish, or that density is not uniformly distributed within the fish / water system? If the first, I know but it's irrelevant to the current discussion; if the second, it is - the fish and the water have the same density, so distribution *must* be uniform. |
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[Dimandja]: You even quote from your link: "a fish fills its bladder to the point at which it displaces a volume of water that weighs what the fish weighs." That means that the fish and the water have the same density. This only changes when the fish is moving higher or lower in the water. Once it is at its required depth, it manipulates its swim bladder so that it, again, has the same density as the water. |
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Well, I think you're *all* right. Every one of you. |
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Viola! Shark Feeding Frenzy Bowl! |
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//Spin a fishbowl at 75,000 RPM for two minutes and you'll observe// |
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You bake some way of safely spinning a fishbowl at 75K rpm, and you'll have better mousetrap manufacturers beating a path to your door. |
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[Dimandja], have you been assuming all along that the fish would impart movement to the sphere simply by bashing into the sides? |
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Next, watch paint drying, for three days. |
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magic eight ball experiment---I swirl it, set it down gently, it begins to move around. (for those unfamiliar, a magic eight ball is a sphere filled with water and a die-like cube with different sayings on it, a little window on the bottom of the sphere allows you to see one message at a time. Currently in says, "outlook not so good") |
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I should think this problem is similar to, and maybe easier to visualize with, a bird in an air-filled sphere. Some questions that could be asked as the bird flies from left to right causing an air current from right to left:
1. The air current would cause an initial force to the left on the sphere, but would it equal the force of the bird as it collides with the right wall?
2. Would the sphere sway left and then right?
3. The air current would swirl around and produce a lesser force on the right wall (+ for the bird) and then even produce a slight headwind for the bird (-) slowing its progress?
4. Though this is a closed system with no friction against outside air and floor, and ignoring production of CO2 and heat, should the bird's potential energy in its muscles be considered?
5. Would the result be different between a gentle, increasing push and an impulse after getting up speed? |
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Other questions that may be of interest:
6. Is the sphere system lighter when the bird is no longer standing on the bottom?
7. Can a strong bird lift the sphere by flapping hard enough with its back against the top?
8. Why is the answer to the last question different than / the same as pushing the sphere forward? |
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When a bird falls, in a sphere... will it roll any distance? |
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For some time now, this conversation has resembled one I heard somewhere about the load-bearing capacities of English vs. African swallows. |
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So the key is using African goldfish? |
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anyway, throughout all this, i can't believe nobody has correlated this idea to an even more awesome idea: (...which a friend of mine once mentioned, but, as it applies in this situation, here it is. i take no credit.) like [lurch] and [notme] had mentioned, use optical sensors on the bowl--when the fish moves in a certain direction, the bowl moves with it. but here's the kicker: i'm not talking on the ground, i'm talking IN THE AIR! that's right, a sealed goldfish bowl suspended from a miniature helium balloon that has miniature fans attached. the fans get their data from the optical sensors and turn on/off accordingly. now the fish has 360 DEGREE range of motion! it's swimming through OUR world, instead of just its own. the interesting thing will be: does the fish learn how its motions translate in the real world and alter them accordingly to move among us? or does it just randomly float around and bump into stuff since it only has a tiny fish brain? i would love to see this tested someday. |
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p.s. this wouldn't really work inside... unless you were in some sort of arena or auditorium. |
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Nice work, squidink! I was close to posting a "Goldfish Bowl Newton's Cradle" idea just to muddy the waters still further on the physics of this debate (and perhaps drown it out entirely with the sounds of breaking glass), but you've restored my will to live. |
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"Why are you crying, Timmy?" |
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"No. I left the window open.." |
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To paraphrase from a classic Carlin routine: |
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"Mom, I lost my goldfish!"
"Well where did you have it last?"
"Hey, if I knew that, I'd still have my goldfish."
"Well it didn't just get up and walk away!"
"Hey, Mom, I think you figured this one out." |
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I thought through the same analogy with a plane. However, the model is complicated by the bird / plane's much larger net density compared to the air it displaces. This is significant because the problem no longer becomes one of an equivalence in net density displacement between fish and water, but primarily of an internal conservation of momentum of the entire bowl, given a displacement in mass that the goldfish model doesn't see, because of the water's sameness in weight as the net fish. (fish net?) |
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An impulse for a bird is more likely to get the air bowl rolling, however, it will not stay rolling. Impulses are almost always more effective at overcoming static friction than the same energy spread over more time. |
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A second difficulty arises when you compare the difference in Reynolds numbers between air and water, and so the simplified models would use different factors, effectively making a comparison tough to work out. |
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The bird / air bowl problem is in some way functionally equivalent to the explosion of a non-propelled firecracker, in that the directional momentum of every chunk of mass involved in the system must be maintained. Pieces fly off in random directions with seemingly random velocities, but if you add them all up vectorially, the net directional change is 0. |
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Hence, if the ball is stationary and the bird starts flying to the left, yes, the ball will counteract by moving right. If we discount frictional differences of impulse vs. steady force, when the bird hits the wall, the whole system will come back to its original starting point, and probably a little past because of impulse and frictional effects. |
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The bird will never be able to lift the bowl by flapping its wings, if the bowl is sealed. That's me trying to lift myself up by my own hair. |
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Also note that a sealed bowl is mechanically different than an open bowl with fresh, stagnant air sources, which may complicate my whole analysis here. In the sealed bowl, a cyclone of some sort will be created, which may, like the fish problem, create its own effects, one of which would be to instantly ground the flight of the bird, as the air which he's flying against is being replaced by air that is directionally unfavorable to his lift. He's like a Harrier that's sucking in it's own exhaust and crashlands.
The only way he'll get off the ground and stay there would be to be inside a ball that is large enough such that a steady-state current is never created throughout the system. |
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Summarizing per FJ's numbers: |
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1. Sortof. They would be mathematically equivalent. However, the larger impulse wins out at the beginning due to the percentage of force lost to overcoming static friction. |
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2. Yes. conservation of momentum of the system would dictate that it landed where it started. |
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3. The bird won't be able to maintain lift for very long, due to an induced downdraft. |
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4. No. It's functionally equivalent to the firecracker problem. |
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5. After getting up to speed, a gentle, increasing push is favorable over an equivalent impulse, due to non-rigid body energy losses. |
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8. It's nearly the same. The difference lies in the fact that in the horizontal direction, we've taken it as a given that the bird is suspended by some magic. If a bird jumps off of the bowl bottom (which is in contact with the ground, obviously), and rams himself into the top, then the bird has something to push against which is outside the system, and the bowl will jump up as a result of his impulse. However, the bird will not be able to lift the bowl by flying upwards, as he won't be able to fly at all, and if he did, then the force of his downdraft against the bottom of the bowl is exactly the same as his push against the top. Similarly, a fish cannot push a ball by forcing himself against the edge, since he's pushing back on the water to do it. |
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But the bird model is not quite the same as the fish model, for the reasons I've described above. Fluid dynamics problems usually need to be taken one at a time and are rarely transferrable between one system and another, as the math models used to solve the system will be highly dependent upon the properties of the fluids involved, the speeds involved, the pressures, etc, which is why we have weird mathematical things like Reynolds numbers, Mach numbers, Cauchy numbers, etc. We first find a few of those 'dimensionless numbers,' and then decide what type of aero-dynamic model fits the scenario based upon the numbers generated by the first go. |
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[The only way he'll get off the ground and stay there would be to be inside a ball that is large enough such that a steady-state current is never created throughout the system.] Woot! You solved that problem. So the bird could create a cyclone that would not directly be effecting the air directly around it. So we could also say that a sealed ball the size of a SUV, holding say, a mako shark (I hate goldfish), would be moving from the sharks own movements. Now, as said with the gyroscopic movements, if the shark did only swim counterwise or clockwise (1 direction only), due to a gyroscope not rolling in a straight line, the shark would eventually have the ball tilting (as it will start to spin from circular in its own space, to diagonaly), and after a few rotations diagonaly, would start to roll in a very random path. Now brings up the questions of "If a glass sphere on a frictionless surface starts to roll, where does it go?" |
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<roby scratches head and wonders what all the fuss is about. Shirley this is obvious?>
Dimandja's link shows possible answer. Goldie starts at the bottom of the bowl on front side, floating with neutral density, (Goldie mass is 100 g, currently displacing 100 g of water with 100 cc of volume).
Goldie spots shiny mirror outside of sphere. "Let's get a closer look," he/she might think, if it could.
Goldie swims to back bottom side of sphere, expands bladder to increase volume to 200 cc, displacing 200 g of water, though Goldie mass now more like 101 g (original 100g plus a gram for the extra air). Goldie rises, ends up at top back end of sphere. The now displaced 200 g of water redistributes it elf in sphere, changing sphere center of gravity, initiating forward motion. From top back of sphere, a bloated 200 cc Goldie maneuvers to top middle, thereby shifting half the water mass back the other way again, to slow but not stop forward momentum. Goldie then normalizes volume to 100 cc,. swims to front top, then releases air in bladder, reducing Goldie volume to 90cc, thus reducing water displaced to 90 cc and 90 grams, causing 100 g Goldie to sink down to bottom front of sphere. Water in sphere redistributes it elf again, extra mass to front of sphere, changing C of G again, continuing sphere's movement forward. 90 cc Goldie manuevers from bottom front to bottom middle, shifting C of G back again buy by half as much, then normalizes Goldie volume to 100 cc again, swims to back bottom, prepares to reinflate to 200 CC again, and so on until Goldie is close enough to see him/her elf in mirror!
Now, I'm sure my numbers are gross exaggerations of the density changes Goldie is capable of making to rise and sink him/her elf in water. But I think it shows that Goldie could initiate and control change in sphere C of G and thus sphere movement. |
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//Do you mean that the density of the fish is not uniformly distributed within the fish// |
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Yes. Bones, and other structures are denser. Swim bladder is lighter. Net, the fish is able to trim to neutral bouyancy (this works with scuba divers too, but the centrifuge would be too big). |
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I used to operate a centrifuge for separating cellular components. If a (homogenised admitedly) liver cell will separate under these forces, goldie will probably too. You'll be left with fishy bones crumpled at the bottom of the tank, covered in a layer of fishy muscle bits, clear-ish water in the middle and fats and bladder bubbles on the surface. |
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Speed of spin-up is not important. It's the forces at peak speed that do the separating. |
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That reminds me... perhaps we could use one of those magnetic beaker stirrers on a low setting to give Goldie a gentle spin. |
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//Need to add plenty of slime coating to prevent injury to the fish.// |
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Thunbwax, what kind of mucus covering do you propose. I've thought of silicone, but I'm concerned it might dissolve and/or poison Goldie. |
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There's nothing to sphere but sphere itself. |
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// Woot! You solved that problem. // |
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No, I did the opposite. I killed the air model. The induced current is what we were relying on to propel the ball in the first place. |
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// Goldie swims to back bottom side of sphere, expands bladder to increase volume to 200 cc, displacing 200 g of water, though Goldie mass now more like 101 g (original 100g plus a gram for the extra air). // |
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roby got as close as anybody could, however, there's another problem. |
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Where did the displaced water go? If we're assuming that the sphere is closed and water-filled, (which was what I was working on), then Goldie will never expand against the pressure-sealed system. And if the sphere is not completely filled with water, then Goldie must effectually pressurize the air above, which is still tough to do. |
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If the bowl is only half-filled, then what if the fish jumps out of the water, hits the side of the bowl, and splashes back in against the edge? Hmmm... gonna have to think on that one. |
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Schrodinger's fish... paint one half of the outside of the sphere black. Upon swimming to the visible side, the fish may collapse into wave form, causing instant system depressurization and a temporary vacuum in the water. The ball then rolls towards the invisible side... |
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