Counterintuitive Mechanical Devices

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Rackum
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Counterintuitive Mechanical Devices

Postby Rackum » Wed Oct 27, 2010 8:44 pm UTC

As an engineer I love the "oddities" that occur that seem impossible/implausible at first but are actually very real. In that regard, I'd like to start building some of the better ones (smaller scales if necessary, but still working models) to put around my office (DWFTTW, for example) and was wondering if the collective power of the xkcd Science forum could give me some more suggestions. I'd prefer mechanical devices but not absolutely constrained to that type. Thanks in advance.

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Kang
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Re: Counterintuitive Mechanical Devices

Postby Kang » Wed Oct 27, 2010 8:50 pm UTC

I'm not sure whether that's in any way what you are looking for, but all sorts of 'free labour' pseudo perpetuum-mobiles, like clocks powered by temperature changes could qualify. A few different models are explained (and the originals illustrated) in Perelman's Zanimatelnaya Fisika, which can be found in an older post of XKCD's own blag.

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Re: Counterintuitive Mechanical Devices

Postby doogly » Thu Oct 28, 2010 1:49 am UTC

The south pointing chariot gets significant badass points.
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Minchandre
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Re: Counterintuitive Mechanical Devices

Postby Minchandre » Thu Oct 28, 2010 2:27 am UTC

I like the tube that you put warm air into, and you get hot and cold air out.

http://en.wikipedia.org/wiki/Vortex_tube

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LucasBrown
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Re: Counterintuitive Mechanical Devices

Postby LucasBrown » Thu Oct 28, 2010 4:33 am UTC

One of my favorites is the Peltier module--technically, it's an EM demo, but you can hook up the wires to a motor or something like that.

If you hook it up to a motor, you might as well have it hooked up to a homopolar motor and get two awesomely counterintuitive devices in one demo.

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Cynwulf
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Re: Counterintuitive Mechanical Devices

Postby Cynwulf » Thu Oct 28, 2010 4:59 am UTC

It isn't terribly counterintuitive, but I've found the Stirling engine to be one of the coolest (durp, no pun) heat engines that is given very little attention. It operates on such a low delta T that you can power one from nearly any tiny heat source, and so it could appear to be "powered by nothing." In fact, I've seen cute desktop-miniature models that you can buy or build yourself:

http://www.youtube.com/watch?v=4sV_0xKX4II
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Technical Ben
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Re: Counterintuitive Mechanical Devices

Postby Technical Ben » Thu Oct 28, 2010 12:25 pm UTC

The Crookes Radiometer because I'm sure everyone will think the light pushes the paddles around.

I also like the vortex tube. I'm so using one of those some day to remove the need for a heating system in a house. The simpler version that is, using water pipes, one coiled around the over, to make a vortex, instead of compressed air. You still need a pump or gravity feed, but you get warm water without any heating elements.
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LucasBrown
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Re: Counterintuitive Mechanical Devices

Postby LucasBrown » Thu Oct 28, 2010 6:54 pm UTC

RE Cynwulf's link: I initlally read the clip's title as "a difference engine that runs at low temperatures" rather than "an engine that runs off of small differences in temperature." The resulting double take was, according to my friend Catherine, quite astounding.

As for a mechanical device, you could construct a double inverted pendulum if you have the materials to build the control mechanisms...

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Wnderer
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Re: Counterintuitive Mechanical Devices

Postby Wnderer » Thu Oct 28, 2010 7:12 pm UTC

Build a Feynman Carousel

http://www.analogsf.com/0806/altview.shtml

Another odd motor is the Feynman carousel, discussed in section 17-4 and at the end of section 27-11 of volume II of The Feynman Lectures on Physics. Mixing straight quotes from section 17-4 in with my own descriptive modifications, consider a thin plastic disc supported on a concentric shaft with excellent bearings, able to rotate freely. On the disc is a superconducting coil of wire in the form of a short solenoid concentric with the axis of rotation, in which current is flowing. Near the edge of the disc, spaced uniformly about its circumference, are a number of small metal spheres insulated from each other and from the solenoid by the material of the disc. Each sphere is given an identical electrostatic charge. Everything is stationary; the disc is at rest.

Initially, the angular momentum of this motor is zero, right?

Allow the temperature of the solenoid to rise above the critical point until it is no longer a superconductor. While the current was flowing, there was a magnetic flux through the solenoid more or less parallel to the axis of the disc. Now with the current flow interrupted, the flux must go to zero, and an electric field is induced which will circulate around in circles centered on the axis. This field exerts a force on the charged spheres, tangential to the perimeter of the disk, and all in the same sense. This results in a net torque on the disk and it starts rotating. Since no one pushed the carousel to set it spinning, we conclude that angular momentum now apparent was stored in the magnetic field.

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Re: Counterintuitive Mechanical Devices

Postby skeptical scientist » Thu Oct 28, 2010 7:48 pm UTC

I find the progressive cavity pump bizarre and counterintuitive, but staring at high-res pictures on wikipedia for several minutes is helping me visualize the geometry of it a bit better. I suspect a working model with a clear stator, opaque rotor, and colored liquid would be great for understanding it, if you wanted to build one.
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Rackum
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Re: Counterintuitive Mechanical Devices

Postby Rackum » Thu Oct 28, 2010 7:48 pm UTC

LucasBrown wrote:RE Cynwulf's link: I initlally read the clip's title as "a difference engine that runs at low temperatures" rather than "an engine that runs off of small differences in temperature." The resulting double take was, according to my friend Catherine, quite astounding.

As for a mechanical device, you could construct a double inverted pendulum if you have the materials to build the control mechanisms...

Funny you mention the double inverted pendulum as one of those was an integral part of one of my friend's senior engineering design project (forget what they actually used it to do).

Overall, I'm really liking the ideas so far (the homopolar motor coupled with the thermoelectric device would be really cool and pretty easy). I appreciate the help folks.

skeptical scientist wrote:I find the progressive cavity pump bizarre and counterintuitive, but staring at high-res pictures on wikipedia for several minutes is helping me visualize the geometry of it a bit better. I suspect a working model with a clear stator, opaque rotor, and colored liquid would be great for understanding it, if you wanted to build one.

That one is interesting and I, too, am having some trouble seeing the apparent mechanism ... that's never stopped me from building anything before though, hehe. Building it with a piece of clear plexi pipe with some colored mineral oil would really help see the principle; similarly, an Archimedes' Screw would also look cool done that way.

flamewise
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Re: Counterintuitive Mechanical Devices

Postby flamewise » Fri Oct 29, 2010 3:55 pm UTC

Tippe Tops are gyroscopes that can do an unexpected mechanical trick right on a desk top:

http://www.youtube.com/watch?v=8J6nG4PygFU

Technical Ben
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Re: Counterintuitive Mechanical Devices

Postby Technical Ben » Fri Oct 29, 2010 9:37 pm UTC

skeptical scientist wrote:I find the progressive cavity pump bizarre and counterintuitive, but staring at high-res pictures on wikipedia for several minutes is helping me visualize the geometry of it a bit better. I suspect a working model with a clear stator, opaque rotor, and colored liquid would be great for understanding it, if you wanted to build one.


Watching the animation large, and I've got it visualised. :P
It's a screw pump, but as it's a different shape, it makes sealed compartments. AFAIK.
It's all physics and stamp collecting.
It's not a particle or a wave. It's just an exchange.


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