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POV sphere

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There was a link to a "POV Jump rope", but the idea seems to have vanished. It was, however, probably somewhat similar to this.

If you hold a rope or a length of chain vertically, you can make it swing in such a way that it describes an (approximate) sphere, with a conical bit at the bottom. If such a rope or chain were bedecked with LEDs, and if the handle at the top contained some gubbins to detect the rotation rate of the rope and control the LEDs, you could create a spherical POV device which could display, for example, a terrestrial globe. Powered by a motor, the device could even be used for display or advertising.

MaxwellBuchanan, Aug 16 2012

Ooh, I found it... http://en.wikipedia.org/wiki/Troposkein
The shape assumed when both ends are fixed, at least... [RayfordSteele, Aug 16 2012]

http://www.youtube....watch?v=qRlr784qKEo [2 fries shy of a happy meal, Aug 16 2012]

Here's a better one. http://www.ted.com/..._demos_the_orb.html
[2 fries shy of a happy meal, Aug 16 2012]


       Dang, now you have me curious as to what the actual shape described is. Modified Parabola? Sine wave?
RayfordSteele, Aug 16 2012

       A "pure" curve is just a special case of a parabola.   

       A segment of a circle, spinning round the observer's head sufficiently fast, would work. Actually, several segments would work better.   

       It would look a bit like a hairdryer ...   

       Don't nod your head.
8th of 7, Aug 16 2012

       The curve of a hanging rope or chain is called a "catenary". If you need a mnemonic for it, just think of it as the grin on the cat that ate the canary.
Vernon, Aug 16 2012

       No, not a pure catenary, but perhaps a modified catenary, since the rope is not just supporting its inertia at every point, but it's weight and also the weight of the sections of rope below it.
RayfordSteele, Aug 16 2012

       What does pov mean?
zeno, Aug 16 2012


       There are at least two pov skipping rope ideas still existing here and I've seen the real thing on a video once. I'll see if I can find it.
<couple seconds later>
It's also possible to create more than one spinning spherical shape with only one rope.

       Persistence of vision.
RayfordSteele, Aug 16 2012

       Yes, POV is persistence of vision - the second link illustrates it nicely.   

       The shape: it won't be a catenary. It's hard to describe in words but, relative to a vertical line downwards starting from your hand, the spinning rope arcs out to one side then back to the centreline, and then continues for some distance on the other side. As the rope spins, the lateral force (centripugal) varies in proportion to the distance from the vertical axis; also, the gravitational force acts parallel to the vertical axis.   

       For a catenary, the force (usually due to gravity) is uniform and usually at right angles to the line joining the two ends.   

       //It's also possible to create more than one spinning spherical shape with only one rope.// It is? Surely not if the spin speed is kept constant? (I know that you can make multiple spheres, with the rope describing several S-turns rather than just a single curve, but that's a different jar of newts.)
MaxwellBuchanan, Aug 16 2012

       //It's also possible to create more than one spinning spherical shape with only one rope.// It is? Surely not if the spin speed is kept constant?//
hmmm, it would be S-bends wouldn't it. I just know that there's a little flick thing you can do to make a rope spin more than one circle. the speed stays constant to maintain the new shape. I was daydreaming about this the other day for no good reason wondering what was bugging me about it.

       It's still bugging me, there's an idea in there somewhere I just know it...   

       Try looking round through 360 degrees, you might spot something …
8th of 7, Aug 16 2012

       Like [+]
sqeaketh the wheel, Aug 17 2012


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