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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.
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]
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Dang, now you have me curious as to what the actual shape described is. Modified Parabola? Sine wave? |
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A "pure" curve is just a special case of a parabola. |
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A segment of a circle, spinning round the observer's head sufficiently fast, would work. Actually, several segments would work better. |
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It would look a bit like a hairdryer ... |
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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. |
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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. |
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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>
[link]
It's also possible to create more than one spinning spherical shape with only one rope. |
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Yes, POV is persistence of vision - the second link
illustrates it nicely. |
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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. |
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For a catenary, the force (usually due to gravity) is
uniform and usually at right angles to the line
joining the two ends. |
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//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.) |
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//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. |
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It's still bugging me, there's an idea in there somewhere I just know it... |
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Try looking round through 360 degrees, you
might spot something … |
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