TOYS, TRICKS, & TEASERS
By Donald Simanek
The Spinning Cylinder Illusion
This is a toy, a puzzle, and an illusion all in one, with only one
moving part. No batteries are required. I like simplicity, especially
when it produces puzzling complexity.
» I’ve not been able to track down the origin of Here’s a sequence of photos showing the cylinder
this homemade toy, but it isn’t very well known beginning to spin (Figure C), not quite settled down
outside the community of physics teachers. It’s (Figure D), and finally spinning in one place with
a kinetic illusion, one that depends on physical nearly constant angular speed (Figure E). These
motion to make you see something that isn’t there. photos were taken with a digital camera, but they
A familiar example of a kinetic illusion is the show approximately what appears to the eye.
strobe effect sometimes seen in old movies, causing All these photos are taken from the same camera
the spokes of a carriage wheel to seem to be turning position and distance, and reproduced at constant
in the wrong direction. scale. Therefore lengths on the pictures may be
In its simplest form, this toy consists of a hollow directly compared. Notice that a stable illusion
cylinder of rigid plastic. The version in these photos showing 4 equally spaced dots occurs when the
is 4cm long and 1cm in diameter, with 2mm wall cylinder rotates about its center, and both ends
thickness. It was cut from a piece of polyethylene traverse the same circle, which has a diameter
plastic tubing that happened to be lying on my equal to the length of the cylinder.
workbench. Whatever tubing you use, be sure to The puzzling outcome is that when the cylinder
choose a very straight piece. settles down to a uniform spin rate at a particular
This material can be cut easily with a single-edged location on the table, you see only one color dot
razor blade, scrap of board, and hammer. Use heavy (red, in this case), repeated N times, where N is
gloves and goggles in case the razor blade breaks. the ratio of the cylinder’s length to its diameter.
Or use a hacksaw, then smooth up the cut with fine The dot you see is the one at the end you pressed
sandpaper. Cut the piece initially a bit long, then with your thumb to launch it. You don’t see any
shorten it to optimum length by trial. I’ve made evidence of the dot at the other end (green, in this
small ones in 2-, 3-, 4-, and 5cm lengths. I’ve also case). Launch the cylinder by pressing the green
made them from the barrels of old ballpoint pens, dot end and then you see only the green dot, and
and larger ones from PVC plumbing pipe. not the red one.
Glue or paint a red dot near one end and a green How can we explain this unexpected behavior?
dot at the other end, or use a felt-tip marker to Note that translational motion of the cylinder’s
make distinctive symbols at the ends. center of mass stops just before stable dot patterns
become visible. Note also that what we call the
“stable motion” occurs when the ends of the
cylinder trace a circle.
When you think you’ve got it figured out, test
your understanding by considering this follow-up
question. If you spin this toy on a glass sheet, and
someone looks up at it from below the glass, which
colored dot would that observer see? A glass-topped coffee table is good for this experiment.
What would be the optimum length-to-diameter
The Vanishing Dot
Place the cylinder on a flat surface. It’s best to
choose a surface that won’t dent or scratch, like a
smooth, hard floor. Press down on 1 side of 1 end
with your thumb, as shown in Figure B, until the
cylinder slips away from your thumb and flies off
spinning. It will settle down, revolving about its
midpoint and at the same time spinning about
its long axis (Figure A).
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