F
Fig. F: As the cylinder spins about its long axis, it
traces a circle around the floor. When the green dot
is up, the dot is moving too rapidly to be distinctly
seen. The red dot moves most slowly when it is facing upward, and is therefore briefly visible, as seen
from above. Fig. G: Different decorations can
produce kaleidoscopic patterns. Fig. H: Experiment
with patterns using colored tape.
The motion stabilizes with both ends moving
around a common circle, but the end you launched
is not quite touching the floor, while the other end is
rolling around the circle without slipping. When the
green dot is up, the dot is moving too rapidly to be
distinctly seen. When the red dot is up, it’s moving around the cylinder’s long axis in the opposite
direction to the direction that end of the cylinder
moves around the larger circle. Therefore the red
dot moves most slowly when it’s facing upward, and
is therefore briefly visible, as seen from above.
If the cylinder is 4 times as long as its diameter,
the larger circle is 4 times the diameter of the cylinder. When rolling without slipping is established, the
cylinder rotates about its long axis 4 times for every
time it spins around the large circle, and therefore
we see the red dot 4 times.
When the cylinder is spun very forcefully, it may
for a short while spin with the cylinder elevated at a
large angle, but it soon settles down to the motion
174 Make: Volume
16
G
H
described. There’s a lot of good physics going on
here, involving moments of inertia, gyroscopic
motion, rotational energy, and so forth. If anyone
wants to work that out, please get in touch with me
at
dsimanek@lhup.edu, and I might find space for
your analysis on my web page.
More Illusory Fun
Now that you understand the principle, you can
get creatively arty. If you use a larger-sized white
plastic tube, you can decorate it in various ways so
that when it’s spun, it will produce “kaleidoscopic”
patterns (Figure G). Star patterns are easy. Colored
flexible plastic tape is good for initial experimentation
(Figure H), and more permanent marking may be
used later.
Photography and illustration by Donald Simanek
Donald Simanek is emeritus professor of physics at Lock
Haven University of Pennsylvania. He writes about science,
pseudoscience, and humor at
www.lhup.edu/~dsimanek.
