WHERE DID THE GREEN DOT GO? Fig. A: The spinning cylinder revolves around its midpoint (radius R) and
simultaneously around its long axis (radius r). Fig. B: A small plastic cylinder has a red dot on one end, and
a green dot on the other. Fig. C: Immediately after spinning, the motion is chaotic. Fig. D: As the cylinder
stabilizes, many red dots are visible. Fig. E: When the spinning has stabilized, 4 red dots are clearly visible.
The green dot is not visible.
Photography by Sam Murphy; illustrations by Donald Simanek
ratio for a given number of dot repetitions, if you
used a solid cylinder? (It may not be the same.)
What role does moment of inertia play here?
Friction? Energy? There’s a lot of physics here to
address, and it may not be easy to do.
its red dot end is only slightly above the table. Then
you see a pattern of 3 red dots for a longer time.
During the transition you do not observe a stable
pattern of 4 dots, except perhaps fleetingly. Why
5 and not 4? Could it be that the lower end is still
sliding on the floor? This would be consistent with
the observed shorter time that this motion is stable.
Some Curious Observations
A 3: 1 version with ¾" PVC pipe works best for the
following experiments. This larger version also Solution
seems to exhibit less translational motion from the Another thing I like about this toy is that it combines
point of launch before stabilizing, enabling you to do physics and physiology. Initially, because of the
the experiments on a smaller surface area. manner of launching, the cylinder spins about its
Spin the 3: 1 cylinder by launching it forcefully at own long axis and slides on the floor (Figure F, next
the red dot end, and observe it from the side. For a page). Because the end you launched (red dot) is
while, it spins with the red dot end rather high in the also propelled sideways, the other end (green dot)
air. During that time you may see a brief but stable drags on the floor. The friction with the floor slows
pattern of 5 red dots. The cylinder’s spin slows until the cylinder’s motion and also changes its direction.