Pipe in a Bottle

A Demonstration of Persistence of Vision and Rolling Motion
by John C. Lahr
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Materials:

A 1-liter clear-plastic soda bottle with the label removed.

5 or 6-inch long piece of 1/2-inch white plastic (PVC) pipe.
(This pipe has 5/8 outside diameter and will just fit through the top of the bottle.)

A black marking pen.

Preparations:

Write a few words on the pipe. The writing can wrap all around the pipe. Drop the pipe into the bottle.

To Do:

Shake the bottle with a circular motion so that the pipe spins around on the inside. Once it is spinning, a good way to keep it going is to hold the bottle in a horizontal position, with the bottom of the bottle against your stomach, while keeping the pipe spinning with one hand on the top. Look down at the pipe spinning within the bottle and notice that all of the words can be read.

What's going on?

Because of "persistence of vision" an image that is focused on the back of you eye (your retina) does not go away instantaneously. Even though only one side of the pipe can be seen at any instant, the image from an instant before appears to still be there. For this reason, you can see all of the words written on the pipe as if they were "unwrapped" and printed on the inside surface of the bottle.

The only portion of the pipe that is not blurry at any instant is the part that is in contact with the inside of the bottle. The words can not be seen on the pipe as it rolls past the far side of the bottle because when the pipe is in this location the edge facing you is moving very fast and that the letters just appear as a blur.


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