Phenomenon Of The Disappearing Dot! – Is it Physics, or Math, or Perception?


Lee Walkerby: Lee Walker and Partnership for Learning.com, LLC

Use a hack saw to cut a 2¼ to 2½  inch length of ¾ inch outside diameter PVC pipe.  The point is for the tube to be three times the length of its diameter. While 2¼ inches is more precise, it is fine to fudge just an extra ¼ inch. Trust me, it’s close enough.  Next put a green dot at one end and a red dot on the other (see photo). I like to drill two small depressions and put the paint in those two shallow holes (don’t drill through).

PhysicsQuest 2010: Spectra's Force Disappearing DotSet the cylinder down in front of you on a nice smooth surface.  You will need plenty of room as you develop the operational skills, so a reasonably smooth desk or table top should be fine.

Now, place the tip of your index finger on the red dot. It works just fine on the green one, but let’s have the first run match the PhysicsQuest 2010: Spectra's Force Disappearing Dotphotographs. If you force down your finger as you pull it slightly back toward you, the cylinder will spin rapidly around a horizontal axis in your direction. You can visualize the action around this axis by imagining the cylinder seen from one end so that it would look like a spinning wheel.

The two drawings below show how the cylinder, viewed from the end, rolls (spins) around a horizontal axis in a direction toward the finger thatsnaps downward (though sliding away at the same time).  As this is going on, (see the drawing on the left) the end that had the fingertip on it, rotates around a vertical axis in a direction away from the fingertip. Read the rest of this entry »