We all enjoy the surprise of center of mass demonstrations: flat, seemingly symmetrical bodies that do not balance at their geometrical centers because of pieces of metal let into their surfaces. These eccentrically-located weights can be seen on both of the disks below. The upper writing on the disk at the left reads "Center of Motion and Gravity", and the writing in the center reads "Center of Magnitude". The disk spins smoothly about the upper pivot, which is not in the geometrical center. This apparatus is in the collection of the National Museum of American History of the Smithsonian Institution.
The apparatus at the right was bought by the Denison University Physics Department on August 29, 1905 from the Central Scientific Company of Chicago.
My own favorite center of mass demonstration is home-built. A plywood disk, about 50 cm in diameter, has a 25 cm diameter hole cut out of one side, running from the center to the edge of the disk. The center of mass is one sixth of the radius over from the center of the disk.
REF: Thomas B. Greenslade, Jr., "Center of Mass Demonstrations using Ceiling Tile", Phys. Teach., 30, 240-241 (1992)
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