Center of Mass

Irregular lamina with marked center-of-mass tossed in air.

What it shows:

The center of gravity fixed in (or outside) the object always orients itself with minimum potential energy on a vertical line below the support point. When an irregular shape is thrown into the air, it is seen to rotate about its marked center of gravity or center of mass (COM).

How it works:

We have several irregular lamina to suspend and/or throw in the air. They are (1) an amoeba shaped piece of masonite pegboard, (2) a cut-out map of the U.S. glued onto a 1/2" plywood board, (3) a U-shaped object made out of 3/16" plywood, (4) an asymmetric polygon shaped polystyrene sheet, and (5) an irregular shaped piece of rug which can be safely thrown, spinning in the air, and caught.

center of mass

The center-of-mass of an irregular shaped lamina is found by supporting it from various points. The lamina is hung from any of its suspension points with the plumb bob hanging from the same. A chalk line is drawn along the vertical plumb line. This is repeated from one or two other suspension points. If reasonable care is exercised, the chalk lines intersect at a single point -- the COM.

center of mass

Setting it up:

The suspension mechanism is nothing more than a simple assembly of standard lab clamps and rods attached to the lecture table. The various suspension "points" on the lamina are actually holes, any of which are slipped over the anchor rod of the suspension assembly. The plumb bob is any suitable weight on a string with a loop at the other end for quick hanging and removal from the anchor rod.

irregular lamina


The COM of the U.S. map is somewhere in Kansas (very likely where Dorothy and Toto reside). The U-shaped object's COM is in the center of the U, not within the object itself. A large movable dot (made out of Velcro™ and fluorescent tape) can be stuck anywhere on the rug lamina. When the rug is thrown spinning into the air, the dot orbits around the COM (unless it has been stuck at the COM location).