Demos: 1J-21 Center of Gravity of Irregular Lamina
A variety of wooden shapes (of uniform thickness and density) is suspended from two or more points in succession. A hanging weight suspended from the same point determines the downward direction, the line of which is marked by chalk. Where the lines cross is the center of gravity of the object.
Directions: Choose one of the objects and suspend it from a predrilled hole. When it comes to complete rest, carefully draw a chalk line coincident with the hanging string. The center of gravity is along this line. Repeat, using another hole to suspend the object from. Where the two chalk lines cross is the center of gravity.
Suggestions for Presentation: Begin by showing the students a mass suspended from a string. Why does it hang straight down? Show that in equilibrium the center of gravity is always located below the suspension point. For example, a pendulum always returns to this position when it finally comes to rest. So when the suspended lamina come to rest, the CG must be directly below the suspension point, as determined by the chalk line. When using something like the cut-out of the state of Indiana, for example, point out that this is the center of gravity of the state only if it were of uniform thickness and density, which it isnt. Finding the CG of a state cut-out is entertaining but not terribly useful!
With regard to the cut-out of the crescent moon, emphasize that the CG does not have to be at a point where there is mass.
Applications: A method of locating approximately the balance point of an object.