cardboard circle
cardboard rectangle
piece of cardboard
ruler
scissors
You can easily find the center of a rectangle. Draw its two diagonals. The point where they intersect is the center. Mark the center on the cardboard rectangle.
To find the center of a circle, use your ruler to find the widest part. Draw a line, the diameter, across the widest part of the cardboard circle. Now turn you ruler 90 degrees and find the widest part of the circle in this direction. Draw a line. Where the two lines intersect is the center.
The center of these shapes is also known as the center of gravity. Try sliding each shape slowly off of the edge of your desk. Where is the center (of gravity) when the shape starts to fall?
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Now let's use this idea about the center of gravity to find the center of an irregular shape. Cut one out of a piece of cardboard. Slowly slide it off the desk. Just as it starts to fall catch it and hold it in place. Draw a line on the bottom along the edge of the desk.
Turn the shape and repeat this step. Mark another line just where the shape starts to fall. Where these two lines cross is the center.
Try pushing the shape off the desk several times and note where the center is when the shape starts to fall.
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