Greenwood press

12
CIRCLES

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the circumference of them. If a wheel has a diameter of 32 inches, then its cir-
cumference, or distance around, is 
32π inches ≈ 100.5 inches.
In addition to distance traveled by an automobile, the circumference of cir-
cles is used in several applications. A trundle wheel is a device used to measure
distances that are too long for a tape measure. A marking is placed on the wheel
so that it clicks for one complete revolution. A trundle wheel can be made in any
size, although it is convenient to make one with a diameter of 31.8 cm, because
then its circumference will be 1 meter (circumference is the product of 
π and the
diameter of the circle). Therefore, as you push the trundle wheel, every click that
is recorded on the odometer means that the wheel has gone around once and you
have traveled 1 meter. 
Another useful application of circumference is to determine the age of cer-
tain trees. The girth, or thickness, of trees increases as they grow older. A fallen
tree often shows a large group of concentric rings, where each ring represents a
year of its life. Since the tree gets thicker during its lifetime, the number of rings
is proportional to its circumference. Therefore, a functional relationship can be
created to estimate the age of a tree based on its circumference. This means that
a measurement of the circumference of a tree can give an indication of its age
without having to chop it down and count its rings.
The area of a circle is useful to determine the price of circular foods that have
the same height. For example, pizzas are often advertised according to their
diameter. A pizza with a diameter of 12 inches might sell for $10, and a pizza