10.8: Circumference
What if you wanted to find the "length" of the crust for an entire pizza? A typical large pizza has a diameter of 14 inches and is cut into 8 or 10 pieces. Think of the crust as the circumference of the pizza. Find the circumference. After completing this Concept, you'll be able to solve this problem.
Watch This
CK-12 Foundation: Chapter10CircumferenceA
Guidance
Circumference is the distance around a circle. The circumference can also be called the perimeter of a circle. However, we use the term circumference for circles because they are round. The term perimeter is reserved for figures with straight sides. In order to find the formula for the circumference of a circle, we first need to determine the ratio between the circumference and diameter of a circle.
Investigation: Finding \begin{align*}\pi\end{align*}π (pi)
Tools Needed: paper, pencil, compass, ruler, string, and scissors
- Draw three circles with radii of 2 in, 3 in, and 4 in. Label the centers of each \begin{align*}A, B\end{align*}
A,B , and \begin{align*}C\end{align*}C . - Draw in the diameters and determine their lengths. Are all the diameter lengths the same in \begin{align*}\bigodot A\end{align*}
⨀A ? \begin{align*}\bigodot B\end{align*}⨀B ? \begin{align*}\bigodot C\end{align*}⨀C ? - Take the string and outline each circle with it. The string represents the circumference of the circle. Cut the string so that it perfectly outlines the circle. Then, lay it out straight and measure, in inches. Round your answer to the nearest \begin{align*}\frac{1}{8}\end{align*}
18 -inch. Repeat this for the other two circles. - Find \begin{align*}\frac{circumference}{diameter}\end{align*}
circumferencediameter for each circle. Record your answers to the nearest thousandth. What do you notice?
From this investigation, you should see that \begin{align*}\frac{circumference}{diameter}\end{align*}
From this Investigation, we found that \begin{align*}\frac{circumference}{diameter}=\pi\end{align*}
Example A
Find the circumference of a circle with a radius of 7 cm.
Plug the radius into the formula.
\begin{align*}C=2 \pi (7)=14 \pi \approx 44 \ cm\end{align*}
Depending on the directions in a given problem, you can either leave the answer in terms of \begin{align*}\pi\end{align*}
Example B
The circumference of a circle is \begin{align*}64 \pi\end{align*}
Again, you can plug in what you know into the circumference formula and solve for \begin{align*}d\end{align*}
\begin{align*}64 \pi= \pi d=14 \pi\end{align*}
Example C
A circle is inscribed in a square with 10 in. sides. What is the circumference of the circle? Leave your answer in terms of \begin{align*}\pi\end{align*}
From the picture, we can see that the diameter of the circle is equal to the length of a side. Use the circumference formula.
\begin{align*}C=10 \pi \ in.\end{align*}
Watch this video for help with the Examples above.
CK-12 Foundation: Chapter10CircumferenceB
Concept Problem Revisited
The entire length of the crust, or the circumference of the pizza is \begin{align*}14 \pi \approx 44 \ in\end{align*}
Vocabulary
A circle is the set of all points that are the same distance away from a specific point, called the center. A radius is the distance from the center to the outer rim of the circle. A chord is a line segment whose endpoints are on a circle. A diameter is a chord that passes through the center of the circle. The length of a diameter is two times the length of a radius. Circumference is the distance around a circle. \begin{align*}\pi\end{align*}
Guided Practice
1. Find the perimeter of the square. Is it more or less than the circumference of the circle? Why?
2. The tires on a compact car are 18 inches in diameter. How far does the car travel after the tires turn once? How far does the car travel after 2500 rotations of the tires?
3. Find the radius of circle with circumference 88 in.
Answers:
1. The perimeter is \begin{align*}P=4(10)=40 \ in\end{align*}
\begin{align*}C=10 \pi \approx 31.42 \ in\end{align*}
2. One turn of the tire is the circumference. This would be \begin{align*}C=18 \pi \approx 56.55 \ in\end{align*}
3. Use the formula for circumference and solve for the radius.
\begin{align*}C&=2 \pi r\\ 88 &=2 \pi r\\ \frac{44}{\pi}&=r\\ r&\approx 14 \ in\end{align*}
Practice
Fill in the following table. Leave all answers in terms of \begin{align*}\pi\end{align*}
diameter | radius | circumference | |
---|---|---|---|
1. | 15 | ||
2. | 4 | ||
3. | 6 | ||
4. |
\begin{align*}84 \pi\end{align*} |
||
5. | 9 | ||
6. |
\begin{align*}25 \pi\end{align*} |
||
7. |
\begin{align*}2 \pi\end{align*} |
||
8. | 36 |
- Find the radius of circle with circumference 88 in.
- Find the circumference of a circle with \begin{align*}d=\frac{20}{\pi} cm\end{align*}
d=20πcm .
Square \begin{align*}PQSR\end{align*}
- Find the length of the diameter of \begin{align*}\bigodot T\end{align*}
⨀T . - How does the diameter relate to \begin{align*}PQSR\end{align*}
PQSR ? - Find the perimeter of \begin{align*}PQSR\end{align*}
PQSR . - Find the circumference of \begin{align*}\bigodot T\end{align*}
⨀T .
- A truck has tires with a 26 in diameter.
- How far does the truck travel every time a tire turns exactly once?
- How many times will the tire turn after the truck travels 1 mile? (1 mile = 5280 feet)
- Jay is decorating a cake for a friend’s birthday. They want to put gumdrops around the edge of the cake which has a 12 in diameter. Each gumdrop is has a diameter of 1.25 cm. To the nearest gumdrop, how many will they need?
- Bob wants to put new weather stripping around a semicircular window above his door. The base of the window (diameter) is 36 inches. How much weather stripping does he need?
- Each car on a Ferris wheel travels 942.5 ft during the 10 rotations of each ride. How high is each car at the highest point of each rotation?
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chord
A line segment whose endpoints are on a circle.Circumference
The circumference of a circle is the measure of the distance around the outside edge of a circle.diameter
A chord that passes through the center of the circle. The length of a diameter is two times the length of a radius.pi
(or ) The ratio of the circumference of a circle to its diameter.Congruent
Congruent figures are identical in size, shape and measure.Radius
The radius of a circle is the distance from the center of the circle to the edge of the circle.Regular Polygon
A regular polygon is a polygon with all sides the same length and all angles the same measure.Image Attributions
Here you'll learn how to calculate the circumference of a circle.