Lesson 19

Distinguishing Circumference and Area

Let’s contrast circumference and area.

19.1: Filling the Plate

About how many cheese puffs can fit on the plate in a single layer? Be prepared to explain your reasoning.

A picture of a tray with covered with one layer of circular cheese puffs. On top of the cheese puffs is a circular plate with 1 cheese puff placed in the center of the plate. 

 

19.2: Card Sort: Circle Problems

Your teacher will give you cards with questions about circles.

  1. Sort the cards into two groups based on whether you would use the circumference or the area of the circle to answer the question. Pause here so your teacher can review your work.
  2. Your teacher will assign you a card to examine more closely. What additional information would you need in order to answer the question on your card?
  3. Estimate measurements for the circle on your card.
  4. Use your estimates to calculate the answer to the question.

19.3: Visual Display of Circle Problem

In the previous activity you estimated the answer to a question about circles.

Create a visual display that includes:

  • The question you were answering
  • A diagram of a circle labeled with your estimated measurements
  • Your thinking, organized so that others can follow it
  • Your answer, expressed in terms of \(\pi\) and also expressed as a decimal approximation

19.4: Analyzing Circle Claims

Here are two students’ answers for each question. Do you agree with either of them? Explain or show your reasoning.

  1. How many feet are traveled by a person riding once around the merry-go-round?

    Image of a person on a circular merry-go-round.
    • Clare says, “The radius of the merry-go-round is about 4 feet, so the distance around the edge is about \(8\pi\) feet.”
    • Andre says, “The diameter of the merry-go-round is about 4 feet, so the distance around the edge is about \(4\pi\) feet.”

  2. How much room is there to spread frosting on the cookie?

    Image of a plate of circular-shaped cookies.
    • Clare says “The radius of the cookie is about 3 centimeters, so the space for frosting is about \(6\pi\) cm2.”
    • Andre says “The diameter of the cookie is about 3 inches, so the space for frosting is about \(2.25\pi\) in2.”

  3. How far does the unicycle move when the wheel makes 5 full rotations?

    Image of a person on a unicyle.
    • Clare says, “The diameter of the unicycle wheel is about 0.5 meters. In 5 complete rotations it will go about \(\frac52\pi\) m2.”
    • Andre says, “I agree with Clare's estimate of the diameter, but that means the unicycle will go about \(\frac{5}{4}\pi\) m.”


A goat (point \(G\)) is tied with a 6-foot rope to the corner of a shed. The floor of the shed is a square whose sides are each 3 feet long. The shed is closed and the goat can't go inside. The space all around the shed is flat, grassy, and the goat can't reach any other structures or objects. What is the area over which the goat can roam? 

Square, labeled "shed." Diagonal line segment labeled 6, ends at point G.

 

Summary

Sometimes we need to find the circumference of a circle, and sometimes we need to find the area. Here are some examples of quantities related to the circumference of a circle:

  • The length of a circular path.
  • The distance a wheel will travel after one complete rotation.
  • The length of a piece of rope coiled in a circle.

Here are some examples of quantities related to the area of a circle:

  • The amount of land that is cultivated on a circular field.
  • The amount of frosting needed to cover the top of a round cake.
  • The number of tiles needed to cover a round table.

In both cases, the radius (or diameter) of the circle is all that is needed to make the calculation. The circumference of a circle with radius \(r\) is \(2\pi r\) while its area is \(\pi r^2\). The circumference is measured in linear units (such as cm, in, km) while the area is measured in square units (such as cm2, in2, km2).