# Engaging students: Circumference of a circle

In my capstone class for future secondary math teachers, I ask my students to come up with ideas for engaging their students with different topics in the secondary mathematics curriculum. In other words, the point of the assignment was not to devise a full-blown lesson plan on this topic. Instead, I asked my students to think about three different ways of getting their students interested in the topic in the first place.

I plan to share some of the best of these ideas on this blog (after asking my students’ permission, of course).

This student submission again comes from my former student Daniel Littleton. His topic, from Geometry: computing the circumference of a circle.

C3: How has this topic appeared in the news?

On January 29, 2014 an internet based publisher of medical news, news-medical.net, published an article as to the link between waist circumference and health risk factors. The article is entitled Waist Circumference Measurements Help to Detect Children and Adolescents with Cardiometabolic Risk. This study was conducted in Spain and concluded that including the measurement of waist circumference in clinical practices, in conjunction with traditional height and weight measurements, will allow an easier detection of risk factors for cardiometabolic disorders in children. Waist circumference is measured by placing a tape measure at the top of the hip bone and wrapping the tape around the body level with the navel. This measurement is the circumference, and this measurement can be used to determine the radius and diameter of a human body by knowing that circumference is equivalent to $2 \pi r$ where $r$ is the radius of the circle. This is certainly not the first use of waist circumference in determining health risk factors published in a medical article, however this is a very recent example. This story may be found at the following link: http://www.news-medical.net/news/20140129/Waist-circumference-measurements-help-to-detect-children-and-adolescents-with-cardiometabolic-risk.aspx.

A1: What interesting word problems using this topic can your students do now?

One example of an engaging word problem utilizing the concept of a circumference is as follows. “Aliens have invaded earth and they are establishing colonies on Earth. You are a member of the human resistance and you need to plant explosive traps for the alien soldiers. You know that you have enough materials to build one large bomb, one mid-size bomb, and one small bomb. The large bomb has an explosive diameter of 100 feet. The blast radius of the small bomb is one-fifth the distance of the large bombs diameter. The mid-size bomb has a blast radius that is 20 feet greater than the radius of the small bomb. What is the blast circumference of each of your bombs?” This problem requires the manipulation of both forms of the formula for circumference, $C=2\pi r$ and $C= \pi d$ where r is equal to the radius and d is equal to the diameter. The circumference of the large bomb can be calculated directly from the information provided in the problem. The circumference of the small bomb requires manipulation of the data provided. First, the diameter of the large bomb is divided by five. This determines the blast radius of the small bomb which can be used to determine the circumference. The blast radius of the mid-size bomb is determined by adding 20 feet to the blast radius of the small bomb, and then using this radius in the formula for circumference. The solutions for the circumference are as follows. Large bomb: $100\pi$ or 314.16 feet, Small bomb: $\latex 40\pi$ or 125.66 feet, Mid-size bomb: $60\pi$ or 188.50 feet. I believe that this problem would present an intriguing challenge to the students.

A2: How could you as a teacher create an activity or project that involves your topic?

An engaging activity that involves the determination of circumference will always need to include the manipulation of circles. One creative way to create circles is to form them through bubbles. The title I have chosen for this activity is “Bubblelicious Circumference.” This activity will require the following materials: bubble solution, straws, rulers, paper, and pencil. First the students will clear their desk surface, after which the instructor will pass out the straws, rulers, and pour approximately one tablespoon of bubble solution on the students’ desk. The instructor will also place a small container with bubble solution inside of it on the students’ desk. The students will first dip the end of the straw that will not go into their mouths into the container with bubble solution inside. Next, the students will place the wet end of the straw into the bubble solution on their desk and gently blow air into the bubble solution. The students will continue to blow air until a bubble forms and pops on their desk. Once the bubble pops it will leave a ring of liquid on the surface of their desk in a near perfect circle. The students will then use the ruler to determine the diameter of the circle that is on their desk. This measurement can then be used to determine both the circumference and the radius of the circle. The students will repeat this process at least 10 times, and as many times as the allotted time for the activity will allow. The circumference data will be recorded for each circle formed by each bubble blown on a piece of paper.

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