# Engaging students: Finding the volume and surface area of prisms and cylinders

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 Angelica Albarracin. Her topic, from Geometry: finding the volume and surface area of prisms and cylinders.

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

For finding the surface area of prisms and cylinders, I as the teacher would create an activity centered around using the nets of these figures to better visualize this concept. In my experience, many students do not struggle with the computational aspect of finding the surface area of prisms and cylinders, but rather, they tend to forget to calculate the area of all the faces of such figures. When a student views these three-dimensional figures on paper, it can be easy to forget some faces as not all of them can be illustrated, requiring the student to have an accurate depiction of the figure already in mind. By having students work with nets, they will have some guidance in calculating the surface area of prisms and cylinders. Additionally, having the students construct each intended figure with the net can also help students develop a better understanding of the composition of prisms and cylinders.

A project I could use as a teacher in order to help students understand volume of prisms and cylinders would be to have the students create their own drink company. I could provide the students with several models of different styles of cans they could use and have them find the volume of their selected can as a requirement. I think this would be a fun way to not only allow to students some creative freedom but also provide practice calculating the volumes of various prisms and cylinders. Students would have to consider aspects such as how much liquid one container holds over another, how portable the shape is, and how will others drink from it. Students could also find the surface area of their drink cans in order to see how much material would be needed to print a label that would fit around each can.

How can this topic be used in your students’ future courses in mathematics or science?

Finding the volume and surface area of prisms and cylinders provides a basic background for students to start exploring more complex shapes such as spheres, cones, and pyramids. However, in Calculus I, this topic is taken further with the introduction of integrals and the concept of finding the area under irregular curves. Later down the line, students will also learn about volumes of solids of revolution. For rounded curves, an approximation for such solids is comprised of taking the sum of the volume of many cylinders; the more cylinders there are, the closer the approximation will be to the true volume. An image of this is shown below:

Continuing with the theme of solids of revolutions, Calculus II is when students must find the surface area of these solids. To approximate the surface area, we take the surface area of frustums that can be formed under the curve. Frustums are similar to cones as they both have circular bases, but instead of coming to a point, a frustum also has a circular top. As before, the greater the amount of frustums used in the approximation, the closer the calculated value is to the true surface area. The formula for the surface area of a frustum is $A = 2\pi r h$ A = where $r =(r_1+r_2)/2$. Frustums are unique in that both circular bases are different. In the case that the bases are the same, the formula for $r$ becomes $r =(2r_1)/2 = r_1$,  in which case the formula for surface area becomes $A = 2\pi r h$  which is exactly the formula for the surface area of a cylinder. Below is an image of the surface area approximation of a solid formed by revolution:

What interesting things can you say about the people who contributed to the discovery and/or the development of this topic? (You might want to consult Math Through The Ages.)

The ancient Greeks are responsible for naming many of the figures and solids we commonly see in Geometry. For example, the word “prism” comes from the Greek word meaning “to saw”, which comes from the fact the cross sections (or cuts) of a prism are congruent. The word “cylinder” also comes from Greek, specifically from the word that means “to roll”. In addition, the Greeks were also “the first to systematically investigate the areas and volumes of plan figures and solids”. One of the most famous of these Greeks is the mathematician Archimedes who is directly responsible for the approximation of the area of a circle, the approximation of pi, the formulas for the volume and surface area of a sphere, and a technique called the “method of exhaustion”, which was used to find areas and volumes of figures in a manner similar to that of modern calculus. Archimedes viewed his discovery of the formula for the surface area of a sphere as his greatest mathematical achievement and even instructed that it be remembered on his gravestone as a sphere within a cylinder.

Another mathematician who developed techniques that bore similarities to modern calculus was Italian mathematician Bonaventura Francesco Cavalieri. While his discoveries pertained to finding the volume of objects, he was able to use are of cross sections to show that “two objects have the same volume if the areas of their corresponding cross-sections are equal in all cases”. This came to be known as Cavalieri’s Principle, but it is important to note that Chinese mathematician Zu Gengzhi had previously discovered this principle hundreds of years before Cavalieri. The next biggest advancement in this topic is attributed to integrals and making sense of the idea of finding the area under a curve. An approximate method for finding the area of a figure with an irregular boundary was developed known as Simpson’s Rule which had previously been known by Cavalieri but was rediscovered in the 1600s.

References:

https://amsi.org.au/teacher_modules/area_volume_surface_area.html