# Solids of Revolution

## Objective

SWBAT visualize 3-D solids of revolution and find their volumes

#### Big Idea

Close Encounters of the Third Dimension...In this lesson, students see 2-D figures transform into 3-D solids.

## Visualizing Solids of Revolution

10 minutes

The idea of rotating a two-dimensional region to create a three-dimensional solid is a new concept for most students. Even when the concept is explained well, it is still difficult for many students to visualize it. Thankfully, we have technology that makes our jobs as teachers a lot easier.

I use one such technology aid to start this lesson. It is a resource from Wolfram Alpha and its full citation is shown here:

Contributed by: Lyle Cochran

The following video explains how I use this demonstration to introduce this lesson.

## Guided Practice: Solids of Revolution Problems

20 minutes

To begin this section, I give each student a copy of Guided Practice_Solids of Revolution. The problems on this handout deal with rotating rectangles about their sides and right triangles about their legs. The more complex problems students will do later in the lesson involve rotations that are merely compositions of these fundamental rotations.

The two problems on the handout each have an (a) portion and a similar (b) portion. I model the (a) portion for students and then I give them time to complete the (b) portion on their own. After they have completed the (b) portion, I have them compare answers and process with their seat partners before I show the correct process and answer.

## Solids of Revolution Problem Solving

40 minutes

In the previous two sections, I've been careful to equip my students with the fundamental knowledge and skills they will need in order to tackle the problems in this section.

My intent in this section is for students to gain experience with the process of problem-solving, which someone once described as what you do when you don't know what to do. For this reason, I will not be giving answers to the problems during this classroom session. I want students to take risks, collaborate, make sense of things, persevere, etc. If they know I'll soon be giving answers, some students hesitate to take the full onus of the problem-solving upon themselves.

So I have my students move the desks so that they are seated side by side with their seat partners. Then I give each student a copy of Problem Solving _Solids of Revolution I give the students 40 minutes to work collaboratively on the  problems. I walk around the room looking for students who are seriously stuck. I'll try to ask these students open ended questions to try and move them forward. For example, I will usually ask, "What lengths can we infer on the diagram besides what's been given?" or "How might you compose or decompose this figure into simpler figures?" and sometimes it just "Explain to me how this figure is going to rotate."

In any case, as I said, I will allow students to struggle through the problems and use each other as resources. If they don't finish in class, they will need to take the problems home to complete. When students have had enough time to finish (inside and outside of class), I will post Solids of Revolution Worked Solutions online for students to learn from and to check their work.