Cylinder Volumes (Part 1)

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Objective

SWBAT understand the relationship between surface area and volume of a cylinder.

Big Idea

Which cylinder is larger? Students make conjectures, then do the math.

Launch

10 minutes

I begin this lesson by projecting an image of two real cylindrical Sinkholes. I tell the class that a sinkhole is a depression or hole in the ground caused by some form of collapse of the surface layer. Sinkholes are often shaped like a cylinder. The dimensions of these two holes are given below each picture. I ask:

Assuming the sinkholes are perfect right cylinders which sinkhole is larger? Which sinkhole holds more soil if they were filled completely?

I ask my students to write down their reasoning before they share their ideas. I say, "I'll call on a few of you to make a prediction, but I want to hear a clear explanation of your prediction." Explaining their conjectures helps students think about the math that is inherent in the situation. It also encourages students to realize that there is more to a mathematical solution than a confident guess.

My experience is that quite a few students will conclude that the holes will hold the same amount of soil because the dimensions of both holes are the same, just flipped. This would work for a rectangle, for example, so it is good reasoning based on prior experience. Others will conclude that the deeper sinkhole held more soil, because it is deeper. I am looking to see how many students consider the radius as the more significant dimension.

As students write, I walk around listening, being careful not to share information. I want students to feel that I think they can do this on their own. I also ask students to try to work without technology, particularly technology that contains (or searches for) formulas. The objective here is for students to explain their reasoning. It is important practice with a skill that will be used over and over in this unit.

Exploration Activity

35 minutes

Next I pair up my students with their shoulder partner and hand them two sheets of printer paper (8.5" by 11"). I ask that they roll one paper the long way to make a cylinder and tape it. The other paper should be rolled the short way and taped at the edges as well. After the cylinders are done, I hand each pair the following Cylinder Activity Instructions, and ask that they read and discuss each instruction carefully.

I walk around listening and observing students working. The following are short narratives for each question.

Question 1: Many of my students may not recall the Lateral Surface Area formula 2πrh. I tell these students to search for it. Some students will realize or recall, that the lateral surface area is the length times the width of the sheet. I ask that they be discrete and allow the other students to work on their own. Even after obtaining the formula, some students may complain that they don't know how to find the radius. I don't give any info and motivate students to continue figuring it out. I may say..."you really don't need the radius here, to get the LSA".

After students answer Question 1, I stop and address the class. Some may not have answered or may have done so incorrectly, so I call on one of the students who figured the formula quickly without the radius and ask that he/she explain their reasoning to the class. I expect the student to show how the circumference of the base is actually the width of the rectangular paper.

It is important for students to fully understand why the Surface Area is the same for both cylinders, and, to be able to explain it concretely and using mathematical abstractions (i.e., a formula). This dual approach is a key to the lesson, and in fact, the unit.

Question 2: For Question 2, I expect students who said that both sinkholes were the same size will once again conclude that both cylinders have the same volume because the same piece of paper is used for both cylinders. I believe that this is a misconception, a stubborn one! Following the Warmup, a multiple choice question can be used productively to survey the results and once again ask for an explanation. I usually ask for explanations from students who favor a, b, and c when we discuss this question.

Question 3: Here, I ask students to search for the formula, Πr2h. But, I allow students to struggle with figuring out how to obtain the radius when they know the volume and the height. If students get stuck, and can't seem to figure things out, I will use the following hint: Can you use the circumference as in Question 1 to find the radius.  An important conversation occurs after the formula is applied as students react to the discovering that the volumes are different.

Question 4: This question gives students the opportunity to write an explanation of their current understanding in their own words.

Question 5: I will take some time here to help students formulate a clear explanation. Some, but not all, will be thinking about the fact that the radius is squared, so it is more of a factor in the numerical value of the volume than the height. So, one can expect a wider sinkhole to hold more soil, if one is asked for an estimate.

Closure

10 minutes

I close this lesson with an EXIT QUIZ. I tell my students that it is a formative assessment. I say, "I'll make corrections for you and I will not give you a number grade." I want to take the pressure off students, but I encourage them to give their best explanation. They try their best knowing that they will eventually receive feedback on whatever they don't understand. Please read my After teaching this lesson... reflection for more details.