# Lesson 21

Cylinders, Cones, and Spheres

## 21.1: Sphere Arguments

CCSS Standards

• 8.G.C.9

### Warm-up: 5 minutes

The purpose of this warm-up is to catch errors students are making in calculating the volume of a sphere. Monitor for students who use the formula for a cylinder or cone, who use $r^2$ instead of $r^3$, or who forget to include $\pi$ as a factor in the computation.

### Launch

Arrange students in groups of 2. Tell students that if they have a hard time visualizing this sphere, they can sketch it. Give students 1–2 minutes of quiet work time followed by time to discuss their responses with their partner.

Conceptual Processing: Eliminate Barriers. Allow students to use calculators to ensure inclusive participation in the activity.

Fine Motor Skills: Peer Tutors. Pair students with their previously identified peer tutors and allow students who struggle with fine motor skills to dictate the directions for sketching a sphere as needed.

### Student Facing

Four students each calculated the volume of a sphere with a radius of 9 centimeters and they got four different answers.

• Han thinks it is 108 cubic centimeters.
• Jada got $108\pi$ cubic centimeters.
• Tyler calculated 972 cubic centimeters.
• Mai says it is $972\pi$ cubic centimeters.

Do you agree with any of them? Explain your reasoning.

### Student Response

Details about student responses to this activity are available at IM Certified Partner LearnZillion (requires a district subscription) or without a subscription here (requires registration).

### Activity Synthesis

For each answer, ask students to indicate whether or not they agree. Display the number of students who agree with each answer all to see. Invite someone who agreed with $972\pi$ to explain their reasoning. Ask students if they think they know what the other students did incorrectly to get their answers. (To get 108, Han and Jada likely used $r^2$ instead of $r^3$, and Tyler probably didn’t realize that multiplying $\frac43 r^3$ by $\pi$ is necessary.)

## 21.2: Sphere’s Radius

CCSS Standards

• 8.G.C.9

### Optional activity: 5 minutes

The purpose of this activity is for students to think about how to find the radius of a sphere when its volume is known. Students can examine the structure of the equation for volume and reason about a number that makes the equation true. They can also notice that $\pi$ is a factor on each side of the equation and divide each side by $\pi$. Both strategies simplify the solution process and minimize the need for rounding. Watch for students who substitute a value for $\pi$ to each side of the equation, who use the structure of the equation to reason about the solution, or who solve another way, so strategies can be shared and compared in the whole-class discussion.

### Launch

Allow students 3–4 minutes work time followed by a whole-class discussion.

Conceptual Processing: Processing Time. Provide the image to students who benefit from extra processing time to review prior to implementation of this activity. Also, allow students to use calculators to ensure inclusive participation in the activity.

### Student Facing

The volume of this sphere with radius $r$ is $V=288\pi$. This statement is true:

$$288\pi =\frac43 r^3 \pi.$$ What is the value of $r$ for this sphere? Explain how you know.

### Student Response

Details about student responses to this activity are available at IM Certified Partner LearnZillion (requires a district subscription) or without a subscription here (requires registration).

### Activity Synthesis

The purpose of the discussion is to examine how students reasoned through each step in solving for the unknown radius. Ask previously identified students to share their responses.

Consider asking students the following questions to help clarify the different approaches students took:

• “$\pi$ appears on both sides of the volume equation. Did you deal with this as a first step or later in the solution process?”
• “How did you deal with the fraction in the equation?”
• “If the final step in your solution was solving for $r$ when $r^3=216$, how did you solve? If you found that $r^3$ was a different number, how did you solve?”

## 21.3: Info Gap: Unknown Dimensions

CCSS Standards

• 8.G.C.9

### Activity: 20 minutes

In this info gap activity, students determine and request the information needed to answer questions related to volume equations of cylinders, cones, and spheres.

The info gap structure requires students to make sense of problems by determining what information is necessary, and then to ask for information they need to solve it. This may take several rounds of discussion if their first requests do not yield the information they need (MP1). It also allows them to refine the language they use and ask increasingly more precise questions until they get the information they need (MP6).

Here is the text of the cards for reference and planning:

### Launch

Tell students that they will continue to refresh their skills of working with proportional relationships. Explain the Info Gap structure and consider demonstrating the protocol if students are unfamiliar with it.

Arrange students in groups of 2. In each group, distribute a problem card to one student and a data card to the other student. After you review their work on the first problem, give them the cards for the second problem and instruct them to switch roles.

Conceptual Processing: Eliminate Barriers. Demonstrate the steps for the activity or game, in which a group of students and staff play an example round while the rest of the class observes.

### Student Facing

Your teacher will give you either a problem card or a data card. Do not show or read your card to your partner.

If your teacher gives you the problem card:

1. Silently read your card and think about what information you need to answer the question.
2. Ask your partner for the specific information that you need.
3. Explain to your partner how you are using the information to solve the problem.
4. Solve the problem and explain your reasoning to your partner.

If your teacher gives you the data card:

1. Silently read the information on your card.
2. Ask your partner “What specific information do you need?” and wait for your partner to ask for information. Only give information that is on your card. (Do not figure out anything for your partner!)
3. Before telling your partner the information, ask “Why do you need that information?”
4. After your partner solves the problem, ask them to explain their reasoning and listen to their explanation.

Pause here so your teacher can review your work. Ask your teacher for a new set of cards and repeat the activity, trading roles with your partner.

### Student Response

Details about student responses to this activity are available at IM Certified Partner LearnZillion (requires a district subscription) or without a subscription here (requires registration).

### Activity Synthesis

Select several groups to share their answers and reasoning for each of the problems. If any students made sketches with labeled dimensions, display these for all to see. In particular, contrast students who used volume formulas versus those who remembered that the volume of a cone is half the volume of a sphere with the same dimensions (radius and height).

Consider asking these discussion questions:

• For students who had a Problem Card:
• “How did you decide what information to ask for? How did the information on your card help?”
• “How easy or difficult was it to explain why you needed the information you were asking for?”
• “Give an example of a question that you asked, the clue you received, and how you made use of it.”
• “How many questions did it take for you to be able to solve the problem? What were those questions?”
• “Was anyone able to solve the problem with a different set of questions?”
• For students who had a Data Card:
• “When you asked your partner why they needed a specific piece of information, what kind of explanations did you consider acceptable?”
• “Were you able to tell from their questions what volume question they were trying to answer? If so, how? If not, why might that be?”

## 21.4: The Right Fit

CCSS Standards

• 8.G.C.9

### Activity: 10 minutes

In this activity, students once again consider different figures with given dimensions, this time comparing their capacity to contain a certain amount of water. The goal is for students to not only apply the correct volume formulas, but to slow down and think about how the dimensions of the figures compare and how those measurements affect the volume of the figures.

### Launch

Arrange students in groups of 2. Allow 5 minutes of quiet work time, then a partner discussion followed by a whole-class discussion.

Social-Emotional Functioning: Peer Tutors. Pair students with their previously identified peer tutors.

### Student Facing

A cylinder with diameter 3 centimeters and height 8 centimeters is filled with water. Decide which figures described here, if any, could hold all of the water from the cylinder. Explain your reasoning.

1. Cone with a height of 8 centimeters and a radius of 3 centimeters.
2. Cylinder with a diameter of 6 centimeters and height of 2 centimeters.
3. Rectangular prism with a length of 3 centimeters, width of 4 centimeters, and height of 8 centimeters.
4. Sphere with a radius of 2 centimeters.

### Student Response

Details about student responses to this activity are available at IM Certified Partner LearnZillion (requires a district subscription) or without a subscription here (requires registration).

### Activity Synthesis

The purpose of this discussion is to compare volumes of different figures by computation and also by considering the effect that different dimensions have on volume.

Ask students if they made any predictions about the volumes before directly computing them and, if yes, how they were able to predict. Students might have reasoned, for example, that the second cylinder had double the radius of the first, which would make the volume 4 times as great, but the height was only $\frac14$ as great so the volume would be the same. Or they might have seen that the cone would have a greater volume since the radius was double and the height the same, making the volume (if it were another cylinder) 4 times as great, so the factor of $\frac13$ for the cone didn’t bring the volume down below the volume of the cylinder.

## 21.5: New Four Spheres

CCSS Standards

• 8.G.C.9

### Cool-down: 5 minutes

Students synthesize the material they have learned about using the volume of a sphere formula by sorting different representations of the equation.

### Student Facing

Some information is given about each sphere.  Order them from least volume to greatest volume. You may sketch a sphere to help you visualize if you prefer.

Sphere A: Has a radius of 4

Sphere B: Has a diameter of 6

Sphere C: Has a volume of 64$\pi$

Sphere D: Has a radius double that of sphere B.

### Student Response

Details about student responses to this activity are available at IM Certified Partner LearnZillion (requires a district subscription) or without a subscription here (requires registration).

## Student Lesson Summary

### Student Facing

The formula

$$V=\frac43 \pi r^3$$

gives the volume of a sphere with radius $r$. We can use the formula to find the volume of a sphere with a known radius. For example, if the radius of a sphere is 6 units, then the volume would be

$$\frac{4}{3} \pi (6)^3 = 288\pi$$

or approximately $904$ cubic units. We can also use the formula to find the radius of a sphere if we only know its volume. For example, if we know the volume of a sphere is $36 \pi$ cubic units but we don't know the radius, then this equation is true:

$$36\pi=\frac43\pi r^3$$

That means that $r^3 = 27$, so the radius $r$ has to be 3 units in order for both sides of the equation to have the same value.

Many common objects, from water bottles to buildings to balloons, are similar in shape to rectangular prisms, cylinders, cones, and spheres—or even combinations of these shapes! Using the volume formulas for these shapes allows us to compare the volume of different types of objects, sometimes with surprising results.

For example, a cube-shaped box with side length 3 centimeters holds less than a sphere with radius 2 centimeters because the volume of the cube is 27 cubic centimeters ($3^3 = 27$), and the volume of the sphere is around 33.51 cubic centimeters ($\frac43\pi \boldcdot 2^3 \approx 33.51$).