Lesson 19

The Volume of a Cone

Lesson Narrative

In this lesson students start working with cones, and learn that the volume of a cone is $$\frac13$$ the volume of a cylinder with a congruent base and the same height. First, students learn a method for quickly sketching a cone, and the meaning of the radius and height of a cone. Then they watch a video (or if possible, a live demonstration) showing that it takes three cones of water to fill a cylinder with the same radius and height. At this point, it is taken as a mysterious and beautiful fact that the volume of a cone is one third the volume of the associated cylinder. A proof of this fact requires mathematics beyond grade level.

Students write the volume of a cone given a specific volume of a cylinder with the same base and height, and vice versa. Then they use the formula for the volume of a cylinder learned in previous lessons to write the general formula $$V= \frac13\pi r^2 h$$ for the volume, $$V$$, of a cone in terms of its height, $$h$$, and radius, $$r$$. Finally, students practice computing the volumes of some cones. There are opportunities for further practice in the next lesson.

Learning Goals

Teacher Facing

• Calculate the volume of a cone and cylinder given the height and radius, and explain (orally) the solution method.
• Compare the volumes of a cone and a cylinder with the same base and height, and explain (orally and in writing) the relationship between the volumes.

Student Facing

Let’s explore cones and their volumes.

Required Preparation

For the Which Has a Larger Volume activity, it is suggested that students have access to geometric solids.

During the From Cylinders to Cones activity, students will need to view a video. Alternatively, do a demonstration with a cone that could be filled with water and poured into a cylinder.

Student Facing

• I can find the volume of a cone in mathematical and real-world situations.
• I know the formula for the volume of a cone.

Glossary Entries

• base (of a prism or pyramid)

The word base can also refer to a face of a polyhedron.

A prism has two identical bases that are parallel. A pyramid has one base.

A prism or pyramid is named for the shape of its base.

• cone

A cone is a three-dimensional figure like a pyramid, but the base is a circle.

• cross section

A cross section is the new face you see when you slice through a three-dimensional figure.

For example, if you slice a rectangular pyramid parallel to the base, you get a smaller rectangle as the cross section.

• cylinder

A cylinder is a three-dimensional figure like a prism, but with bases that are circles.

• prism

A prism is a type of polyhedron that has two bases that are identical copies of each other. The bases are connected by rectangles or parallelograms.

Here are some drawings of prisms.

• pyramid

A pyramid is a type of polyhedron that has one base. All the other faces are triangles, and they all meet at a single vertex.

Here are some drawings of pyramids.

• sphere

A sphere is a three-dimensional figure in which all cross-sections in every direction are circles.

• surface area

The surface area of a polyhedron is the number of square units that covers all the faces of the polyhedron, without any gaps or overlaps.

For example, if the faces of a cube each have an area of 9 cm2, then the surface area of the cube is $$6 \boldcdot 9$$, or 54 cm2.

• volume

Volume is the number of cubic units that fill a three-dimensional region, without any gaps or overlaps.

For example, the volume of this rectangular prism is 60 units3, because it is composed of 3 layers that are each 20 units3.