3
Faces and edges
Learner’s Book 1: Unit 10, pp. 143–144
GoalCount the faces and edges of cubes, cuboids and pyramids, and know which shapes stack.
New words
face · a flat side of a 3D shapeedge · where two faces meetbase · the face a shape stands on
Explanation
A face is a flat side. An edge is where two faces meet. A cube has 6 faces and 12 edges. A cuboid also has 6 faces and 12 edges. A pyramid with a square base has 5 faces: 1 square and 4 triangles. It has 8 edges. Other pyramids can have a different base, such as a triangle. Shapes with flat faces stack well. A sphere has no flat face, so it would roll off.
| cube | cuboid | square pyramid | |
|---|---|---|---|
| faces | 6 | 6 | 5 |
| edges | 12 | 12 | 8 |
Worked examples
Count the faces of a cube: top, bottom, front, back, left and right. That is 6 faces.
A pyramid with a square base has 1 square face and 4 triangle faces. 1 and 4 make 5 faces.
A cube on top of a box is steady. But a sphere at the bottom of a tower would roll away, because it has no flat face.
Class activity
Build and count: take a box (for example a tissue box). Count its faces by touching each one. Then run your finger along every edge and count them. Put a small sticker on each edge you count so you do not count one twice.
Practice
1
How many faces does a cuboid have?
6
2
How many edges does a cube have?
12
3
A pyramid has a square base. How many faces does it have in all?
5
4
You want to build a tower. Should a sphere or a cube go at the bottom? Why?
The cube. It has flat faces so it stays still. The sphere rolls.