Three-Dimension (3-D) Objects
Geometry · Three-Dimension (3-D) Objects
CBC Grade 5 Mathematics — Three-Dimension (3-D) Objects
Lesson objectives
By the end of this topic, a learner should be able to:
- Describe common 3-D objects found in the environment
- Identify the 2-D shapes that make up the faces of 3-D objects
- Appreciate the use of 3-D objects in everyday life
Three-Dimension (3-D) Objects
A flat shape has length and width. A 3-D object has length, width and height as well. It takes up space, so you can hold it.
a) Flat and solid
A drawing of a square is flat. You cannot pick it up off the page.
A box is solid. It has thickness, and it fills space.
That is the difference between 2-D and 3-D.
b) The parts
A face is a flat surface you can put your hand on.
An edge is where two faces meet.
A vertex is a corner where edges meet. More than one is called vertices.
c) Common 3-D objects
| Object | Looks like |
|---|---|
| Cube | a dice, a sugar cube |
| Cuboid | a matchbox, a brick |
| Cylinder | a tin, a drum |
| Cone | a funnel, an ice cream cone |
| Sphere | a ball, an orange |
| Pyramid | a tent with a square base |
Look around the room. Nearly everything you can pick up is one of these. Some are several joined together.
d) The flat shapes inside
Every 3-D object is built from flat shapes.
A cube has six faces, and every one is a square.
A cuboid has six faces, and they are rectangles.
A cylinder has two circles, one at the top and one at the bottom. A curved surface joins them.
A square pyramid has a square at the bottom. Triangles go up from it to a point.
e) Finding the shapes
Pick up a real object and look at each face in turn.
Put a face flat on the paper and draw round it. The shape you have drawn is one of its faces.
Do that for every face. Then you have all the flat shapes it is made from.
f) Curved objects
A sphere has no flat faces at all. A ball has no corners and no edges.
A cone has one flat circle and one curved surface. It has one point at the top.
You cannot draw round a curved surface, because it will not lie flat.
g) Where this is used
A shopkeeper stacks boxes. A builder lays bricks. A packer fills a carton. A potter shapes clay.
:::checkpoint Check yourself
- What makes an object 3-D rather than 2-D?
- What flat shape makes every face of a cube?
- How many circles does a cylinder have?
- Why has a sphere no edges? :::
Definitions
- 3-D object — a shape with length, width, and height.
- Face — a flat or curved surface of a 3-D object.
- 2-D shape — a flat shape with only length and width.
Must remember
- A cube's faces are all squares; a cuboid's are rectangles.
- A sphere has no flat faces at all.
- A cylinder has flat circular ends but a curved connecting surface.
- Naming the 2-D shapes within a 3-D object helps describe and compare shapes.
Conclusion
Every 3-D object here can be partly understood through the 2-D shapes making up its faces.
Bridge to practice
The exercises ask you to describe solids and name the 2-D shapes in them. Handle a real box or tin if you can; counting faces on a real object is far easier than imagining it.