Geometry of 3D objects
Classifying 3D objects
In the next few lesson we are going to look at different 3D shapes.
- Lesson 1 - Classifying 3D objects
- Lesson 2 - Prisms and pyramids
- Lesson 3 - Building 3D models
- Lesson 4 - 3D nets
3D shapes are also called solids or solid shapes. 3D shapes have 3 dimensions namely length, breadth(width or depth) and height.
2D shapes have 2 dimensions namely the length and breadth (width)
Here are a few examples of 3D shapes learners should already know:
Key words to remember:
- Three dimensional (3D) - 3D objects have a length, width and height.
- Solid - An object that has 3 dimensions
- Face - the flat surface of a solid
- Edge - where two faces of a polyhedron meet
Polyhedron:
A polyhedron is a solid where all the surfaces are flat. Each of the flat surfaces, has the shape of a polygon and is called the face of the shape. These faces meet at an edge and where three faces meet to form a point is called the vertex.
Not all solids have only flat faces. If you think of the cone and a cylinder, they have both flat and curved faces. The sphere is a solid that only has a curved face.
Example of an object with flat faces only
Example of flat and curved faces
We can also calculate the volume for 3D shapes. In grade 7 we only learn the volume of a cube and a rectangular prism.
2 Basic prisms
Cube:
A cube has 6 faces, 8 vertices and 12 edges.
To calculate the volume of a cube with sides 5cm, we can say that the formula is side x side x side or sxsxs or lxwxh
The volume of a cube with side equal to 5cm will be 5 x 5 x 5= 125 cubic cm.
We multiply all 3 dimensions together, that's why the unit of measurement is in cubic units.
Rectangular prism:
A rectangular prism also have 6 flat faces, 8 vertices and 12 edges.
To calculate the volume of rectangular prism we can use the formula:
Volume = l x w x h
= 6 x 3 x 4
= 72 cubic centimetres.
The following activity can be given:
1. Write down the 3D object that would make a good model for each of these items.
- 1. A tennis ball
- 2. A six-sided die
- 3. A door wedge
- 4. A mattress
- 5. The pointed end of a pencil
- 6. A can of fruit juice
- 7. A box of tissues
2. Name the following solids
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