Classifying 3D objects

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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)

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Here are a few examples of 3D shapes learners should already know:

 

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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

Example of curved faces only

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:

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:

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