Most classrooms are built in the shape of a rectangular prism. You will probably find yourself inside a polyhedron at school!

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1 3 D OBJECTS Properties of 3 D Objects A 3 Dimensional object (3 D) is a solid object that has 3 dimensions, i.e. length, width and height. They take up space. For example, a box has three dimensions, i.e. length, width and height. Any 3 D object made from polygons is called a polyhedron. A polyhedron is made from planes which have flat surfaces. We say one polyhedron, but two or more polyhedra. They have faces, edges and vertices. Most classrooms are built in the shape of a rectangular prism. You will probably find yourself inside a polyhedron at school! A can is an example of a solid which is not a polyhedron because it has curved surfaces. A polyhedron has flat surfaces. There are 3 main points on a 3 D Shape: Vertex (vertices) this is the corner of the shape. It is where the sides or straight edges meet. It is the point where 3 or more edges meet. Face this is the flat surface of the shape, between the edges. It is the side of the shape. Version 1: November 2013 Copyright My Cyberwall

2 Edge this is the straight line where the two faces meet. An edge can be curved or straight. However, a sphere is the exception to this, as it has no edges or vertices. It is a round shape like a ball. Version 1: November 2013 Copyright My Cyberwall

3 Types of 3 D Objects In Grade 4 and 5, we studied the following 3 D objects. We will recap on these and learn more about an octahedron and a tetrahedron. Object Example Shape of Faces Faces Vertices Edges Cube Squares Rectangular prism Squares and rectangles Triangular prism Triangles and rectangles Cylinder Circular and round Sphere Circular Pyramid Triangles and square Version 1: November 2013 Copyright My Cyberwall

4 Object Example Shape of Faces Faces Vertices Edges Cone Circle, curved surfaces Octahedron Triangles Tetrahedron Triangles Remember that pyramids get their names from the polygons that form their bases. So the base of a rectangular pyramid is a rectangle and the base of a square pyramid is a square. Octahedron plural octahedra, is a solid shape that has 8 flat faces. A regular octahedron is one that has 8 equilateral triangle faces. Version 1: November 2013 Copyright My Cyberwall

5 Tetrahedron plural tetrahedra is a solid shape with four sides. All four sides are triangular in shape. The regular tetrahedron has faces that are equilateral triangles. Comparing a Tetrahedron and a Pyramid Let's look at the similarities and differences between a tetrahedron and a pyramid. Tetrahedron Rectangular Pyramid Number of faces 4 5 Shape of faces Triangles 1 rectangle 4 triangles Vertices 4 5 Edges 6 8 Shape of Base Triangular Rectangular Version 1: November 2013 Copyright My Cyberwall

6 Making 3 D Models and Nets We can make models of prisms and pyramids using straws and plasticine. The straws will represent the edges of the prisms and pyramids and the balls of plasticine are the vertices of the prisms and pyramids. Example We will make a model of a rectangular pyramid from straws and plasticine. Step 1 make the base by pushing the ends of 4 short straws of equal length, into 4 balls of plasticine. Step 2 push the 4 long straws into each ball of plasticine on the base. Step 3 complete your pyramid by joining the 4 long straws together with a ball of plasticine at the top. Step 1 Step 2 Step 3 Version 1: November 2013 Copyright My Cyberwall

7 Nets A net is a flat shape that we can fold up to make a solid object. Some objects have several different nets. A cube is an example of a shape that has several different nets. A net shows us what the object looks like once it has been opened up and flattened out. The following diagram represents the net of a rectangular pyramid shown. We will now make a triangular prism using a net. To make this triangular prism, follow the instructions below: Step 1 First copy the net onto squared grid paper. Step 2 Using a pair of scissors, cut along the solid lines. Step 3 Now fold along the dotted lines. Step 4 Finally glue the shaded parts and stick them to the edges, matching the letter. Version 1: November 2013 Copyright My Cyberwall

8 3 D Objects in Our Environment There are many examples of 3 D objects in our environment, for example, a loaf of bread, a tissue box and a brick are all the shape of a rectangular prism. Blocks of flats and offices are often constructed in the shape of huge rectangular prisms or even hexagonal prisms. Many South Africans live in round houses (cylindrical form), with roofs in the shape of cones. Containers in the shape of cubes or rectangular prisms are used for transporting goods. Can you identify 3 D objects in your environment? Version 1: November 2013 Copyright My Cyberwall