Pyramids are polyhedra because they are made up of plane faces. Spheres are not polyhedra because they are curved.
Pyramids
If a line that is fixed at a point, called the vertex, is moved
around the perimeter of a polygon, then a pyramid is formed.
The following solid is pyramid.
Pyramids are named after their base. So, the pyramid shown above is
an example of a rectangular pyramid.
Some other pyramids are shown below.
A pyramid is said to be right when its vertex is directly above
the centre of the base. The cross-sections parallel to the base
are the same shape but have different sizes.
The net of the above pyramid consists of:
- a rectangular base; and
- four triangular faces
Note:
- The cross-sections parallel to the base have the same shape as
the base but different sizes.
- The net of a pyramid consists of a base polygon and a number of
triangular faces.
- A regular pyramid has a base that is a regular polygon and has faces which are isosceles triangles.
Spheres
If a semicircle is revolved about its diameter, then a sphere is
formed.
The following solid is a sphere.
The cross-sections of a sphere are circles. The cross-sections that
pass through the centre of the sphere are called great circles whilst
other cross-sections are called small circles.
A hemisphere is a half sphere.
Key Terms
pyramid, vertex of a pyramid, base of a pyramid, rectangular pyramid, triangular pyramid, hexagonal
pyramid, octagonal pyramid, right
pyramid, net of a pyramid, regular
pyramid, sphere, centre of a
sphere, great circles of a sphere, small
circles of a sphere, hemisphere |
