Cones and cylinders have curved surfaces as shown below. So, they are
not prisms or polyhedra.
Cones
If one end of a line is rotated about a second fixed line while keeping
the line's other end fixed, then a **cone** is formed. The point
about which the line is rotated is called the **vertex **and the base of
the cone is a circle.
A cone is said to be **right** when the vertex is directly above the
centre of the base.
The **net** of a three-dimensional object is a representation of its
faces in two dimensions.
The **net** of a cone consists of the following two parts:
- a circle that gives the base; and
- a sector that gives the curved surface
Cylinders
If a line is rotated about a second fixed line while keeping both lines parallel, then a **cylinder** is formed.
Note:
- The fixed line about which the line is rotated is called the
**axis
of symmetry**.
- The base of the cylinder is a circle and the top of the cylinder is
also a circle.
- A cylinder is said to be
**right** when the line joining the centre
of the base and the centre of the top is perpendicular to the base of
the cylinder.
- The
**cross-sections** parallel to the base are circles and are all
identical.
The **net** of a cylinder consists of three parts:
- One circle gives the base and another circle gives the top.
- A rectangle gives the curved surface.
Key Terms
cones, vertex of a
cone, right cone, nets, net
of a cone, cylinders, cylinder's
axis of symmetry, right cylinder, net
of a cylinder |