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**Definition**: Rigid Body

A **rigid body** is one that does not deform during its motion.
The
distance between any two points in the rigid body remains
fixed.

**Idea: **Angles to Describe Rotation

In order to completely describe the rotation of a rigid body about
a fixed axis,
*O* , it is sufficient to give the angle, , between the
position vector of any point in the rigid body (for example the
point *P* in the Fig. 7.1), and some arbitrary, fixed
reference line.

**Definition**: Radian

The angle expressed in **radians** is defined as the
ratio of
the arc length, *s* , swept out by the angle, to the radius, *r* ,
of the corresponding circle:

radians = .
| (1) |

**Note: **For a complete revolution, *s* = 2*r* is the circumference,
so that the conversions between revolutions, radians and degrees
are given by:

1 revolution = 2 radians = 360 degrees.
| (2) |

1 radian = 360/2 degrees = 57.3 degrees.
| (3) |

10/9/1997