Coordinate systems and reference frames exist for the pleasure and convenience of the
engineer who defines them. In the next chapters we will freely adorn our systems with
multiple coordinate systems as we see fit, to aid in understanding and solving the problem.
We will denote one of these as the global or absolute coordinate system, and the
others will be local coordinate systems within the global framework. The global system
is often taken to be attached to Mother Earth, though it could as well be attached to
another ground plane such as the frame of an automobile. If our goal is to analyze the
motion of a windshield wiper blade, we may not care to include the gross motion of the
automobile in the analysis. In that case a global coordinate system attached to the car
would be useful, and we could consider it to be an absolute coordinate system. Even if
we use the earth as an absolute reference frame, we must realize that it is not stationary
either, and as such is not very useful as a reference frame for a space probe. Though we
will speak of absolute positions, velocities, and accelerations, keep in mind that ultimately,
until we discover some stationary point in the universe, all motions are really relative.
The term inertial reference frame is used to denote a system which itself has no
acceleration. All angles in this text will be measured according to the right-hand rule.
That is, counterclockwise angles, angular velocities, and angular accelerations are positive
in sign.