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velocities, and accelerations so determined are termed absolute. It is
not always possible or convenient, however, to use a fixed set of axes
to describe or to measure motion. In addition, there are many
engineering problems for which the analysis of motion is simplified
by using measurements made with respect to a moving reference
system. These measurements, when combined with the absolute
motion of the moving coordinate system, enable us to determine the
absolute motion in question. This approach is called a relative-motion
analysis.
The motion of the moving coordinate system is specified with
respect to a fixed coordinate system. Strictly speaking, in Newtonian
mechanics, this fixed system is the primary inertial system, which is
assumed to have no motion in space. For engineering purposes, the
fixed system may be taken as any system whose absolute motion is
negligible for the problem at hand. For most earthbound engineering
problems, it is sufficiently precise to take for the fixed reference
system a set of axes attached to the earth, in which case we neglect the
motion of the earth. For the motion of satellites around the earth, a
coordinate system is chosen with its origin on the axis of rotation of
the earth. For interplanetary travel, a coordinate system fixed to the
sun would be used. Thus, the choice of the fixed system depends on
the type of problem involved.
There are many cases, however, where the path of motion for a
particle is complicated, so that it may be easier to analyze the motion
in parts by using two or more frames of reference. For example, the
motion of a particle located at the tip of an airplane propeller, while
the plane is in flight, is more easily described if one observes first the
motion of the airplane from a fixed reference and then superimposes
(vectorial) the circular motion of the particle measured from a
reference attached to the airplane.
Will enter basic concepts and definitions which use at
consideration of relative motion of particle, what is represent in Fig.
2-21.
Absolute motion. The movement of the particle B relation to
the common origin O of the fixed X, Y, Z reference frame is named
absolute motion.

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