Relative motion is the calculation of the motion of an object with
regard to some other moving or stationary object. Thus, the motion is not calculated
with reference to the earth, but is the velocity of the object in
reference to the other moving object as if it were in a static state.
For example, a person sitting in an airplane is at zero velocity
relative to the airplane, but is moving at the same velocity as the
airplane with respect to the ground. Relative motion is a concept, and
its calculation occurs with relative velocity, relative speed, or
relative acceleration.