On occasion objects move within a medium that
is moving with respect to an observer. For example, an airplane usually
encounters a wind - air that is moving with respect to an observer on the
ground below. As another example, a motorboat in a river is moving amidst a
river current - water that is moving with respect to an observer on dry land.
In such instances as this, the magnitude of the velocity of the moving object
(whether it be a plane or a motorboat) with respect to the observer on land
will not be the same as the speedometer reading of the vehicle. That is to say,
the speedometer on the motorboat might read 20 mi/hr; yet the motorboat might
be moving relative to the observer on shore at a speed of 25 mi/hr. Motion is
relative to the observer. The observer on land, often named (or misnamed) the
"stationary observer" would measure the speed to be different than
that of the person on the boat. The observed speed of the boat must always be
described relative to who the observer is.