Relative Motion

Whenever you measure the velocity of an object, that velocity is always with respect to some reference. Even if there is no explicit reference, you superimpose one everytime you watch an object in motion.

Consider an ice floe moving across your field of view as you look down from some high elevation above a body of water.

This ice floe appears to be moving up and to the right at some speed, say, 2.5 m/s.

Pretending, for the moment, there is any real meaning to the individual symbols (v_Ice and v_Screen), you are seeing the v_Ice with respect to the v_Screen.

Their difference v_Ice - v_Screen describes (v_I,S) the velocity of the floe with respect to your field of view, the screen.

v_I,S = v_Ice - v_Screen

To save space I will write this as,

v_I,S = v_I - v_S.

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