... A vector , that's what. ... Graphically, each stretch on the ground or velocity can be represented by an arrow in its direction, its length giving the magnitude--for instance, an arrow AB 200 mm (milimeters) long for the airplane's motion and another one BC 40 mm long, in the same direction, for the wind. ... Just as one can combine two vectors into one--their sum--so it is also possible to move in the opposite direction--given a single vector, find two vectors whose sum it is. ...