One of the most confusing concepts for young aerodynamicists is the relative velocity between objects. Aerodynamic forces are generated by an object moving through the air, but the air itself can also move.
Aerodynamic forces depends on the square of the velocity between the object and the air.
To properly define the velocity, it is necessary to pick a fixed reference point and measure velocities relative to the fixed point. In this slide, the reference point is fixed on the ground, but it could just as easily be fixed to the aircraft. The air in which the aircraft flys can move in all three directions. In this figure, we are only considering velocities which occur perpendicular to the flight path but parallel to the ground. The effect of wind along the flight path has been considered in the previous slides.