An animated version of this slide is also available. One of the most confusing concepts for young aerodynamicists is the relative velocity between objects. A fixed object in a still fluid does not generate aerodynamic forces. To generate lift, an object must move through the air, or air must move past the object.
Aerodynamic lift depends on the square of the velocity between the object and the air.
Now things get confusing because not only can the object be moved through the air, but the air itself can move. 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 to the ground, but it could just as easily be fixed to the aircraft. It is important to understand the relationships of wind speed to ground speed to airspeed. For a reference point picked on the ground, the air moves relative to the reference point at the wind speed .