In Fig. 1.13, the point of view is assumed to be that of an observer fixed with respect to the atmosphere with the vehicle moving past with speed V = |V| in the direction of its velocity vector V. It often is simpler for an observer to be moving with the vehicle. Then, the airflow relative to the vehicle is in the direction – V at a sufficiently great distance upstream of the body—that is, far enough upstream that the effect of the presence of the vehicle has not yet affected the relative flow of the air particles. This change in point of view is useful and often referred to as a Galilean Transformation. As long as there are no acceleration effects present (i. e., no vehicle acceleration relative to the airmass or angular motion about the mass center), then the force system on the body can be taken to be independent of the choice of coordinate frame. This is a great convenience in aerodynamic modeling because it is often the case that the flow problem is best described in terms of the gas motion relative to the body.