To help me clarify my title's question, let me quote the text from Fundamentals of Aerodynamics, 5th Ed., by Anderson:


“the aerodynamic drag on a vehicle is the sum of drag due to the pressure distribution exerted over its surface, called pressure drag Dp, and the drag due to shear stress exerted over its surface, called skin friction drag Df.”


“The net result is the production of drag; this is called the pressure drag due to flow separation and is denoted by Dp. In summary, we see that the effects of viscosity are to produce two types of drag as follows:

Df is the skin friction drag, that is, the component in the drag direction of the integral of the shear stress τ over the body.

Dp is the pressure drag due to separation, that is, the component in the drag direction of the integral of the pressure distribution over the body.

Dp is sometimes called form drag. The sum Df +Dp is called the profile drag of a two-dimensional body. For a three-dimensional body such as a complete airplane, the sum Df + Dp is frequently called parasite drag.”

To me, "drag" is a term that is synonymous with "force". Perhaps the term "drag" is used because the fluid force tends to "drag" the solid body in the same direction as the force vector the fluid imparts on the solid?

Now to my main question. These forces, the skin friction and form drags, are the forces the fluid imparts on the solid, and their orientation is identical to the orientation of the (bulk) fluid velocity vector. From Newtons 3rd law, $\vec F_{12} = -\vec F_{21}$, there are reactionary forces that a fixed (in space & time) body would impart on the fluid. How are these forces referred? I.e., what names are given to these forces?


1 Answer 1


Drag itself is a reactionary force - reactionary to the object moving through a medium. Newton's 1st law states that an object (in this case air) will remain at rest or in uniform motion in a straight line unless acted upon by an external force. As the air and a moving body collide, the body slows down and the air starts moving. The air will have to be moved out of the way by some force and the reactionary force is experienced by the moving body as drag due to Newton's 3rd law.

As I understand, the title's question is already answered in one of your quotes: "Dp is sometimes called form drag."


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