Classroom discussions of aerodynamic reactions can raise relevant questions, such as why a spinning ball follows a curved flight path, or why an airplane wing provides lift.
Such questions are usually answered by reference to the Bernoulli Theorem and associated assumptions about the acceleration of air on the affected surfaces. This approach is unsatisfactory for three reasons. First, it can lead to questions which cannot be answered. Second, the Bernoulli relationship between fluid velocity and static pressure is applicable to the confined flow of fluids through pipes. Third, the movement of an object through the atmosphere does not result in the acceleration of air around the object.
A more effective approach can be based on the following rationale:
1. Bernoulli demonstrated that an increase in the velocity of a fluid that is flowing through a pipe will result in a decrease in the static pressure of the fluid.
2. The reduction in static pressure occurs because the increase in the velocity of a fluid decreases the time that is available for the molecules of the fluid to impact on the walls of the pipe.
3. It can be shown that whenever a fluid is moving relative to a surface, or a surface is moving relative to a fluid, an increase in relative velocity will result in a decrease in the pressure that the fluid exerts on the surface.
4. The reduction in the pressure that the fluid exerts on the surface will result in the fluid being forced into intimate contact with the surface by the fluid molecues that are further away from the surface.
5. The resultant "adhesion" of fluids to curved surfaces and the reduction in the fluid pressures that are exerted on the surfaces are manifestations of the Coanda effect.
This line of reasoning makes answers to questions such as those raised above almost self evident. The spinning of the ball results in a Coanda type decrease in the air pressure that is acting on the side of the ball that has the higher effective velocity through the atmosphere. Similarly, the movement of an airplane wing through the atmosphere results in a Coanda type reduction in the air pressure that is acting on the top of the wing. In both instances, the result is the development of a pressure differential. In the case of the ball, the differential is between the two affected sides of the ball and an unbalanced sideward force pushes the ball off course. In the case of the wing, the differential is between the upper and lower surfaces of the wing and an unbalanced upward force is applied to the bottom of the wing.
The above rationale is discussed in detail in a new web page:
This web page also:
1. presents examples of the manner in which the Bernoulli/acceleration rationale involves assumptions and presumptions that are obviously invalid,
2. provides background information which supports and augments the Coanda/constant velocity rationale, and
3. gives examples of the misconceptions and fallacies regarding aerodynamic reactions that are perpetuated in articles that appear in publications and on the internet.