Wednesday, May 24, 2017

Air Power

Our choice of “Air Power” for this post evokes multiple images. Are we talking about  military strategy, an aviation technology, or a cleverly named industry? Few search engines offer air power as a synonym for air pressure. When we deal with multiple aspects of the topic of air pressure, however, we suggest power is an appropriate alternate term. Air pressure is a dynamic, powerful force to be reckoned with in many aspects of human experience. Some do not understand the causes and effects of air pressure. The current post investigates the possibility of “Air Power” as a synonym for air pressure. 

Events in our world are explained with a healthy measure of questioning and investigation. Explanations rely on scientific method for answers to our questions. Natural curiosity of children and adults is not age limited. Answers are provided by experiments performed in the science classroom and in everyday life. Solid learning is provided by skillfully presented demonstrations and the help of the instructor to explain difficult or mysterious results.

The power or pressure of invisible air surrounding us provides fascination and wonder. In a physics classroom instructors may stress definitions and distinctions for terms they use. (One reference book listed 14 different definitions for power and a similar number of definitions for pressure—an illustration of the richness and sometimes challenging aspect of the English language.) Use of “power” in this context relates to the total output of energy available. In this sense, the “power” of air is almost unlimited on a planet such as Earth enveloped by an air blanket. In contrast, “pressure” could relate to an effect experienced at a specific location. For example, in describing the results of our “crushed can” experiment (4-30-17 post) we focused on the effect of air “pressure” at only one location—the spot where our can demonstration took place. The “crushed can” experiment, however, could be performed anywhere and everywhere on the Earth simultaneously.

A famous effect related to the pressure and power of air was discovered by Daniel Bernoulli (1700-1782), a mathematician and scientist. He reported on the phenomena that air pressure in fluids such as air is reduced when the fluid is in motion. The effect he described is known today as Bernoulli’s Principle. Bernoulli explained that air always flows from a region of higher to lower air pressure. We first relate several simple lab experiments or classroom discussions related to the principle.

In keeping with our preference for somewhat spectacular demonstrations, we describe several which seem to contradict common sense. We took an old fashioned thread spool with an open shaft through the middle and placed a piece of oak tag on the open end. After sticking a straight pin through the paper and into the spool to stabilize the cardboard, we forced our breath into the spool from the opposite end. Without an air flow gravity causes the cardboard to fall to the floor. When the air flow began, the oak tag defied gravity and remained on the spool. When we blew harder, the stiff paper oak tag adhered to the spool even more tightly. When the student stopped blowing into the spool, gravity took over and pulled the cardboard to the floor. 

A ping pong ball remained suspended indefinitely in a vertical stream of air above a hair dryer. The rapid air flow from the hair dryer possessed less air pressure compared with the motionless air outside. Higher pressure flowing from still air to moving air kept the ball within the moving, lower pressure air. A dangerous application of Bernoulli’s Principle occurs in city subway systems. Subway trains rushing by at high speed creates a moving air flow. Air pressure behind the passengers is higher in the motionless air. Therefore, one feels “pushed” toward the moving train. “Stay behind the line” is a multi-purpose safety warning. For a similar reason, drivers on interstate highways feel themselves being pulled toward a large truck if it passes closely at high speed.

Finally, our last example comes from modern aviation. Airplane wings are constructed with a slight curve over the top but are flat on the bottom. On takeoff, when sufficient air speed is achieved, the pilot pulls back on his flight control stick in order to begin the ascent. According to Bernoulli’s Principle, faster moving air exerts less pressure than slower moving air. Over the top of the wing the air is forced to travel a little faster than air at the bottom in order to rejoin as the wing slices through the air. Therefore, airplanes are “lifted” into the air by greater air pressure acting on the bottom of the wing. Many other complex issues are involved in the science of aeronautics. The principle described by Bernoulli was posed long before airplanes became a reality. I link the following post from 5-20-15 for our readers’ enjoyment. Bernoulli articulated a principle that may have preserved my life in 1952:

The phenomenon of air power or air pressure supplies a spiritual object lesson. The power of air pressure is available over the entire area of Earth. At any moment in time, the power of air and air pressure is virtually limitless. The God of Creation has designed hundreds of systems by which humanity and all other living things enjoy a rich physical existence. God also makes spiritual power available. His physical and spiritual power is limitless and independent of the constraints of time, space, matter, and energy in which we exist. God created our dimensions of reality, but he exists in a realm beyond our dimensions as well as within our temporal realm.