Air Pressure Curriculum

Section 3—Lesson 9: Relational Causality and Bernoulli's Principle

Background Information

Using Relational Causality to Explain Flight

Students have observed planes overhead, and many have traveled in them, yet most of them have little understanding of how a plane can fly. This is not surprising. It took centuries for humans to achieve flight. Daniel Bernoulli discovered in the 1700's that the pressure in a fluid decreases as the speed of the fluid increases in a steady flow. It follows that faster moving air exerts less pressure than slower moving air. Bernoulli's principle applies to fluids moving in a steady flow. However, if the flow speed is too fast, the flow can become turbulent and Bernoulli's principle no longer holds.

Air has to travel farther over curved surfaces than straight surfaces. Given a flat surface and a curved surface that connect the same two points, air traveling over the curved surface will go a longer distance than air traveling over the flat surface. In order to compensate for this longer distance, the air traveling over the curved surface must move faster. That is why airplane wings have curved surfaces on top and straight surfaces on the bottom. The faster moving air on the top exerts less pressure than the slower moving air on the bottom, and the wing moves up. This causes lift. Lift is the net upward force that results from the differential between downward and upward pressures on the wing.

Bernoulli's Principle is Counter-intuitive

Bernoulli's principle is counter-intuitive for most people. The cognitive difficulties appear to relate to how people think about the wind. When we think of faster moving air, we tend to picture winds as they behave in a hurricane. When reasoning about wind, students often confuse what causes wind with the effects of wind. Winds are caused by air in areas of higher pressure moving towards areas of lower pressure. Pressure is omni-directional. However, the wind that results when air moves from areas of higher pressure to areas of lower pressure does have a direction, is forceful, and results in the push that we experience if we are in the path between the higher and lower areas of air pressure. Most students have strong experiential knowledge of the effects of the wind. They tend to carry clear images of the high-speed winds in hurricanes as forceful and powerful. Merely telling students that wind is caused by air moving from areas of higher and lower pressure and that pressure is not forceful, or that faster moving air exerts less pressure than slower moving air, will not convince them to see it differently. Engaging students in discussion to help them to see the role of pressure (in causing winds) and of force (as the effect of the direction of the wind) in the equation, and to differentiate between the two, will help them to see why the concepts are implausible to them. This, in turn, can help them to reason about Bernoulli's principle. When thinking about the lower pressure in faster moving air, encourage students to focus on the pressure exerted in all directions rather than the force exerted in the direction of the movement of the air.

As difficult as it is to understand, Bernoulli's principle is key to understanding many pressure-related events that occur everyday. These include weather phenomena such as hurricanes, tornadoes, and gales, as well as understanding car design, a curve-pitched baseball, and why the shower curtain often presses against your legs when you take a hot shower.

In this lesson, students will complete several activities to lead them to understand the effects of moving air on various-shaped surfaces. Lift will then be explored as a relational causal model, in that lift is made possible by the relationship or difference between the pressure on the upper and lower wing. Without this relationship, lift would not be possible.