Lesson Two: What is the Bernoulli principle?
Return to Mini-Course: How do planes fly?
Before we get to know what the Bernoulli's Principle is, let us try two interesting experiments. Before you conduct each experiment, think about what would hapen. After you have finished the experiments, compare the actual results with your original thoughts. Think about why.
Activity1
Follow the steps below:
Step1:Hold two pieces of thin paper vertically a short distance apart.
Step2:Blow down into the space between them.
Step3:Write down your findings and think about why.
Experient result
It seems amazing that the two sheets of paper move closer together.
Activity2
Follow the steps below:
Step1:Hold one end of a small sheet of paper in both hands. The best type of papers to use are the thin sheets used in magazines.
Step2:Keep the held edge horizontal while the other end sags under its own weight.
Step3:Blow steadily over the top of this horizontal edge.
Step4:Does the lower end of paper go down or up? Think about why.
Experient result
It is hard to believe that the lower end of the paper moves upwards slowly.
The Bernoulli principle
Bernoulli's principle, sometimes known as Bernoulli's equation, holds that for fluids in an ideal state, pressure and density are inversely related: in other words, a slow-moving fluid exerts more pressure than a fast-moving fluid. Since "fluid" in this context applies equally to liquids and gases, the principle has as many applications with regard to airflow as to the flow of liquids.
In both activities, pressure is reduced where air is moving quickly.
See more Bernoulli's principle experiments: https://www.youtube.com/watch?v=sIrJOrTAJjg
See more Bernoulli's principle definition: https://en.wikipedia.org/wiki/Bernoulli%27s_principle
Thinking
Now think about how airplanes get lift in the air? Does the air up the plane move faster than that under the plane? How does it work?
Discuss your ideas with your classmates in the discussion forum.
References
http://www.nuffieldfoundation.org/practical-physics/bernoulli-experiments-sheets-paper