Explaining the Phenomenon
In an earlier chapter, In Dying you Live, I demonstrated that you could place a disk against a leaf blower and it will be "sucked" up against the blower. This chapter will attempt to explain how this phenomenon works. (The following chapter, Setting up the Demonstration, tells you how to set up the demonstration.) The Smithsonian Aerospace Museum (Washington D.C.) has a similar exhibit to demonstrate how air flowing over a plane’s wing causes it to rise. The following is the explanation they provided:
Personally, I don’t feel this does a good job explaining why this happens. For those who have studied physics, I think this phenomenon has something to do with the Bernoulli’s Principle. This principle states, in part, "As the speed of a fluid increases, the pressure decreases." In other words, fast moving air exerts less pressure than slow moving air. The reason this happens is, "More energy is used up as the molecules accelerate. This leaves less energy to exert pressure, and the pressure thus decreases." (If this is "clear as mud," please read on.)
We often see this happen without even realizing it. When you first turn on the shower, the shower curtain gets "sucked in" toward the spraying water. When a car drives fast down a street, the leaves that are sitting on the side of the road get sucked in behind it. In a sense, the car creates a vacuum in the surrounding area.
You can try this principle out for yourself. Cut a piece of paper that is about 2 by 6 inches long. Hold the narrow end next to your lower lip with the other end hanging down. Blow across the top of the paper and you’ll find that the paper rises up. The fast moving air on top of the paper creates a lower pressure zone compared to the slow moving air on the bottom of the paper. The low pressure on top pulls the paper up.
It is this principle (Bernoulli’s principle) that lifts airplanes up in the air. As air flows over a wing at great speeds, it creates a "low pressure" zone above the wing (compared to the air pressure below the wing). It is the low pressure (or vacuum) on top of the wing that pulls the plane up. Although I won’t bore you with the details, the wings are designed with some special features that enhance this process.
If you were to blow fast moving air across the top of the disk, the low pressure on top of the disk would cause it to rise up. At this time you may be saying, "Okay, I understand that to a point, but how can the disk rise up when you have so much pressure blowing straight down on it?" Before I explain that, take a look at the diagrams below.
When the disk is brought next to the platform, all of the air is flowing sideways. This sideways moving air is moving incredibly fast and as a result, it is creating a very low-pressure zone. This low-pressure zone is so powerful that the upward pressure is larger than the downward pressure. Thus, the disk rises. See the diagram below.
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