Bernoulli Levitator
Suspend an object in the air by blowing down on it
The Bernoulli principle explains how atomizers work and why windows are sometimes sucked out of their frames as two trains rush past each other. You can choose from two versions of this Snack-- small or large.
Small Snack
- A large wood or plastic thread spool
- An index card
- A pushpin
- Optional: Drinking straws
Large Snack
- A hair dryer or vacuum-cleaner blower
- A stiff paper or plastic plate
- A cardboard box with one side somewhat larger than the plate
- A pushpin
Small Snack
(5 minutes or less)Trim an index card to a 3 x 3 inch (7.5 x 7.5 cm) square. Push the pushpin into the card's center.
If more than one person is going to use this, construct the following sanitary version: Cut a 2 inch (5 cm) long piece of straw for each person. At each person's turn, have him or her push one end of the straw into the hole in the spool of thread. If any straw does not fit, cut a 1/2 inch (6.25 mm) slit near the end of the straw and push it into the spool.
Large Snack
(5 minutes or less)Cut the flaps off the top of the box, and turn the box so that the opening faces to the side. Put the side of the box that is larger than the plate on top, and cut a hole in the center slightly smaller than the outlet of the hair dryer or vacuum hose. Stick a pushpin through the center of the plate.
Small Snack
(5 minutes or more)Hold the card against the bottom of the spool with the pushpin sticking into the spool's hole. The pushpin keeps the card from drifting off to the side.Blow strongly through the hole in the top of the spool and let go of the card. If the card falls at first, experiment with different sized cards or spools until you can make the card hang suspended beneath the spool.
Large Snack
Turn on the blower and direct it down through the hole. If you use a vacuum cleaner, be sure to use it as a blower. If you use a hair dryer, turn the heat off if you can. (If you can't, the hair dryer may overheat and automatically turn off. It will work again as soon as it cools down.)Bring the plate up toward the hole from below. Contrary to what you might expect, as the plate approaches the hole it will be sucked up and held in place by the air blowing down. The pushpin should keep the plate from drifting off to the side.
When you blow into the spool or through the box, the air goes through the opening, hits the card or plate, and accelerates outward. The energy needed to accelerate the air comes from the energy stored as compression of the gas, so the gas expands, and its pressure drops.
As air (or any other fluid) accelerates, its pressure drops. This is known as the Bernoulli principle. In the small version of this Snack, the air rushing between the spool and the card exerts less pressure on the card than the still air underneath the card. The still air pushes the card toward the spool and holds the card up against gravity. In the larger version, the same principle is at work, holding the plate up against the hole in the box.
In an atomizer, or perfume sprayer, you squeeze a rubber bulb to squirt air through a tube. Because of the Bernoulli principle, the air rushing through the tube has a lower pressure than the surrounding atmosphere. Atmospheric pressure forces the perfume up an intersecting tube into the low-pressure airstream. The perfume is pushed out of the tube and sprays into the air as a fine mist.
The air rushing through the space between two moving trains also has a lower pressure, due to the Bernoulli principle. Sometimes, the higher pressure, stationary air inside each train forces some of the trains' windows out of their frames.
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