The Bernoulli principle explains how atomizers work and why windows are sometimes sucked out of their frames as two trains rush past each other. Choose from two versions of this Snack—small or large.
- Index card
- Pushpin or thumbtack
- Large wood or plastic thread spool or a small wooden block with a hole drilled through it
- A stiff paper or plastic plate
- Cardboard box with one side (not the top) that is somewhat larger than the plate
- Hair dryer or vacuum-cleaner blower
- Pushpin or thumbtack
- Trim an index card to a 3 × 3 inch (7.5 × 7.5 centimeter) square.
- Push the pushpin into the card’s center.
- If you are using a plastic thread spool, make sure the paper label is intact except for a small hole in the middle. If you don't have a thread spool, you can use a wooden block with a small hole drilled through it (see images below).
Large version (see diagram below):
- Cut the flaps off the top of the box and turn the box on its side so that the opening faces to one 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 and set this assembly inside the box.
Hold the card against the bottom of the spool or wooden block with the pushpin or thumbtack 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.
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, not suction. If you use a hair dryer, turn the heat off if you can, to avoid the dryer overheating.
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 (click to enlarge diagram below). 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 above 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 air stream. 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 train windows out of their frames.