Bernoulli's Principle: Moving Air Results in Lower Pressure

The Idea

Here is a simple challenge: hang two empty soda cans from a string, separated by a few inches, and then blow them apart. Whether you use your lungs or a hair dryer, the result will be the same.

What You Need

• 2 empty soda cans
• 2 strings
• blow dryer

Method

1. Attach the strings to the can and hang from a support (such as a ring stand).
2. With the blow dryer (or your own breath if you are good at blowing out birthday candles), direct a stream of air between the cans. Avoid blowing so hard that the cans start bouncing around, which will only serve to confuse the issue.
3. Observe what happens (Figure 43-1).

Expected Results

Instead of blowing the cans apart, the air stream will drive them together. In fact, the harder you blow, the greater the force that moves the cans together.

Why It Works

What is going on here was explained by Bernoulli. A key consequence of the principle that bears his name is the faster air moves, the lower the air pressure. This is the mechanism for airplane lift. The contour of the airplane wing directs air above the wing at a higher velocity, resulting in a lower pressure above the wing. A spoiler on a race car does the same thing, but upside down. A sailboat takes advantage of Bernoulli's principle by directing the air in front of the sail at a higher velocity. This produces a greater force on the wing and enables the sailboat to move at a greater velocity than the wind driving it.

Other Things to Try

A stream of moving air from a blow dryer keeps an object, such as a ping-pong ball, suspended in mid air. See Figure 43-2. This is another example of Bernoulli's principle. The faster moving air in the center results in a pressure gradient that draws the ball into the air steam above the blow dryer.

Another way to explore Bernoulli's principle is to take a sheet of paper and hold it horizontally in front of your mouth. The paper will droop down in front of you. Blowing across the top of the paper will reduce the pressure above the paper, causing it to defy gravity and straighten out horizontally.

If you place a dowel in the center of a roll of toilet paper and direct a blow dryer across the top, you can run out the entire roll! If anyone complains about the mess, just say you had to do it to prove Bernoulli's principle. (Then, of course, please clean up the mess.)

The Point

According to Bernoulli's principle, moving air results in lower pressure.

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