Have you ever said of someone who was talking too much, “He’s just an old windbag”? Actually, windbags can be fun (think balloons), and they allow us to learn about an important property of moving air called Bernoulli's Principle.

Bouncing Balloon

For this experiment you will need:

     • an electric hair dryer with circular nozzle
     • a balloon
     • a table tennis ball

First, blow up the balloon and tie off the end. Hold the balloon out at arm’s length and let go of it. Does it stay there, or drop?

Next, hold the balloon above your head at arm’s length, then blow hard at it as you let go. Can you keep the balloon in the air?

Now hold the hair dryer in one hand, turn it on, and point it up toward the ceiling. Place the balloon in the stream of moving air, and let go. Does the balloon fall to the ground or stay up? Why does this happen?

Experiment further with the hair dryer. If you tilt the nozzle slowly a little to one side, does the balloon stay in the air stream? Can you bring the nozzle back to vertical and make the balloon follow? How far can you tilt the nozzle before the balloon falls? What causes the balloon to stay in the stream of moving air?

Finally, try placing a table tennis ball in the air stream created by the hair dryer. Does it float? If your hair dryer is powerful enough, the ball will float in the stream just like the balloon. Can you place both the balloon and the table tennis ball in the air stream at the same time? Which object must be placed on top for them to both remain floating? Why is this the case?

When you first let the balloon go, it sinks because it is denser than the surrounding air. (Why do balloons filled with helium rise?) However, the balloon is just a little denser than air, so you were able to keep it from falling by blowing on the bottom of the balloon, either by mouth or with the hair dryer. The column of moving air pushes on the bottom of the balloon, forcing it to rise. The effect is the same in either case, but the hair dryer doesn’t run out of breath.

But what happened when you tilted the nozzle of the hair dryer? If the balloon were simply held up by the column of air, it would have fallen down when you tilted the nozzle. Instead, the balloon followed the column of air when it moved—so something else must be happening.

The reason the balloon stays in the moving stream of air has to do with Bernoulli's Principle. Bernoulli's Principle says that the pressure decreases inside a stream of flowing air. When the balloon begins to move out of this low pressure stream, the higher pressure of the air in the room pushes it back into the moving stream. When you tilt the nozzle, the pressure in the room is still high enough to push the balloon back into the air stream, even if the moving air stream is at an angle. Eventually, at a large tilt, the force of gravity will become greater than the force of the air pressure holding the balloon in the stream, and the balloon will fall.

Let’s consider an alternate explanation: could it be that the balloon is held up by the heat of the warm air from the hair dryer? This is a reasonable guess, since we know that hot air balloons float. In fact, hot air balloons have been around since 1783. Hot air balloons are able to float high in the air, because when the air inside a hot air balloon is heated, it expands, making it less dense than the surrounding air. This causes the balloon to float up to a higher altitude, where the air is less dense.

How can you test this explanation? If you can, turn off the heat on the hair dryer, and repeat the experiment. Are you able to keep the balloon afloat without the use of hot air? Actually, when you think about it, the heat from the warm dryer makes the air outside the balloon warmer than the air in the balloon. So this explanation isn’t correct.

Wind Tube

For this part of the experiment you will need a WindTube from www.teachersource.com (also available as a Bernoulli Bag from www.arborsci.com).

Stretch out the WindTube, and have someone else hold the closed end while you hold the open end. How many breaths does it take you to inflate the WindTube?

If you inflated the WindTube like most people inflate a balloon or paper bag, then it probably took a lot of breaths. If you use Bernoulli's Principle however, you can inflate the bag with a single breath!

Get all of the air out of the WindTube, and again have someone hold the closed end while you hold the open end. Hold the WindTube about a foot from your face, and hold the open end wide open. Blow one big, long breath (like you are blowing out birthday candles) straight down the middle of the WindTube and close the open end off when you are done. Did it work? How were you able to fill the WindTube with only one breath?

When you blow one breath straight down the middle, your moving breath creates a stream of low-pressure air, which pulls air from the room into the bag with it. Have you ever been on the side of the road when a big bus or truck sped by? You probably felt the rush of air that the vehicle was pulling along with it. Your breath acts in the same way when you blow into the WindTube. It pulls some of the room air with it, so that you are able to fill the entire bag with only one breath.

Daniel Bernoulli was a Swiss physicist who lived from 1700 to 1782 and did pioneering work on the motions of fluids (“hydrodynamics”). A modern application of Bernoulli’s Principle is the shape of airplane wings, which in part generate “lift” by making the air on the top of the wing move faster than the air under the wing.

For more examples of applications of Bernoulli’s Principle, with good explanations, see http://www.nasm.si.edu/exhibitions/gal109/LESSONS/TEXT/TEASERS.HTM