Bernoulli Bags

A set of long and lightweight tube-shaped bags, open on both ends. Approximately a foot in diameter and eight feet long.

Demonstrate how Bernoulli's principle may be used to use a small volume of fast-moving air to move a much larger volume of air.

• Bernoulli's wind bags

Tie one end of the bag and have a helper hold the closed end. With the bag now held horizontally, you can show two ways to inflate the bag:

• If you try inflating your bag by pressing the open end to your mouth and blowing, you'll find that it works… But it'd take a long time and many breaths (10-20 or more, depending on your lung capacity) to fill the bag. You can show this by breathing into the bag once, and then holding the closed end shut and squeezing the air down the tube until it's concentrated in one place. The single breath will only be able to fill a small part of the bag.
• Now, empty the air out of the bag. Hold the open end so that it's open widely, and (with your mouth about a foot away from the opening) breath out vigorously into the opening. The entire bag should now fill with air. You can then close the opening and squeeze the air down into the end to illustrate how much more air entered the bag this time.

• These over-sized grocery bags demonstrate Bernoulli's principle quite nicely. It should be intuitive for most students, that by blowing into a plastic grocery bag they can open it much quicker than if they tried to blow the bag up like a balloon.
• The reason for this being that by blowing into the bags, the air velocity causes a decrease in pressure, which sucks air into the bag.