Windbag - The Bernoulli Bag How many breaths does it take to fill an 8 foot bag? email to friend
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Here's the challenge: How many breaths would it take to blow up an eight foot long bag? Depending on the size of the person, it may take anywhere from 10 to 50 breaths of air. However, with a little practice you will be able to inflate the bag using only one breath!

Materials

The "bag" is actually a long plastic bag in the shape of a tube. While you can purchase "Windbags" from toy stores that sell science experiments, you can make your own long bag using a product called a Diaper Genie refill. It's part of a diaper system that parents use to store diapers. A Diaper Genie refill is commonly available at any major department store.

Experiment
Video

Tie a knot in one end of the bag. Invite a friend to blow up the bag, keeping track of the number of breaths it takes. Then, let all of the air out of the bag. Explain to your friend that you can blow up the bag in one breath.
Have your friend assist you by holding onto the closed end of the bag. Hold the open end of the bag approximately 10 inches away from your mouth. Using only one breath, blow as hard as you can into the bag. Remember to stay about 10 inches away from the bag when you blow.
Quickly seal the bag with your hand so that none of the air escapes. Tie a slip knot in the end of the bag, or let the air out and try again.

How does it work?

The long bag quickly inflates because air from the atmosphere is drawn into the bag from the sides along with the stream of air from your lungs. Here's the technical explanation: In 1738, Daniel Bernoulli observed that a fast moving stream of air is surrounded by an area of low atmospheric pressure. In fact, the faster the stream of air moves, the more the air pressure around the moving air drops. When you blow into the bag, higher pressure air in the atmosphere forces its way into the area of low pressure created by the stream of air from your lungs. In other words, air in the atmosphere is drawn into the long bag at the same time that you are blowing into the bag.

Additional Info

Fire fighters use this principle to quickly and efficiently force smoke out of a building. Instead of placing the fans up against the doorway or window, a small space is left between the opening and the fan in order to force a greater amount of air into the building. Fire fighters call it "Positive Air Flow."