Windbag Wonders

Master Bernoulli's principle and expand the bag with a single breath using air pressure.

Here’s the challenge … How many breaths would it take to blow up a 8-foot-long bag? Depending on the size of the person, it may take anywhere from ten to fifty breaths of air. At the end of the challenge, the person is totally out of breath, wondering why she said yes in the first place. Now imagine the look on her face when you are able to inflate the giant bag using only one breath of air. That’s right … one breath and you win! This is one of my all-time favorite science demonstrations, and it’s guaranteed to make it into your Top Ten list.

Experiment Materials

Experiment Videos

Experiment

1

If you’re using one of the Diaper Genie bags, cut off a section of the plastic tube material that is roughly 6 to 8 feet long. A shorter section of bag (4 to 5 feet long) is recommended for younger kid-scientists.

2

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, squeeze all of the air out of the bag. Explain to your friend that you can blow up the bag in one breath. Chances are, he or she won’t believe you, but that’s all part of the surprise.

3

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. Make the opening as wide as you can with the index fingers and thumbs of both hands. Using only one breath, blow a long, steady stream of air into the bag (just as if you were blowing out candles on a birthday cake). You MUST keep your mouth off of the bag (about 10 inches away from the opening) and keep the opening of the bag as large as possible. As you’ll soon see, the secret is actually in the open space between your mouth and the bag.

4

If you do it correctly, you’ll see the bag rapidly inflate. The trick is to quickly seal the bag with your hand so that none of the air escapes. Tie a slipknot in the end of the bag or let the air out and try again.

How Does It Work

Here’s the quick and simple answer. The long bag quickly inflates because air from the atmosphere is drawn into the bag along with the stream of air from your lungs.

For you science enthusiasts out there, here’s a more technical explanation. In 1738, Daniel Bernoulli concluded 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 drops around the moving air. 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 moving into the bag 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 it.

The Ultimate Windbag Challenge

Announce to your audience that they have 5 minutes to work together to build the largest freestanding Windbag structure they possibly can. The structure must be held up only by the Windbags themselves—no one can physically hold up the structure. Note: It would help to do this activity in a gym, a large ballroom, or outside.

Here’s a tip: loop two rubber bands together to form a “figure eight.” Now hook two Windbags together by slipping the rubber bands over the tied ends of two inflated Windbags. Use more rubber band “figure eights” to connect multiple Windbags and create all kinds of creative structures. It’s a great team-building activity for kids and adults alike.

Real-World Application

Firefighters use Bernoulli’s Principle to quickly and efficiently force smoke out of a building. Instead of placing fans up against a doorway or window, a small space is left between the opening of the building and the fan in order to force a greater amount of air through the building. Firefighters call this “positive air flow.”

Guinness World Record

Steve used Windbags to demonstrate the power of air at the first annual 9News Weather and Science Day at Coors Field in Denver, Colorado on May 7, 2009. As part of Weather and Science Day, Steve Spangler Science was awarded the Guinness World Record for the Largest Physics Lesson, with 5,401 participants using their own Windbags to perform an independent physics activity. Participants had 2 minutes to inflate their Windbags, and the news helicopter hovering above the stadium captured the colorful scene. Danny Girton Jr., official adjudicator for Guinness World Records, was on hand to verify the record-breaking event and presented Steve Spangler and his team with an official Guinness World Record certificate at the close of the day.

Note

The “Windbag” is actually a long plastic bag in the shape of a tube. SteveSpanglerScience. com is your source to purchase the brightly colored Windbags pictured throughout the pages of this activity. There is a real-world version of a Windbag at your local department store. Head to the aisle where baby products are sold and look for a diaper disposal system (commonly referred to as a Diaper Genie). The long plastic bags are sold as refills for the diaper disposal system, and they work very well for this demonstration.

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