A wind bag is like a balloon without any elastic qualities. It’s made out of material that is similar to a plastic grocery bag. It is eight feet long and ten inches in diameter. How many breaths would it take to blow up a bag of this size? 10... 20... 30... more? With a little practice you will be able to blow up the bag in one breath. Try these two exciting activities with your wind bags.
- Bernoulli’s principle
- Air pressure
Big breath of air
Tables, identical, flat, 4–6 feet in length, 2
Wind bags, 4
Warn students to be careful when turning, lifting and lowering tables. Warn them to not get their fingers caught between the tables. Follow all normal laboratory guidelines. Have at least one adult volunteer near the table in case the tap table starts to move.
- Ask for a volunteer. Have the volunteer try to blow up the wind bag keeping track of the number of breaths it takes. In general, it takes anywhere from 10 to 40 breaths to fully inflate the bag.
- Let all the air out of the bag. Explain to your students that you can blow up the bag using only one breath! Of course this challenge seems impossible.
- Ask someone to 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 a sharp burst of air 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. Twirl the bag to seal it. With a little practice, you will be able to inflate the entire wind bag using only one breath. How does it work?
- Spread four wind bags out flat on one table. Have two openings at one end and two openings at the other end.
- Ask for four student volunteers to lift a heavy object. Pick the strongest students in the class to lift a second table and place it upside-down on top of the first table and the wind bags. Assign a student to each leg of the second table. Caution the students to be careful while turning, lifting, and lowering the second table onto the first. Warn them not to get their fingers caught between the two tables.
- Select two of the largest student volunteers to sit on top of the second table. Ask the class, (1) “Is there any way to lift up these two students?” (2) “How much weight are we attempting to lift?” Accept all predictions and guesses before starting the experiment.
- Select four of the smallest students and assign each one to a wind bag positioned between the two tables. Tell the four students to bend down on their knees and grab the wind bag with both hands. At the count of three have all four students start blowing into the wind bag, like they are blowing into a paper bag. Remind them to hold the bag closed after each breath and to not let the air escape from the wind bag. After four or five breaths, the students sitting on the table will be lifted by the wind bag “hydraulic lift.”
- Sanitary Caution: If another group of students is allowed to do the experiment with the same wind bag, cut off 4 to 6 inches of the open end of the wind bag with a scissors.
Correlation to Next Generation Science Standards (NGSS)†
Science & Engineering Practices
Asking questions and defining problems
Developing and using models
Planning and carrying out investigations
Constructing explanations and designing solutions
Disciplinary Core Ideas
MS-PS2.A: Forces and Motion
MS-PS2.B: Types of Interactions
MS-PS3.B: Conservation of Energy and Energy Transfer
MS-PS3.C: Relationship between Energy and Forces
HS-PS2.A: Forces and Motion
HS-PS2.B: Types of Interactions
HS-PS3.B: Conservation of Energy and Energy Transfer
HS-PS3.C: Relationship between Energy and Forces
Cause and effect
Scale, proportion, and quantity
Systems and system models
Energy and matter
MS-PS2-2. Plan an investigation to provide evidence that the change in an object’s motion depends on the sum of the forces on the object and the mass of the object
HS-PS2-1. Analyze data to support the claim that Newton’s second law of motion describes the mathematical relationship among the net force on a macroscopic object, its mass, and its acceleration.
The wind bag quickly fills with air because of a scientific law called Bernoulli’s principle. As air is blown into the bag, the air pressure around the mouth of the bag drops. Bernoulli observed that whenever air moves, its pressure drops. The faster the air moves, the more the pressure drops. As a result, the air in the atmosphere (high pressure) fills the bag as long as the fast moving air (from your lungs) creates an area of low pressure around the mouth of the bag. In this example, high pressure air moves toward low pressure air and the bag fills. This is the same principle that explains why airplanes fly, why baseballs curve, and why a race car needs a spoiler.
Air from the students’ lungs is being forced into the plastic bag. The air is not allowed to escape because the students are holding the bag shut between breaths. The compressed air is exerting air pressure in all directions. As the bags expand, air pressure is causing the inverted table to rise.
Every student in the class can do a small scale version of this experiment by putting a ten-inch-long party balloon under a heavy book.
Air pressure is used to inflate bicycle, car and truck tires. Air safety bags in cars are another practical application of this scientific principle. A typical application of Pascal’s principle for gases and liquids is the automobile lift seen in many service stations and garages. Compressed air exerts pressure on the oil in an underground reservoir. The oil in turn transmits the pressure to a cylinder, which lifts the automobile.
Special thanks to John Mauch, Steve Spangler and Tom Gilmore for providing us with this activity.