Demos: 2D-02 Pascal's Principle (Garbage Bag Blow-Up)

A large lawn refuse or garbage bag is sealed with duct tape and approximately a dozen plastic drinking straws are inserted evenly around the bag. A person (demonstrator or student) sits on the bag and several students are asked to come to the front and blow air into the bag through the straws. After a few moments the the bag inflates and lifts the person off the table.

The pressure exerted by the person blowing into the straw is transmitted to the bag. But the bag has a much greater cross-sectional area than the straw, so the pressure forces exerted on the bag are huge.

Directions: To provide stability, a wooden square is placed on the bag. The person should sit squarely in the center. Have the students blow into the bag. (It doesn’t matter if there are small air leaks, or if the students stop to take a breath. The amount of air exiting the bag is small compared with the amount going in.)

Suggestions for Presentation: If you want to have fun with this one, ask if anyone would like to “blow up” the professor (or another student). Ask for volunteers to blow through the straws. (Incidentally, this would work with only one straw, but the time for it to happen would be insufferably long!)

Applications: Various hydraulic systems. Portable “air jack” systems.

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Last Updated: May 9, 2016 11:44 AM