Demos: 2A-05 Pressure Force (Milk Jug)

A simple illustration of the forces due to air pressure is the crushing of a plastic one-gallon milk jug. A small amount of very hot water is poured into the empty jug and the cap is screwed on very quickly. As the water vapor in the jug condenses, a partial vacuum is created and because of the large surface area, pressure forces cause the jug to collapse.

Directions: Water should be heated ahead of time on a burner. Try to keep the water close to the boiling point. Using a pair of insulated gloves, pick up the beaker of water and pour a small amount in the jug, swirl it around for a second or two and then quickly screw the cap on tight. Set the jug on the lecture desk. In about a minute or so, it will begin to crush visibly.

Suggestions for Presentation: The key to understanding this demo is to establish why there is a partial vacuum established in the jug. Discuss the water vapor condensation and what this does to the internal pressure. Also, it is instructive to point out that the pressure difference can be very small, but the surface area of the jug is relatively large, so considerable forces are generated. (And the jug is made of plastic!)

Applications: Related phenomena include the bulging of small food packets on an airplane. When the plane lands, unopened packages are reurned to their normal size.

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