This paper describes a lab experiment on buoyancy. A plastic garbage bag is filled with helium and fitted loosely into an empty 55-gallon drum. A cardboard piston (Fig. 1) is used to square off the free end of the bag, forming an easily measured right circular cylinder. The gas is at ambient pressure since it is not compressed as it would be in a constricting balloon. Students generally find that the measured lifting force agrees with the theoretical value to within 5%.
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