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%.
REFERENCES
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The original “Baker Street Irregulars” were a pack of street kids who occasionally helped Sherlock Holmes.
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The density of both dry gases at STP can be found at a number of websites, for example, http://www.ucdsb.on.ca/tiss/stretton/chem2/density_of_gases.html. We made the necessary temperature correction. Adjustments for elevation and relative humidity were not made since our experiment was conducted essentially at sea level, where the density of saturated air differs only by about 3% from that of dry air.
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© 2005 American Association of Physics Teachers.
2005
American Association of Physics Teachers
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