Water weight figures in jetpack dynamics
DOI: 10.1063/PT.3.2061
Authors Matthew Vonk and Peter Bohacek, in their Quick Study on jetpacks (Physics Today, January 2013, page 54 ), include in their weight calculations the water in the suspended feed hose. Since the mass of the water is supported by the pressure of the watercraft’s output jet, it ought not to be included. One can easily visualize this by imagining the hose and water standing upright on their own: The hose will fall from its own weight, while the water column will continue to rise upward, thanks to the system pressure behind it.
The skin friction between the upwardly flowing water and the interior surface of the hose further diminishes the effective weight of the suspended hose, although that factor will contribute little to the net force acting downward.
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David Fisher. (1@ep5.tv) ep5.tv, Rochester, New York.
