Abstract
A perpetual-motion machine (PMM) problem is described which the author has used for more than a decade to augment a traditional textbook-centred course. Two proofs for debunking a ‘buoyancy engine’ PMM are given. One solution is based on the principles of fluid statics, requiring the calculation of hydrostatic and buoyancy forces on submerged surfaces. An alternative solution is based on an energy method, requiring the calculation of the work done by a moving boundary. These analyses are well suited for instructional purposes on a first course in fluid mechanics or thermodynamics. The author has found that this type of puzzle can be used to generate a high level of student interest and classroom participation, with only a minor commitment of lecture time.
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