Scallop theorem
= Scallop theorem
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= Purcell scallop theorem
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{synonym}
A <force-free> swimmer with a reciprocal shape stroke cannot achieve net locomotion in an inertia-free <Newtonian fluid>. A single real shape coordinate that retraces an interval gives a reciprocal cycle. The conclusion concerns free swimming; externally imposed translations can instead force and pump the fluid.