Feynman Sprinkler: Eppur si muove

The reverse sprinkler problem has attracted numerous analyses that focus primarily on internal flow dynamics,ignoring the pressure gradients and currents in the surrounding fluid. This paper identifies two primarysources of the observed backward torque: external pressure forces and currents in the surrounding fluid, andincomplete absorption of angular momentum by the sprinkler arm, both ultimately driven by the ambientpressure surrounding the sprinkler. The former is supported by Rueckner’s straight-arm experiment, bya rod-in-draining-tub demonstration described in this paper, and by Wandel’s independent experimentalrediscovery of the Bernoulli-based component of the mechanism (2023); the latter is consistent with thecomprehensive experimental results of Smith et al. (PNAS, 2026), for which the present paper provides thefirst theoretical explanation. A general principle is derived showing that for any sprinkler arm geometry, thenet angular momentum received by the sprinkler from incomplete absorption equals −loss, where loss is theangular momentum carried unabsorbed into the hub, correctly predicting the rotation direction for all sevengeometries studied by Smith et al. A generalization of Jenkins’s momentum-transfer argument clarifies whyinternal flow alone cannot drive the observed backward rotation. A simplified estimate of the lift force at thenozzle entrance illustrates the scale of the external flow effect.