Feynman’s Reverse Sprinkler: Why “Sucking In” Is Not Just Playback of “Spraying Out”

A normal sprinkler spins because water is expelled from bent arms, and the reaction torque pushes the arms the opposite way.

So a natural question is:

If you submerge the sprinkler and suck water in, does it spin the opposite way?

This is the classic reverse sprinkler puzzle (often linked to Feynman, though Ernst Mach discussed it in the 1800s).

Short answer:

• in many ordinary setups, it appears to show little steady motion (maybe a startup twitch),
• but in very low-friction, carefully measured setups, reverse rotation can appear and persist,
• and the mechanism is subtler than naive “time-reversal intuition.”

The Intuition Trap

People often think:

• forward sprinkler: fluid out -> sprinkler rotates one way
• reverse sprinkler: fluid in -> sprinkler should rotate exactly opposite

But that symmetry is not exact in real fluid systems.

Why? Because what matters is not only “in vs out,” but also:

• pressure-force distribution,
• momentum transport through curved tubes,
• viscous losses,
• and geometry-dependent internal flow structure.

So “just run the movie backward” is an incomplete model.

Why Early Experiments Seemed to Show “Almost No Rotation”

Classic demonstrations (including stories from Princeton/MIT setups) often report:

1. Transient kick when suction starts,
2. then near-steady little/no visible rotation.

A useful force-balance picture for idealized steady suction:

• Pressure differences at the intake can pull one way,
• internal momentum redirection in the elbow pushes the other way,
• these contributions can largely cancel in steady state.

That makes the reverse effect look weak and easy to miss unless friction is very low and measurements are careful.

Modern Resolution (2024 PRL): Small Effect, Real Effect

A 2024 Physical Review Letters study (Wang et al.) built an ultralow-friction apparatus and observed robust reverse rotation under suction.

Their key message:

• forward and reverse modes can both be explained via angular-momentum flux ideas,
• but in reverse mode, the effect is subtle and linked to centrifugal flow behavior in curved conduits,
• so the reverse torque is real, but typically much weaker/less obvious in practical setups.

This helps reconcile decades of “it doesn’t move” vs “it does move a little” reports.

A Practical Mental Model

Use this three-part model:

1. Startup transient

   • when flow ramps from zero, imbalance produces a twitch.

2. Steady ideal tendency toward cancellation

   • intake-pressure and internal deflection effects partially offset.

3. Residual real-world torque

   • curvature + finite Reynolds effects + non-ideal flow make cancellation imperfect,
   • in low-friction systems, this residual can create measurable reverse rotation.

So the puzzle isn’t “either 100% opposite or 0% forever.” It’s a near-cancellation problem where tiny terms decide what you actually observe.

Why This Problem Is Useful Beyond Toy Physics

This is a great lesson in systems thinking:

• symmetric storytelling can fail when dissipation and geometry matter,
• near-canceling forces make outcomes sensitivity-heavy,
• “no observed effect” can just mean “effect below apparatus threshold.”

Same pattern appears in control systems, finance microstructure, and performance engineering: tiny residual terms dominate once first-order terms cancel.

If You Want to Demonstrate It Yourself

• Use the lowest-friction pivot/bearing possible.
• Keep geometry well-defined and rigid.
• Measure angle/time digitally (video tracking), not just by eye.
• Compare startup transient vs long-time behavior separately.
• Repeat across flow rates (weak signals can flip visibility by regime).

One-Sentence Takeaway

The reverse sprinkler is a near-cancellation fluid-dynamics problem: steady suction often looks torque-free, but with sufficiently sensitive low-friction setups, a subtle geometry-driven reverse rotation emerges.

References (Starter Set)

• Ernst Mach, The Science of Mechanics (1883) — early discussion of reverse reaction-wheel behavior.
• MIT Edgerton Center, “Feynman Sprinkler” demonstration notes.
• Wang, K. et al. (2024), “Centrifugal Flows Drive Reverse Rotation of Feynman’s Sprinkler,” Physical Review Letters 132, 044003. DOI: 10.1103/PhysRevLett.132.044003.
• APS Physics viewpoint: “Feynman’s Reversed Sprinkler Puzzle Solved” (2024).
• “Feynman sprinkler” overview (historical context and debate summary).