Reverse Sprinkler

Presentation Date: 

Friday, December 18, 2015

Presentation Slides: 

What it Shows

Inspired by Richard Feynman's story in his 1985 book (pp 63-65), Surely You're Joking Mr. Feynman, the demonstration answers the question "which direction does a lawn sprinkler spin if water enters the nozzle rather than being expelled from the nozzle?" The reverse sprinkler spins in the opposite direction of a "normal" sprinkler. "Dissipative effects" has been the hand-waving reason for the past 30 years, but the real reason why it spins in the reverse direction is far from obvious (see Comments, below). It turns out that a sprinkler designed to be "truly neutral" does not spin at all, except for a transient jerk when the air source is turned on or off.

How it Works

An Air Source (normally used with air-tracks) has been modified with a fitting so that one can either blow air out of the sprinkler (making it a "normal" sprinkler") or suck air into it ("reverse sprinkler"). You can see it in action in the short movie:

Setting it Up

The sprinkler and air source can simply be set up on top of the lecture bench. It's a good idea to either clamp or duct tape the base of the sprinkler to the bench because the slightly cumbersome hose that connects it to the air source might pull it off the bench.


A full explanation is given by W. Rueckner, "The puzzle of the steady-state rotation of a reverse sprinkler," Am. J. Phys. 83(4), 296-304 (2015).