Marilyn's Sprinkler Answer Incomplete

Marilyn is Wrong Copyright © 1996-1998 Herb Weiner. All rights reserved.

Ask Marilyn ® by Marilyn vos Savant is a column in Parade Magazine, published by PARADE, 711 Third Avenue, New York, NY 10017, USA. According to Parade, Marilyn vos Savant is listed in the "Guinness Book of World Records Hall of Fame" for "Highest IQ."

In Marilyn's Parade Magazine column of October 6, 1996, she discusses Richard Feynman's reverse sprinkler experiment, in which he attempted to determine which way an underwater sprinkler would spin when water is forced to flow in the reverse direction. In this experiment, the sprinkler did not spin at all, and the experiment ultimately exploded from the pressure of the compressed air being used to drive it.

Dick Bukrey <> writes

Marilyn's account of Feynman's experiment is accurate, as far as I know. However, Feynman's work was by no means the end of the story. A number of people have taken a serious look at the issue, and several papers have been published. The most recent paper that I'm aware of is by Collier, Berg & Ferrell, University of Maryland, and was published in the American Journal of Physics, Vol. 59, No. 4, April 1991, pp.349-355. References to earlier work of theirs and others is included therein. Collier, et. al., concluded that the inverse sprinkler does indeed rotate in the reverse direction. A successful demonstration requires careful attention to detail, especially the elimination of friction.

I became interested in this topic last fall, when I supervised two of our freshmen physics majors who set out to test this concept. For further details, see -

A New Spin on the Problem

Mike Gavaghan <> had these comments:

I've often thought that the cleverness of the problem was the fact that most people instinctively draw the conclusion that the sprinkler must spin one way or the other. Regardless of whatever the correct answer is, few people ever seem to even consider the third alternative: it doesn't spin at all.

While pondering this question, I seemed to go back and forth on my answer. I'd wake up, convinced that the sprinkler spins the same direction, but later change my mind and go to bed convinced it would spin in reverse. Apparently, Feynman's long time friend Ralph Leighton suffered in this way as well!

After reading in Gleick's "Genius" that the sprinkler does not move at all (complete with an explanation), I thought the issue was settled.

Needless to say, I was amazed to read Dick Bukrey's message indicating that physicists at University of Maryland had proved it spins in reverse! In addition, students at Loyola apparently performed the experiment and observed the same. Unfortunately, the link he provided for details has since been updated. I was not able to find more information at that link other than a passing mention of the experiment.

Again, I thought I had convinced myself of the answer, only to be questioning it again!

However, I still hold the belief that the sprinkler does not spin at all - albeit less confidently. Despite the results of the Loyola experiment, another experiment at MIT produced a different result.

Their observations, using air rather than water, support the answer provided by Gleick: the sprinkler does not move at all. I've observed the same, rather unscientifically, with a reversible leaf blower.

So there you have it - a new "spin" on the problem! last updated June 30, 1998 by