And yet it moves (backwards, very slowly)

February 3, 2024 8:03 AM

"A typical lawn sprinkler features various nozzles arranged at angles on a rotating wheel; when water is pumped in, they release jets that cause the wheel to rotate. But what would happen if the water were sucked into the sprinkler instead? In which direction would the wheel turn then, or would it even turn at all? That's the essence of the "reverse sprinkler" problem that physicists like Richard Feynman, among others, have grappled with since the 1940s. Now, applied mathematicians at New York University think they've cracked the conundrum, per a recent paper published in the journal Physical Review Letters".

Bonus pedantry re: the post title: Galileo probably never said "And yet it moves".

posted by jedicus (13 comments total)

Reading just the text of the FPP, my reaction was "geez, just submerge a sprinkler in the bathtub and attach a pump, how hard can it be?" Lo and behold, that's more or less what they did, albeit much more scientific and with math. It seems to me as a lay reader like a neat solution to a surprisingly complicated problem.
posted by Dip Flash at 8:30 AM on February 3 [1 favorite]

But what would happen if, instead of water, we fed the sprinkler with high pressure ground beef? Physicists have grappled with this problem for decades yet little progress has been made towards smoothing it over with the neighbors.
posted by phooky at 9:24 AM on February 3 [8 favorites]

From Surely You're Joking Mr Feynman:
The answer is perfectly clear at first sight. The trouble was, some guy would think it was perfectly clear one way, and another guy would think it was perfectly clear the other way. So everybody was discussing it. I remember at one particular seminar, or tea, somebody went up to Prof John Wheeler and said, "Which way do you think it goes around?"

Wheeler said, "Yesterday, Feynman convinced me that it went backwards. Today, he's convinced me equally well that it goes around the other way. I don't know what he'll convince me of tomorrow!"
posted by BobTheScientist at 9:48 AM on February 3 [4 favorites]

cw unpleasant beef


phooky, not a month goes by without something reminding me of Meat Prices Skyrocket After Cow Smashing Machine Gets All Beefed Up.
posted by Richard Daly at 10:39 AM on February 3 [1 favorite]

one of the contributing author's surname is 'sprinkle' lol

also it appears they're all based in the northern hemisphere, so. . .
posted by logicpunk at 11:50 AM on February 3 [2 favorites]

But what if the sprinkler were on a treadmill?
posted by The Manwich Horror at 1:13 PM on February 3 [7 favorites]

Took me a second to find it, but this fun video from Matthias Wandel starts with a description of just this experiment, and teaches some related ideas as well.
posted by shenkerism at 2:16 PM on February 3 [2 favorites]

That Wandel video was on the blue, right? I assumed this was a follow up post honestly. Which is to say "fascinating".
posted by DeepSeaHaggis at 4:48 PM on February 3

Doesn't look like it. I also opened the post expecting to see his vid.
posted by shenkerism at 4:58 PM on February 3

Coming soon to a missile system near you!
posted by pompomtom at 1:26 AM on February 4

The claim is that the reverse propulsion is driven by the jets from the arms meeting in the not-quite-center of the sprinkler hub. But in that case, it seems like you could cause the "suckler" to distress in either direction via tiny misalignments of the central geometry, which wouldn't affect the "sprinkler" mode of the same device. I don't have access to the actual paper to see whether the author's account for this possible variation in alignment.
posted by fantabulous timewaster at 6:55 AM on February 4

it strikes me that the most straightforward way to test this is to put it through a turnstile and have observers watch as it runs, ideally one inverted observer and one non-inverted observer. make sure to have them wear differently colored armbands and watches so that you can sort of tell them apart but don’t make them too obvious because that’s declassé.

there are a couple of problems left to solve, though:

1. does it work differently if both the sprinkler and the water are inverted?
2. why is everyone mumbling so much i can’t tell what anyone is saying what the hell is going on.

posted by bombastic lowercase pronouncements at 8:28 AM on February 4

The order is; 'engage the caterpillar drive'!

[Everyone starts singing the Soviet anthem]
posted by Fiasco da Gama at 8:12 PM on February 4