How Physicists FINALLY Solved the Feynman Sprinkler Problem - Explained

I think if your sprinkler is underwater then your grass is probably wet enough.

Imagine being so smart that a Problem gets Your name because you could NOT solve it.

When I was a physics grad student in the 80s I disagreed with a professor about an E&M problem - the prof was a real *sshole about it and I was sure I was right. I phoned up Feynman at his home (he was in the directory!) and asked him his opinion. He told me I was right (this story ended up doing the rounds at UCI) and he asked me the sprinkler problem. I gave a few different answers that I said were naïve answers (which are covered in your video!), and that I was unsure. He told me to call him back when I had my answer. Overall we had a 45 minute conversation - I felt very honored. I became disappointed in myself as I never got a fully convincing answer so never called him back, and he died in 1988. I felt like I had failed the great man - until I saw your video today!!!!

Am I the first person to notice that the description of Feynman's experiment is wrong? Actually, he tried to pump air into the top of the carboy to push the water backwards through the tubing; he didn't suck the water out of the tube. Eventually the pressure blew the carboy apart. See "Surely You're Joking, Mr. Feynman" at the end of Part 2: The Princeton Years.

Why didn't they repeat the experiment with internal tubes pointing upwards to cancel the vortex?

"Feynman was keenly aware of his own abilities and almost entirely unburdened with modesty" - the sentence where I clicked Subscribe.

"almost entirely unburdened by modesty." That is the greatest description of Feynman! He wasn't so much arrogant as bereft of any desire to not be arrogant.

6:42 "Sucking is not the opposite of blowing" lol

This seems more a function of the specific design of the sprinkler internals. If the pipes were angled to have the vortices' sizes inverted it could be made to rotate in the other direction.

I wish they would have redesigned the test so the arms of the sprinkler don't have that central cavity for the vortexes to form. They could have brought the two tubes together in an upside down Y with the leg of the inverted Y pointing straight up in the center -- that should eliminate those vortexes that were contributing rotational forces.

Me, skipping randomly on work through the video: 7:37 "[...] Interestingly here due to our slightly imprecise use of language when we describe sucking and blowing [...]" With a humor stuck stil in puberty this line without context humours me a little.

That meniscus bearing is cool. I wonder where this idea came from? Can this be used to create frictionless bearings for more practical applications?

I'm giving you a thumbs up for excellent audio quality, no over powering music and clear responses. Great work here.

It took 140 years to put a sprinkler underwater.

I remember seeing a model of the inverse sprinkler years ago with air being sucked in. The result was that it was sensitive to disturbances and it was possible to get it going in both directions. It wasn't going that far to reduce the disturbances though.

i had a feeling the fan topic was gonna be brought up and lo and behold, 6:42 comes up

I think it's because the geometry of the plenum wasn't specified, and the effect would disappear depending on the plenum's geometry. This seems more like experimental error unless the problem specifically states that the sprinkler has to have this specific plenum geometry. I assumed the sprinkler wouldn't move, but i also assumed the experiment would provide a suction via a 2:1 header with decent flow characteristics rather than dumping asymmetrical flow into an internal volume. Of course it would spin in revere in that case, but arbitrary changes to the internal volume can give any result. Care was taken to isolate the system from pump vibration, meniuscus bearing for lower friction, all this is pretty obvious and what i'd assume would be the setup, but i also would assume that the experiment would account for the internal geometry by using a low turbulence Y connection to the suction. As the problem was being described, i already thought of a siphon and meniscus bearing, as well as a fairly laminar internal structure. The experiment is specifically designed to give this result, and it can give the opposite result if the internal geometry contained baffles, guide fins, a rounded feature in which an axle/pivot bolt runs, or any number of possible configurations. When i design hovercraft hulls i use all kinds of tricks like this to negate lift motor torque by adjusting the plenum geometry.

TLDR: The 100 year old answer of "depends on what engineering choices you pick to have the most effect" is the right one and nothing was actually discovered beyond why small house vacuums often have the intake opening on the side which was also known for quite a while.

These are the types of videos that make me glad to study physics in college. I guessed right in the first part, surmised the opposite in the second part, and I was happy with the result in the third part. Always adapting to new information and ideas.

A while ago I saw a YouTube video that immediately came to mind. My first thought was also that pressure is equally everywhere in every direction, by the way. But the video was about a simple vertical (PVC) pipe connected to a vacuum cleaner. It was mounted to the side of a table, but not actually fixated in place. When the vacuum cleaner turns on, the pipe moves up a bit. Conclusion was that the air that is right next to the pipe gets sucked in with a sling-shot motion and the centrifugal force that came with it, pulls the pipe up. It also heavily depends on the shape of the rim: a well rounded edge pulls less.