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!!!!

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.

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

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

"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.

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.

It took 140 years to put a sprinkler underwater.

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.

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.

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.

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.

What an absolutely great explanatory video! I am a lowly engineer with a MS in fluids--and I could so completely understand this! Seriously, thank you!

Truly wonderful. Not only massively educational and interesting but presented by THE most beautiful, hypnotic imagery. Simply brilliant and full marks to all those involved. Thanks Ben. Cheers 🍺

to the people saying they only got this answer because of the way they designed their sprinkler: they also did all of the calculations and math derivations so you can now predict the movement of many sprinkler designs, not just the one they actually built.

Are you telling me that not _one_ person decided to bend the tubes upward toward the pump-rather than just ending them at cavity where they point at each other-in order to basically remove the vortices entirely? It's like the question hasn't been answered at all, at this point.

A while ago I saw a UKposts 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.

The simplest sprinklers are composed of hoses with holes punctured at regular intervals. Great presentation and delivery of the material. Only 4 minutes in but I appreciate the thought and craftsmanship that has been invested in communicating this problem.

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.

"entry for nominative determinism" slayed me sir. bravo

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.

I think it is possible to analyze the problem in a simpler way by breaking it down. 1/ pumping fluid quickly inside a simple tube with an entry and exit generates strong pushing thrust at the exit, and weak pulling thrust at the entry. this can probably be measured independently using load cells. the thrust can be converted into movement/rotation or a stationary force/torque, it doesnt matter. this is highlighted by jet ski having the jet exit direction controling the thrust, while the intake is directed forward and downward (not straight forward) and doesnt change direction for forward or reverse operation 2/ if you now have several exits, and several entries, the overall thrust will be approximately the sum of the exit thrusts 3/ if exit thrusts cancel each others approximately, then the intake thrusts can become prevalent 4/ if exits streams point at each or at fixed objects other weird turbulence and vortices will happen and create additional secondary effects way more complicated to study and probably cant be predicted without numeric simulation and understood through experimentation 5/ even it the main exit thrusts cancel each other, those secondary effect could still outweight intake thrusts. THIS IS probably the ONLY CONCLUSIION of this experiment? 6/ the rotating part of a sprinkler should be analyzed like a freely rotating system with entries and exits for fluid to be pumped through 7/ the traditional sprinkler has several exits which combined generate a clear torque, stronger than any effec onthe sucking side, the intakes don't matter 8/ the generic sucking sprinkler achieved using any sprinkler, with reversed pumping action, is designed wihout any attention to the blowing side , and because of this, has undetermined behavior 8/ the sprinkler shown in this experiment is seemingly designed to cancel the effects of the blowing side to show the effect of the sucking side (by using symetrical exits, pointing at the center, but failed to do so because asymetrical flows and resulting asymetrical vortices

8:35 of course, Brennan _had_ to work this problem. fascinating 🖖

I remember hearing about a problem that early jet aircraft had, especially those with intakes in the nose. It was called "inlet lift". At high angles of attack the air entering the inlet had to turn a corner, and this created a nose up force. This made stall recovery tough because adding power, i.e. afterburner, increased the effect.

nicely done and brilliantly scripted, illustrated, and produced ty for posting!

Thank you. “Experimental design” questions answered that occurred to me as you were presenting the facts, possible solutions and attempted proofs. Very clearly demonstrated and well explained for a visual, life long learner.

It would be interesting to have the tubes inside the central cavity turn so that they are pointing vertically (up or down) compared with the axis of rotation.

Not so convinced.. since you talked about the opposite effects of sucking and inertial forces in the pipe corners, Reynolds should be an important factor. The pressure gradients involved in sucking are influenced by viscosity, while the force imparted on the tube due to the fluid changing direction are not. I expect it would spin in the normal way at sufficiently high Re.

That was an eye opening demonstration of forces that I never thought about.

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

What a fascinating result. Fluid dynamics - elegantly simple rules that often defy expert intuition.

This was already solved a decade ago at Harvard. They knew about the votices inside the hub and that the rotation depends on the geometry. Wang's contribution is more subtle and the new "solution" is pop science sensationalism

That answer is actually really fascinating, and it just goes to show you that it's not always outside, but what's inside that really counts

That was actually remarkably informative and entertaining. Thank you.

Did Mach's theoretical sprinkler have the attitude of the internal ends of the tubes defined? Thanks to all who work on this problem, it has been spinning around my brain for decades now, since I read Feynman's book.

Next experiment: stop those vortexes from forming within the center of the hub

Thank you for this great insight in how to analyze and tes problems, great stuff.

Thank you for this. Amazing visualizations and explanations. I love having my brain scratched.

7:55 "Timmy, close the window" "Oh, sorry dad."

also, there is no "sucking" only pressure differentials. meaning fluids always get pushed, never pulled.

Thanks for the video! I found the paper too dense for a leisurely read, but this was perfect for my curiosity.

This was real fun to watch. Thank you a lot for this video.

Commenting at 0:53, before watching your report. Many years ago, in the early days of the worldwide web, I was curious about this. I searched online and found groups that reproduced the experiment and even had video! These were done in various ways, e.g. with gas instead of water, and other fluids. Also, the sprinkler arm shape varied. The empirical results were all over the place! Some went one way, some the other, some had no discernible movement. I also found a draft of a paper analyzing the physics, and with simplifying assumptions, concludes that in the steady state there should be no motion. I emailed them with the experiments I had found, and later found that not only had they updated the paper but I got thanked in the list of credits at the end! So, with the forces in opposite directions cancelling, these "simplifying assumptions" will be all that's left. Due to viscosity and friction and the arm shape and whatnot, tiny imperfections cause the cancellation to be less than perfect. Whether the real-world sprinkler moves one way or the other depends on the precise design, the fluid viscosity, friction, resident time inside the arm, and who knows what else. So, if this experiment claims to be the first/only people to try it, they are _so_ wrong. If it claims to be the first detailed analysis of the physics, they are wrong by a few decades. So I wonder what is meant by "breakthrough" and why it's "finally" solved when it was solved more than twenty years ago IIRC. Is this all not on the Wikipedia page, perhaps, so nobody else knows about it? I wonder if ChatGPT knows about it? ...

So, if you added a 90 degree bend pointing up to the suction area then all rotation should stop. Right?

Mesmerising and stimulating : Thank you for sharing

What a banger video! Excellent summary of their paper, there's so much to this

When water is spit out it all goes one direction (due to its momentum inside pipe) but when sucked in, it comes in from almost every direction (except for the direction of the pipe) since its initial momentum is close to zero. This is why “put-put” boats work.

6:40 - yes. Sucking and blowing can be the same thing. However. Context is really important

Fantastic experiement!! Huge thanks to the scientific team and God Bless you!

Fascinating! I spotted the first two forces, but (fluid dynamics being largely over my head) didn't know whether the force due to the pressure difference and the force due to water going around the bend would inherently cancel out. Hadn't even considered the behavior of water inside the hub.

It's obvious that it wouldn't spin if the liquid is drawn uniformally from the system (which can be achieved through inner arrangement of the sprinkler) because there is no net change in the angular momentum of the water. Basically the way it spins depends on how the water is leaving the system, not how it enters. Same as with regular sprinkler. Edit: The answer given in the video is only correct if you want to know what forces do sprinkler arms contribute and ignore everything else. Which is not quite the same as the original question

That sprinkler should be in a museum somewhere with an explanation of the design constraints and how this specific design solves them. That is a thing of pure art

OK I'll play ball and engage because you showed me an interesting problem :) My hypothesis at the start of the video is that the sprinkle-sucker will rotate counter to its above-water counterpart. I visualized the forces of a space ship to arrive at this answer. The water jet of a sprinker has essentially the same properties as a rocket. It's just a jet of water instead of a jet of fire. So, the inverse seems to be the most likely outcome, since we have inverted the forces at play.

Was REALLY looking forward to you going over the "friendly fluid dynamic equations" in Wang's paper.🤭

I don't know who pointed the editor to that Simpsons clip, but you need to give them a big fat raise.

Living in 2024 is phenomenal, in terms of how deeply informative content just casually drops when you may most conveniently want/need it. Just a couple days back I was contemplating the problem in leisure, and after having settled on my answer, looking around the internet for confirmation. To my surprise, there was a paper...lo and behold...from 2024 itself! And to my even greater surprise just now, a UKposts video on the topic tips its hat in my path. Internet was always known to be powerful, but something wonderfully invigorating is making it powerful still!

That laser sheet imaging is one of the coolest things I've ever seen in my entire life

I love science. I was thinking it won't move because suction isn't the reverse of blowing and intake is slow compared to repellent and it's just intaking inside the same material. But then of course, slight inconsistancies in fluent motion causing a slight spinning angle. It's so simple yet so complicated.

"How Physicists FINALLY Solved the Feynman Sprinkler Problem" - I didn't even know it existed.... lol... 😂

Maybe there is no sprinkler??

A wonderful problem. I had not heard of that one. Well done 👍

That was a brilliant video - Thanks!

Yeah, but why would you want to suck anything with a sprinkler...? 🤔😂

I would expect it to spin the other way , but very slowly because if you've ever tried to use a box fan to "suck air" you know that the air isn't sucked out in a clean column . So suction is NOT the inverse of blowing .

Great, I'm currently sitting in my laboratory and as a postdoc I'm continuing to work on my work "Experimental study of thermohaline stairs in double-diffusive convection", but now in a rotating reference system. My experimental setup shows significantly more effects than this actually quite simple Feynman sprinkler problem. There are also many technical solutions in my structure, including the PIV system! Nevertheless, it's nice to see how much thought had to be invested to solve the Feynman sprinkler problem and that it is by no means easy to have to think outside the box 1000 times when the problem becomes even more complex! Thanks for this great video! Dr. Axel Rosenthal from Institute for Astrophysics and Geophysics in Göttingen .

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.

12:59 if you don't want a 13 minute history of sprinklers.

I love that the result is 1 - what any rando would have expected 2 - due to forces that hadn't even occurred to Feynman et al.

Wow, very interesting and a lot more complex than I was thinking it would be!

Perfect to watch at 3am, when you think you'll just peak at the result, but end up staying for the amazing video as a whole!

That is utterly fascinating.

That leaves a couple of design to look at to verify the experiment. 1) Make the tubes perfectly straight. 2) Angle their direction entering the center portion.

Wow! As a former fluid physics student, this was a lot of fun to see if my intuition would hold up. My gut predicted counterrotation, and giving it some thought I predicted a net rotation in the water to impart angular momentum. It was probably a lucky guess though!

Super fascinating great experiment and video

Subscribed. What a great video. I guessed it would not move at all. I did not see Dean flow coming as the solution.

I think the mass difference exiting or entering tube is a crucial reason for the rotation of a sprinkler other that the obvious other reasons and that changes the outcome of suction. It's just amazing they didn't use something else to compare the problem to

Brilliant! Amazing experiment

I used to have an aquarium. When doing water changes you suck the air out of a hose you put in the water. If i remember correctly, this moved the hose "forward" in the direction of the opening first but then when the water hits the backside it basically bounces back, then little to no motion at all.. but it's not free-spinning like the sprinkler anyways. So it depends on which force is stronger then. The forward force of sucking in the water, the backwards force of it hitting the backside of the tube/sprinkler, or if they are the same strength it would not move at all. Dunno if this isnt an oversimplification, but i would assume the reverse to happen as if we run it normally.. it spinning the other way around as if we propelled water from it. Edit: Wow that was a cool explanation, and the green particle demonstration looked brilliant aswell!

It would have been interesting for the researchers to simplify the validation of the force that rotates the "sucking" sprinkler backwards by building a second and third sprinkler that has the arms exiting the the reservoir body at both an obtuse and acute angle relative to the axis of rotation while the arm exit into the open water chamber is in the identical location as the main experiment. This would confirm that changing this specific variable alters the direction of the "sucking" sprinkler, without needing to visually interpret the laser-illuminated particle flows. Very cool and enlightening problem !

Alright, I'm convinced that these vortices impart their momentum on the central body, however I'm not quite convinced yet that the negative pressure at the point of ingress is completely overruled by the positive momentum imparted on these tubes by their internal flow's directional change. How about redoing this setup with a longer straight closed-off tube with a 90deg smaller soldered-on jet, and increase the flow to the turbulent realm? That should mix up the flows enough in the core that eddies should be randomized and cancelled, and due to the lack of bend, it'll only be about the effects of the negative pressure at the jet start, and positive at the reaction wall.

Another fantastic example of how what we think "intuitively" makes sense is actually wrong.

"entirely unburdened by modesty" hahahahaha. I'm stealing that line. Also, I initially assumed, the same vacuum hose/shirt effect was in effect. Fascinating video.

"Sucking is not the opposite of blowing" Indeed, that statement is correct as they are the same action

Great Explanation

Fantastic problem; interesting investigation. I have a problem, for wich I had no luck searching for a good explanation: forces in vacuum. You get a suction cup, press against the well finished surface of something heavy, make vacuum ...and it can be moved upwards. Or remember the historical event at Magdeburg, with horses playing tug of war. Wich forces are present, with such a strong value? Intramolecular?

8:00 Wait a sec, did you use battered side down footage??? Awesome!!!

very informative. Non physicists can see this complicated problems solution ( without the math). ( I hear I forget, I see I remember ( I do, I understand).

absolutely brilliant

They shouldn’t discount the additional rotational force caused by the mass of the water being drawn inwards. The suction could account for a small amount of movement, but the conservation of momentum could do the rest. Like an ice skater pulling their arms in during a spin. Great video!

Good to see Serj doing something since SOD days.

Loved it. Thanks.

14:40. This shows that there can be a torque depending on how the water enters the central drum. This means if you modify the design of the tubes entering the drum, you can make it spin *either* direction, depending on how much angular momentum is acquired by the water exiting the drain. This should probably be viewed as a flaw in the experiment. If you design the drum specifically to prevent the water from acquiring any angular momentum at the drain, it will not spin. As an example, turn the tubes in the drum straight downward so that water must exit without angular momentum.

Admittedly not done the video yet, but looked at a few comments and think I agree with what seems to be the common idea, the direction of rotation should logically be influenced heavily by the orientation and specific design of the head, depending on the orientation and more impactfully, the length of the arms. I've very crudely reasoned that having the bend that offsets the direction of flow closer to the central axis while having longer arms should spin in the same direction as a sprinkler under normal conditions, while something with long arms that have a small bend on the end could possibly spin in the opposing direction if oriented correctly. Maybe I'll add an addendum to this after I'm done. Welp, here is the addendum. Logically, I think that it *should* rotate in reverse for most configurations of the arms, however, if you were to cancel the formation of the central vortices with a design change, based on the specific hydrodynamic properties of the arms of your system, it would be completely possible to generate a net torque on the system, as the pressure differentials would create weak currents that would impart more force to certain surfaces of the arms.

"...And this year's entry for nominative determinism: Brennan SPRINKLE." Haha,, of course; how appropriate! Obviously you can't have a study about sprinklers without this guy, ha!

What a great video and explanation in simple terms. Repetition of the experiment needs to occur, with changes to the sprinkler design. You need to use the same sprinkler under water as you would use out of water for consistency. Perhaps use magnetic frictionless bearings to eliminate any friction. Also the internal cavity where the arms extend from needs to be redesigned to eliminate internal vortexes. Perhaps extend the spinning arms directly down to where the water enters sprinkler , making sure that there are no internal spaces for water to accumulate above the bearing position.. However with a conventional water sprinkler that you would use to irrigate your grass or lawn operating under water with the pump working in reverse. The sprinkler head does in fact work in reverse as proved. Why it does so is a different problem. If the sprinkler is redesigned and used to irrigate grass and doesn’t work as the one shown wouldn’t then have you proved anything anyway?

So, basically the reason it spins backwards is the same reason the outside wheel of an axle in a turn spins faster. The particles begin at the same point and end at the same point; however the ones on the outside of the tube are moving with more velocity. Because of the way the tubes are lined up in this experiment, the higher velocity particles are hitting the lower velocity particles from the opposite tube. This is what's creating the spin. I assumed the sprinkler heads would pull them selves in the opposite direction of flow, but my assumption was more based the velocity change between the particles being sucked in (outside the nozzle) vs the ones inside the nozzle. Similar to how compressors work. Interesting finding for sure.

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

I like to think Feynman would have loved this. The closer you look, the more interesting it becomes!