Maybe, the low pressure caused by the air bending around the upper edges of the straight pipe is pulling it up ?

In the late 70s as I was preparing to leave home for 4 years at Canoe U. and then on to a life in fighter jets. Some High School and Hockey buddies decided we should all go camping. Knowing the trouble this could be I drove myself. Just before Priest Grade outside Yosemite I stopped at a Gas Station/Mini-Mart. I noticed the old brown van ,that was the only other vehicle in the gravel lot, had a bunch of white squares all the way around the van under the window line. Each square had a Feynman diagram. A lady came out carrying bags of groceries and fumbling with her keys. I offered to take at least one of her bags and she handed me one and unlocked her door. As I handed her the bag I asked why she had Feynman diagrams all around her van? She responded, “well, I am Mrs. Feynman”. About that time, her husband came out the door carrying 2 more bags, which he too handed one to me. Then after adjusting his gear took the bag, turned around and said “Tanks”. (Not a typo). She then presented her husband with my question, to which he responded, “Hi, I’m Mrs. Feynman’s husband. Call me Richard”. I responded “Call me Richard, too”. We talked about 10 minutes, he asked Me lots of questions. I attended one of his lectures a few years later. He remembered me, told the Navy Brass I was one of the best helpers he’d ever had. I got an A in Physics that year.

Unfortunately, the comments are so full of random guesses, its hard to figure out which is right -- but I did figure it out in the end.

Matthias, you're an absolute gem.

I love the curiosity you have combined with your determination to measure and explore truth. Wonderful!

I recently read, "Surely You're Joking, Mr. Feynman!" A wonderful book I recommend to all. Great video, Matthias!

This is so interesting. I'm a mathematics graduate but I've always been fascinated by physics. I especially love seeing physical experiments with surprising results. Thank you for the video! I really enjoyed it.

Watched your videos as a teenager. Loved your approach to building bandsaws. Now I’m 30 and just stumbling upon your videos again. I’m glad to see you still making the awesome videos you make. Inspirational.

Just loving the direction this channel takes. Popularisation of science. Thanks !

Man, I love engineers!

I always enjoy the rabbit holes!! Thank you Matthias.

I really appreciate your curiosity and thinking. I actually built a dish for my wifi repeater to get signal to an outbuilding. It's been a few years now and was a great solution. Much thanks for your content!

Fascinating! And Feynman was a fantastic guy indeed... More than brilliant! Anyway, thanks, Matthias! 😊 Stay safe there with your family! 🖖😊 And happy holidays!

Now you've got me thinking about what the possible applications are for this.

Thank you for making me think this morning and engaging my brain.

I never imagined getting a lesson on fluid dynamics on this channel. Super fascinating!

Everything you do is fascinating!

Try a clear tube and fine smoke source to "see" what is going on in the tube.

really cool video, your explanations are very intuitive.

Dude! I actually understand this now! Thank you so much. Great experiment.

Consider if you pack the short tube with straws, would this tend to reduce or eliminate any possible recirculation effect? Rather than trying to visualize it with small threads, maybe try to remove it entirely and test? Great video as always. Thank you.

This explains how to repel black holes with a simple vacuum cleaner. Thanks.

Awesome, what cool effects. Fluids behave in mysterious ways.

Thats so cool! And it explains what I experienced just yesterday! I have 120mm dust collection tubing (ridged steel tubing) in my workshop and am currently upgrading my "main line" to 160mm. I have a little reducer piece to go from 160 down to 120mm and thats obviously a cone as well (About 100mm long). I was trying to - for fun - push the reducer piece the wrong way into the 160mm tube (So the 120mm side facing inside of the 160mm tube). And I just couldn't push it in! It kept coming out! I thought for sure it would get sucked in very hard and would be very loud with the air going through the now relatively small opening. Very fascinating indeed! thanks for sharing!

This was an amazing video. I really enjoyed it, though I understand very little of the math / physics / fluid dynamics behind it. For some reason, the way you presented this one or the problem itself perhaps, I found incredibly enjoyable. Thanks!

That sir is the most interesting thing I've seen all week.

the Wandel effect 😂. Keep it up dear sir 🙏i really enjoy the wits and creativity.

I'm neither a scientist, engineer or mathematician but I have a fondness for experiments of this type and greatly enjoyed the video.

Fascinating. Fluid dynamics is (are?) amazing.

Welcome to the new "Mr. Wizard's World". I love these videos. I would love to hear what The Physics Girl would have to say about this phenomenon...hopefully she can recover much sooner than later.

Incredible. I just learned a few things about fluid dynamics.

Really enjoyed this!

That was unexpected. I should not have watched this so late in the evening. Now I'll be laying in bed, wide awake thinking about it all night long.

I thought the conclusion was going to be that the fast moving air started trying to drag air in from the outside, and somehow that was going to cause the motion. I guess that could be ruled out by making a diaphragm around the moving tube, such that all of the air would be forced to go through the tube, but the tube would still be able to move up and down. But with the tiny forces involved, the diaphragm would itself be adding a variable that would probably still be significant.

All very curious. Interesting stuff.

Truly random stuff. Yet so fascinating.

You make physics fun! The TV series Mr Wizard was one of my favorites when I was a kid.

I think that if you increase the wall thickness of the short piece of piping (and make a nice half-circle of the rim cross-section) it may become apparent: the suction on the top rim will pull it up - and as soon as it lifts and exposes the bottom rim, the same will happen there and it will be pulled down again. That would be my guess.

Fun experiment. I love playing around with things like this in the shop. There is also a phenomenon called the Coanda effect with respect to flow and curved surfaces.

So very interesting!

As a retired electrical engineer I would never have thought I would be interested in a video about obsure effects of fluid dynamics......but I was!

Best. Video. Ever. The Feynman sprinkler. Yay.

Interesting as always.

The tube doesnt swing forward much because of the reaction force of the air going round the corner is cancelling out the -ve thrust. When air is flowing out the reaction force adds to the +ve thrust enhancing the effect. No idea about the straight side case at the end though.

Good ole Bernoulli. Explains so much

What an interesting phenomenon, physics is often very counter intuitive!

always interesting stuff.

Interesting one Matt.

Cool concept to investigate. Now I really want to know.....

I'm a fluid dynamicist by trade and training, and my best guess is fluctuating inflow from the gap between the "free" cylinder/cone and the fixed one. Air starts flowing in through the gap, and an interaction between that inflow and the main inflow sets up some kind of eddy or turbulence that exerts a force on the tubes. Would be interesting to see some smoke and a laser sheet used to visualise the flow in a few 2D planes to confirm what's happening. Fun little experiment!

I love listening to people who know way more on a subject than I do. I have no idea what's going on, but I'm along for the ride.

Fascinating!

Thanks! I never thought of centrifugal force as a way of creating lift.

That’s brilliant so interesting.

I think the behavior you see makes more sense if you change your perspective. The fan is lowering the pressure inside and at the end of the tube. The surrounding air in the room is at a higher pressure and so that surrounding air is pushing the air near the end into the tube. It explains why the "reverse sprinkler" doesn't turn backwards and also why you see that separation. The air from the room is trying to squeeze through the crack into the low pressure area inside the tube/pipe. The air in the high pressure zone is trying to find the easiest path to the low pressure zone. Thanks for making these kinds of videos. I'm trained in mechanical engineering but this kind of stuff is non-intuitive, even after studying. The simple "grade-school" or common sense explanations are often wrong.

When the straight tube is closed to the end of the vacuum hose, the entire column of air is below ambient pressure because you are pulling a vacuum. This means that there is a force pulling the entire column forward towards the atmospheric pressure. Since the only articulating part of the machine is the end of the tube, it is pulled forward by the low pressure.

From woodworking to fluid dynamics... Insert "that escalated quickly" meme here

This is quite interesting from a horn loud speaker. We use horns to better couple the acoustic energy to air and to make the speaker more efficient. Basically the reverse of what the bell opening does.

This is so amazingly interesting. I’m watching this and try to get my spouse to come and watch and she goes. That’s nice dear. Clearly we have different interests :). Thanks Mattias

These are my favourite kinds of videos. Hope it gains some traction and someone digs deeper.

Fascinating.

the most astonishing thing for me is that you build an acurate 90 degree elbow out of paper towel rolls

This is what the internet is for! Thank you.

My first thought was entrainment. The high pressure atmosphere wants to push into that gap where the low pressure stream is, and if it's not able to form an airtight seal, there will always be a crack where it can start expanding and blow open. When the gap opens enough for the atmosphere to rush in, the velocity into the gap increases, and the pressure equalizes with the cone or tube. then the air current is able to pull the two pieces together again. I'm curious what would happen if you use a straight tube that's smaller than the inlet? (Mounted to something so it doesn't get sucked through.) Maybe there's an optimum size where the inlet would exert the most force on the tube, while still allowing atmospheric entrainment.

The suction causes a vacuum around the entire hose and nozzle since air is compressible/decompressible. It is the same fluid within the system as outside the system - not air pressure on non- compressible liquid. This prevents the forward motion as the overall space around the piple loses pressure. The boucing is when the pipe is in an air pocket so free-falls back to position. From the original position the rebalanced pressure starts the process again. Layman speaking with limited experience of an open-faced helmet at 140mph.

I think the reason the vibe repels the vacuum is that the vacuum is sucking in air from the outside of the cone and the tube. Same thing with the cone rising, is sucking the air from around the cone and the shape of the cone is what causes it to rise.

3:00 is neat. it makes sense that the air in the tube provides resistance...but in unintuitive ways: when pressed up close, it seals well and the straight horizontal tube air begins to speed up. This lowers the pressure in the tube, and creates the pull force...but then as it reaches steady state the pressure at the elbow and the pressure in the tube are equal-ish. ...but then the air has a high inertia as it hits the elbow and provides a large leftward force. Once the seal breaks there is a big glut of air available to the low pressure elbow, so the elbow is drawn forward once again starting the oscillation.

Feels like one for Steve Mould. Would be interesting to see you guys collaborate on this.

Matthias the momentum change from restriction with tube or cone into vacuum is the greatest. As the vac tube moves away from the straight or cone the vena cava of the air flow reduces the momentum change and pendulum effect of weight moves vac tube back in proximity to the restriction and the system oscillates back an forth.

The movement of the suction pipe with the knee in front of the fixed straight pipe and the straight pipe above the suction use different effects for the movement. 0. The Bend pipe don't move much because it will suck in air from all directions around the opening of the pipe. Only a small section in the middle will make the pipe move forward. 1. The "loose bend pipe" experiment oscillated because of the weight of moving air that was changing direction and with that impulse. The air gets accelerated in the straight section of pipe and the enters the bend section, it has to change direction. It has to loose the horizontal momentum. This pushes the pipe away from the straight section. The movement in the fixed pipe stops and the bend pipe falls back into the default position. It starts to oscillate. 2. You're sucking air through the short loose end of a pipe that is not exactly the same diameter or orientation than the pipe sucking the air, these misalignment can result in wired forces pulling or pushing on the loose pipe. My guess here is that some of air is hitting the suction pipe not exactly straight causing sight changes of air movement and with that turbulence in the suction pipe that results in an repelling effect as long as the loose pipe section is close enough. As the pipe moves away the air flow changes and the loose pipe gets sucked towards the fixed pipe. EDIT: changed some wired wordings and typos. It's hard to write in a non native language after midnight.

Fluid dynamic master class :)

I have not read all the comments so this may have been covered. I believe the lift is being generated by eddy currents as air gets sucked in from outside the tube. This air initially has less resistance and the tube is pushed away as it rushes in, then gravity overcomes the tube’s momentum and it falls so the process repeats. Adjusting the variables should act to control what can be viewed as a machine.

That's a great demonstration for college level course. Unfortunately my fluid professor was so bad at teaching that they did away with it and changed the requirement. I had him for thermodynamics. I can attest to how bad he was. Incredibly gifted researcher though, just was not organized enough to put together a coherent lecture

Thought provoking!!

I'll preface this with: I'm not an expert, but I'm at least an engineer. I wonder if there could be some resonance going on in the long extractor pipe - maybe adding and removing the tube causes pressure waves that cause the air to accelerate and decelerate in the tube, and so cause the 'bouncing' we see. The fast-moving air in the short tube would suddenly come up against the slow-moving air in the extractor pipe and cause it to be pushed away. It'd probably be possible to disprove this by shortening the extractor fan pipe or adding a weight to the short tube. Fascinating video as always! Looking forward to the update🤞

I think the air flowing through the pipe extension can only move so fast so the unrestricted air from outside can force it's way in which pushes the main pipe back but only so far before gravity takes over

i believe that the high velocity air being channeled though the toilet paper tube contain an amount of velocity pressure. When it hits the elbow, or any restriction, it causes a reverse pressure wave. I believe that you have rediscovered "water hammer".

8:30 This would come from what you mentioned earlier in the vid at 6:00 about the fluid having to turn and having a centrifugal force sort of effect edit: I just found your new vid, and turns out I was right. You answered your own question without even realizing it. Very cool science

This is the sort of curiosity I feel society is losing in the younger generation. Superb content, and inspiring!

How cool!!

Between this and Veritasium's new video, my mind is blown today by two things that I would have thought "this is obvious" before being proven wrong.

I wouldn't be surprised if I saw some of the big science channels taking their turn to explain this phenomena in a couple of weeks.

With your extra pipe, it is porpoising like a last year F1 car!

Very interesting. Aerodynamic is always complicated.

For the vertical case I think it could be a reaction force; the air at the side of the pipe detaches from the walls (coanda) and there's an opposite force as it does that. That would be why it is such a small effect.

The tube is affected by turbulence, spinning air at lower pressure, that draws it towards it, but the air coming from behind filling the hole is keeping the tube down, it's all about equilibrium.

This is cool :) it looks a lot like an RC oscillator if we were to analyse it like an electrical circuit

When someone starts talking about eddy currents, I can't help remembering the great Douglas Adams. "Eddies," said Ford, "in the space-time continuum." "Ah," nodded Arthur, "is he. Is he." ... "What?" said Ford. "Er, who," said Arthur, "is Eddy, then, exactly, then?" Ford looked angrily at him.

I have 8.5 years of experience designing turbo machinery, is just Bernoulli. The gap in between pulls air inward which separates both ends. Try inverting the suction cone you will see is not the slope of the first section is the gap that you have between both sections. The inlet also is pulled upward, you are right! Bernoulli again.

my guess paused at 4:30 is that the L shaped tube pulls air through the cone, which then creates a force on the bottom of it, which pushes it back (we saw how much it was affected by small breezes just from blowing on it a minute before), and it's able to be pushed back since it's not attached.

In regards to the comment about people saying you would lie about this.... I mean... What POSSIBLE motivation could this hypothetical person come up with??? I mean..... fluid dynamics.... WOWZA! That has GOT to be a top trending subject on KZfaq right now 😂😂😂 Matthias is definitely cutting up paper towel tubes and hot gluing them back together solely for the money and attention Enjoyable as always, even if I never dreamed I'd be thinking so hard about how a fluid moves through an opening in my life.

Before watching the end, I'm guessing the major force is the inertia of the air exerting a force on the outside bend. It doesn't matter which way the air flows, as the pipe is always moved to the direction of the outside bend as the air exerts force when having to change direction.

18:30 Off hand I'd say because the straight tube will form a vena contracta at the inlet which behaves exactly like a bellmouth would even if there is no physical material shape of the bellmouth. The flow coefficient for a plain pipe like that could be ~.5 so it will behave like a bell mouth of half that diameter. Since there is no actual surface to suck against it will suck against the air in the vena contracta which will in turn pull upwards on the tube walls as it recirculates in the vena contracta.

Fascinating! Try attaching little threads to the *outside* of the short straight tube; I'll bet you'll see airflow moving upward around the outside in a pulsing oscillation. This upward airflow occurs so long as the tube's bottom is resting on the vacuum inlet beneath. But the internal downward flow creates low pressure that overcomes the weight of the tube and support arm, creating a lifting force--upward flow--on the outside of the short tube (Coanda Effect). This breaks the upward flow (Coanda Effect) around the outside of the tube when it lifts off the vacuum inlet, and the tube settles back down. The motion develops into an oscillation.

I remember making circular paper airplanes that were amazingly efficient. No idea why they worked so well.

On the vertical tube going up, could it be that the area of the tube wall thickness under it, is enough tocreate the lifting force? The top will have higher speed air passing on top of it, but the bottom no. As it raises, the forces equalize, as more air passes thru the gap. Maybe make walls thicker, think like a holed cylinder. Love this videos man. You bring curiosity, explanation, experimentation, and thinking.

Flow in the pipe, the speed increases with the length. A long enough straw and you blow into it, the exit speed at the other end would be speed of sound. Basically makes a invisible cone inside the straight pipe.

For your vertical test, my thought is that it's a variation where the air is primarily moving the fastest. At the start, air is pulled through the top of the tube, which creates a low pressure region above the tube. Even though the cross sectional area of the tube is really small, the force of the atmosphere is huge with a relatively small pressure difference, and the force on the bottom edge of the tube pushes it up. As it moves up, air starts to come from the interface where they were previously joined, which makes a low pressure region there (or at least reduces the difference in pressure compared to the top), and the tube is forced back down. You tried the disc on the outlet by itself, but on your vertical test, what if you put the disc on the bottom of the tube (at the interface with the vacuum) - i would expect the effect to be larger if the above is true.

I made videos about this for space craters and mountains...heres the experiment...level of dust in a layer ( about 1 inch)...put the vacuum above and lower it slowly ( min was a 2 inch vac tube)...it will create a mountain as it is pulling in air literally Horizontally in a vortex (not really from the vertical ,as it is in a vortex ( is this a lower pressure situation?)) the vortex goes INTO the vacuum tube so you create a mountain...so in my opinion, the paper rises and floats ON the air going IN. ( and perhaps some rise from a negative pressure in the vortex)

when in doubt, the answer is low pressure probably! lol i never fully understood fluid dynamics to that degree and luckily i don't have to for my work. i've learned that localized low pressure in a fluid can cause all kinds of very strange behaviors though. its similar to the "add a chunk of rubber" rule to solve vibration problems. i don't really know exactly how or why harmonics work, but i know gluing blocks of rubber to something will change its properties and make harmonic problems go away haha