How fast moving water “defies” gravity

I've been putting off making this video because it's a really interesting phenomena but I was never sure of the explanation I think I've worked it out now I'm not totally sure but I thought I'll make the video anyway and then you can decide in the comments what you think about my explanation so I've got this garden hose and the head is set to power jet mode all the water's coming out just a small hole in the front there and just to demonstrate how powerful the jet is I've got this plastic tray it's made from the bottom of a hummus carton and I can launch that into the air what's really interesting is if I point the whole thing downwards like this you'd think with the extra help of gravity would travel even faster away from the jet what's this it sticks to it you can move it around [Music] that's what I want to try and explain to try and explain it I want to show you what I think is the same effect but with air instead of water and for this you need a hairdryer and some polystyrene and a second sheet of post I mean like this if you're going to do this make sure you've got a hairdryer where you can turn the heating element down to zero so you blow the air out I can feel the jet of air pushing my hands down but if I lift it up to a certain point it gets sucked up [Applause] so there you go the poorest timing is defying gravity as some people like to say of course it's not it's just there's some force counteracting gravity pushing up let's see if we can figure out what that force is think about the air flowing out of the hairdryer it's being forced to travel through the cavity between the two layers of polystyrene because of the air pressure produced by the hairdryer which I'm going to assume is fixed so I've created this animation of the particles in the air this animation is missing certain details like it doesn't show any thermal energy because the temperature of the air is somewhere above absolute zero it has some thermal energy so really these particles in the air should be bouncing around in all different directions hitting each other and that sort of thing so what you're really seeing is the average smoothed out motion of the particles ignoring thermal motion I've created the animation in such a way that the density of particles is the same everywhere so if you look at the density out here towards the edge it's the same as the density towards the middle that's because I'm assuming that the pressure is the same everywhere and if you want the pressure to be the same everywhere you need the density of particles to be the same everywhere and you know this intuitively from playing with an air filled syringe if you compress the syringe you increase the density of the particles inside but you also increase the pressure you feel that increased pressure as the syringe pushing back against your hand but is it right to assume that the pressure is the same everywhere probably not and actually we've neglected to think about the velocity of the gas particles because velocity doesn't change like an object's velocity will stay the same unless that object is acted on by a force like if an object is drifting through space it will continue to drift at a fixed velocity unless a force acts upon it and that's true of the particles in this gas as well and we're interested in the sort of net velocity the outward drift velocity of the particles as opposed to the thermal velocity of the particles so in my animation to keep the density of the particles the same I've had to slow them down as they travel outwards but let's for a minute assume that the particles aren't being acted on by force so they wouldn't slow down their velocity would stay the same what would it look like so here's a different version of the animation where the velocity of the particles don't change and you can see how the density changes as you move out from the center there's just more space around the edges than in the middle so there are fewer particles to go around per unit volume the reality is probably somewhere between the two animations but the point is the density of particles is going to go down somewhat as the air streams outwards and it seems like at least for the case of the hair dryer and the polystyrene that the drop in pressure is enough so is that the difference in pressure between the air inside the cavity and the air underneath the cavity is great enough to overcome the weight of the post I mean pulling it down so just so you've got the whole picture then you've got normal atmospheric pressure underneath the post I'm pushing it up you've got the pressure inside the cavity pushing it down but that's less than atmospheric pressure and then you've got the weight of the post own pulling it down and they're all in balance so the post Dome doesn't fall my theory is it's the same principle of play with the water jet and the humus tab which explains why you can't get it to work with just the end of a hose you need to have a small jet embedded within a flat plate look like we've got here and we have the same thing with that power jet hose connector that I was using a lot of people are probably now shouting at the screen Bernoulli it's just Bernoulli say Bernoulli and I get that I like shouting Bernoulli as well but I didn't use Bernoulli to explain what was going on here and the reason is like Bernoulli's principle is a derived principle what I mean by that is you can look at the underlying principles of fluid dynamics and you can derive this thing and you can say look by rearranging all these equations and stuff I can show that this thing is always true this simple relationship between pressure and velocity is always true and that's really useful if you're doing fluid dynamics but it's I don't think it's useful for giving someone a truly intuitive understanding of what's going on one last thing I want to show you that's somewhat related but probably has a slightly different explanation take a hand blender like this one and a container that is slightly larger than the hand blender put some water in there and turn it on [Applause] the effect is actually really strong using these travel scales it looks as if the pulling force is equivalent to the weight of almost a kilogram under gravity but is it the same effect as the downwards jet of water maybe not actually the blades of the hand blender act like propellers they draw liquid upwards so it's different in that way I don't know I feel like it's got something to do with surface tension as well at least service tension would certainly play a part probably does in the the downwards water jet phenomena as well anyway let me know what you think in the comments [Applause] [Music] [Applause] I built those animations in JavaScript and html5 canvas which is something I didn't know how to do until I learnt these days I really see the benefit of taking online video classes and I'm really happy that the sponsor of this video is the best provider of video classes that I've been able to find which is Skillshare an online community of learners that has thousands of classes covering creative and entrepreneurial skills you know in the old days if I'd had a challenge like this I would have tried to hack something together with code I found online and eventually I get it to work but I wouldn't understand why or how but this time I started by taking the get creative with html5 canvas course on Skillshare and yeah of course I still Nick code online because that's how you code but the process of getting that code to do what I wanted it to do was much quicker because I truly understood how the coding works and that's what I see as the benefit of online video classes it's like an acceleration that gets you to where you want to go quicker and it's a speed boost that stays with you like the next time I need to solve a problem like this it will be quick again because I still understand you might also like their basics of web design layout class or if animation is your thing they have classes in other technologies like After Effects it's really affordable ten dollars a month for an annual membership but because they're sponsoring this video you can get a two month free trial if you go to school Dutch forward slash Steve Moll three that's school Dutch forward slash Steve mold three the link is also in the description check it out today I hope you enjoyed this video if you did don't forget to hit subscribe and I'll see you next time [Music]

Channel: Steve Mould

Views: 875,436

Keywords: Explained, understand, air, pressure, hose

Id: TrZyuCh9df0

Length: 9min 52sec (592 seconds)

Published: Thu Jul 25 2019

Reddit Comments

[u/Fmeson]

The examples are different as he says. I believe the immersion blender holds the glass up out of a simple suction force. The blender acts like a pump, pumping water quickly upwards. This creates a lower pressure below it and everything is pulled upwards, trying to compensate for the lower pressure. Gravity then returns the water to the bottom of the beaker and the cycle continues. This works as long as you have enough water that it gets replenished by falling water continuously. It’s basically a water version of holding up a sheet of cardboard with a vacuum cleaner.

This doesn’t actually require surface tension either as the lower pressure creates a suction force either way.