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.
Uh, so
I've got this, uh, garden hose and the head is set to power jet mode.
All the water is coming out of 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 humus 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,
It would travel even faster away from the jet.
But watch this:
It sticks to it.
You can, sort of, move it around.
Isn't that weird?
So that's what I wanna try and explain.
[Hushed] Ah my god
To try and explain it, I want to show what I think is the same effect
But with air instead of water.
And for this you need a hairdryer and some polystyrene
Uh, and a second sheet of polystyrene, like this.
If you're gonna 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 hand down.
But if I lift it up to a certain point
It gets sucked up.
So there you go, the polystyrene 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 intuitively this 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.
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 a 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 centre.
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 gonna go down somewhat
As the air streams outwards.
And it seems like, at least for the case of the hairdryer and the polystyrene,
That the drop in pressure is enough
So 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 polystyrene pulling it down.
So, just so you've got the whole picture then,
You've got normal atmospheric pressure underneath the polystyrene, 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 polystyrene pulling it down.
And they're all in balance,
So the polystyrene doesn't fall
My theory is it's the same principle in play with the water jet and the humus tub.
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,
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, uh, I get that, I like shouting "Bernoulli" as well.
Um, but I didn't use the Bernoulli to explain what was going on here
Uh, and the reason is
Bernoulli's principal is a derived principal.
Um, 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
I don't think it's useful
For, um, giving someone a truly intuitive understanding of what's going on.
One last thing I wanna show you that's somewhat related
But probably has a slightly different explanation:
Take a hand blender, like this one,
And, uh, a container that is slightly larger than the hand blender
Uh, put some water in there
And turn it on:
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 kilogramme
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 dunno.
I feel like it's got something to do with surface tension, as well.
At least, surface tension would certainly play a part.
Probably does in the downwards water jet phenomena, as well
Anyway, lemme know what you think in the comments.
I built those animations in JavaScript and HTML5 canvas,
Which is something I didn't know how to do, until I learned.
These days, I really see the benefit of taking online video classes,
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I hope you enjoyed this video.
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