Welcome back to Smarter Everyday.

This is Steve Mould.

- Hi.

- You've seen him, you've seen his face,

when he did the chain, what's the deal?

- Well, it's called The Mould Effect.

- [Destin] Is it really?

- [Steve] Honestly?

Because yeah, some scientists named it that in a paper.

- I have always wanted something named after me.

It would be something stupid,

if it was like the Sandlin,

anyway, long story short, we are in an argument.

And we are gonna settle it with internet physics videos,

which is the appropriate way to settle an argument.

If you've never seen a nerd fight,

they look just like this.

Steve has proposed the building

of a spiny water apparatus

and he's telling me he thinks

the water's gonna sling out of it in a certain direction.

Now, Steve seems like a pretty smart guy,

if you can look past the British accent and all that,

but I just don't agree with it.

Somewhere along the argument we decide

that it's going to take a phantom camera

to figure out exactly what's going to happen

with this device.

But at the end of the discussion,

I started to think that maybe

Steve has already built this thing

and I'm just getting played.

Anyway, this is how that discussion ended.

- We made an agreement.

And what did you say about being in Alabama?

- If you make a handshake in Alabama--

- Yeah.

- It's a contract, all right.

- And we already did this, right?

- Shut up.

(Steve laughing)

I am on a train in London,

about to go see Steve Mould.

I have the high speed camera.

I've been tricked, I'm pretty sure.

Whenever I visit someone whose videos I watch on YouTube,

I'm always really excited to see

where they film in real life.

Okay, so this is the studio, right?

(laughs) It looks so much bigger on YouTube.

- [Steve] Yeah, where's the red pipe?

- [Destin] Everyone always says it looks smaller but-

Once we were in Steve's back garden,

we set up the slow motion camera

and finalized the terms of our disagreement.

- To state the question simply,

if you spin it and then you take a snapshot,

what does the snapshot look like?

- [Destin] Okay, so which direction is it spinning?

- So your description was this.

You said that, "The water would come out,

and then it would kind of go like this,

and it would be back here like that," okay?

So we take a snapshot and it looks like that.

- [Destin] Well, I think that happens

but I think it also goes in the direction of rotation.

- Okay, this is just a way to think about it

in two steps, okay?

Because if the question is too complicated,

just simplify it and then you might get there.

Okay so this, the jets are just coming out like this, okay?

- [Destin] Okay.

- And so we're going to spin it this way.

And then now, maybe, you have an intuitive understanding

of what's going to happen.

And when you take a snapshot,

Destin, what do you think?

- [Destin] So we have angular momentum.

Angular momentum there, right?

- [Steve] Yeah.

- [Destin] And so I would think that

as a particle comes out here,

it has some sidewards velocity.

That's the term, right?

Sidewards?

- Sidewards, yeah.

- [Destin] Yeah, yeah, yeah (laughs).

- [Steve] Let's say tangential.

- [Destin] Well, I'm just gonna say sidewards.

(Steve laughing)

I'm from Alabama.

So it's got some sidewards direction here.

And so I think what's going to happen

is you're going to see a curve get thrown out like that.

- Okay, so you think that it's spinning in this direction,

the curve is going to go like that?

- [Destin] Yeah, I think so.

- Okay (laughs).

- [Destin] Am I wrong?

- That's like the opposite of most people's intuition.

- [Destin] Why?

- Because, okay well, most people would say,

look, it's going to come out here,

and it's going to trail behind.

- [Destin] Oh.

- That'd be most people's intuition.

- [Destin] Well, I think it's-

- Well, should we see what happens?

Let's do this then.

Like this is the simpler puzzle

so let's just try this one.

- [Destin] Yeah okay, let me set the phantom up.

(Destin laughing)

Am I wrong?

- I think so, yeah.

- [Destin] You think I'm wrong?

- I think you're wrong even about

this (laughs) the simpler version.

- Okay so here we go,

the first question is what happens with

just straight water coming out?

And you think what's going to happen?

- So I think you're going to get this curve of water

that trails the motion of the spouts.

- But, yeah, I think it's going to trail

the motion of the spout, but it still has motion going

in the direction of rotation.

- It does, but,

it does--

- You're trying to make me look stupid,

is what you're doing.

- [Steve] I just gonna get absolutely soaked.

(both laughing)

- [Destin] I'm going to get my feet up.

(both laughing)

Okay I'm ready.

- [Steve] Ready, Ready?

- [Destin] Yeah.

(Steve laughing)

Oh that's pretty.

- All right, there you go.

- [Destin] That's really pretty.

Okay, so let's take a look at that slow motion,

and see why Steve and I both agree, and disagree.

- [Steve] So yeah, it's kind of, it's trailing.

That's what I meant by trailing.

It's behind the direction of motion, right?

- [Destin] Okay, granted. Granted.

But any individual particle

is actually going out and away.

- Yeah, it is yeah.

- So I'm not wrong.

(both laughing)

So this is a language problem.

If you look at it from the snapshot view,

you could say it's trailing.

But I'm an American, and I think about what that particle

is seeing and experiencing on its own,

and Steve's wrong, in a British sorta

always right, kind of way.

As water exits the spout,

let's track any one droplet,

and try to draw what it does.

There are two things happening here.

Number one, it's being shot out of the spout.

That gives it an initial velocity in this direction.

Number two, it's being slung around the wheel by rotation,

which means when it exits the spout,

it also has this velocity going in this direction.

Now that we have those two components laid out,

if you add the arrows together,

that gives you the final direction of the water

when it leaves the pipe.

And if you'll notice, it's not actually trailing the pipe,

it's moving in a straight line forward.

If you were to draw a circle in your mind

around any drop that's on the screen right now,

and follow it,

you'll see that it's moving in a straight line

forward from the direction that it left the pipe.

It looks like it's trailing, but it's not.

That's an optical illusion.

- So, we're now going to have the water jets

firing into the center.

- [Destin] You made this just to, just to get on to me.

- [Steve] Yes.

This was just to get onto your YouTube channel.

(both laughing)

- [Destin] Well it worked.

(both laughing)

So, my guess is,

if we follow the same logic that we had before,

and we have the rotational velocity, omega,

times the radius, 'r',

then that's going to make this go in this direction.

But the water jet is going to be coming up

and at you like this,

so I would expect the snapshot.

Oh god, I see the problem now.

(both laughing)

So when we start it,

it's going to all go to the middle,

but as you start to accelerate,

I think it's going to,

the snapshot's going to look like it's lagging behind,

and then as we get faster and faster,

it's going to look like its going forward?

- Okay, that's interesting.

So at some, there's a certain speed you think where,

the speed of rotation, where it's going to

come back to the middle.

- [Destin] We're talking about the snapshot right now?

- Yeah, yeah.

- [Destin] I think that's possible.

- Okay, yeah.

What I like is,

and what most people's intuition is,

you would expect it to lag behind, right?

- [Destin] I think it's going to start lagging behind,

for a snapshot, and then it'll speed back up.

- Okay, well let's find out.

Okay I'm going to turn the water on.

Are you clear?

- [Destin] Oh that's beautiful.

- [Steve] Is it hitting in the middle?

Nice.

- [Destin] Almost. It wants to.

We can tune it right?

- [Steve] Yeah.

- [Destin] Oh that's pretty.

(Steve laughing)

All right.

- [Steve] You ready?

- [Destin] Yeah.

- [Steve] Okay.

- [Destin] Oh, that's pretty dude!

- Yeah, it's cool, huh?

- [Destin] Ah!

That's awesome.

It feels weird, like it looks--

- [Steve] It just doesn't look right, does it?

- [Destin] No.

It doesn't look right at all.

- Okay, I'm just going to try and go as fast as I can,

just with my hand.

- [Destin] But if you go really slow,

does it lag at all?

- No as soon you start to move it.

- [Destin] It always go forwards.

- As soon as you start to move it,

it goes forward.

- Okay, I was wrong, but,

Steve was right (laughs).

Let's run the slow-mo.

(creepy upbeat music)

Okay what we're seeing here is fascinating.

Part of it's obvious, part of it's not so obvious.

You remember before when we said

you had two velocity components, right?

Number one, we have the water that's

shooting out of the spout, which gives it

a velocity in this direction.

And number two, it's being slung around by rotation,

which gives it velocity in this direction.

Just like before, let's add those arrows up,