(smacking)

- What's up, I'm Destin, this is Smarter Every Day.

This is the tip of a bull whip

and that crack you hear is this breaking the sound barrier.

My question is why or how?

Like, if you think about it,

your arm's never leaving your body

and something's going faster than the speed of sound

in just a few hundred milliseconds and over several feet.

That's a big deal.

OK, now would be an excellent time for me

to explain April, April Choi.

So April is an engineer first and foremost.

I think all this whip business

is just a reason for you to explore--

- Fluid dynamics? - Fluid dynamics.

(cackling) I really do.

So April on the internet, you may have seen her,

Guinness Book of World Records whip stuff,

she's good with whips.

But what's really interesting about April, is your brain.

- There's something in fluid dynamics

known as the no-slip boundary condition.

That means air molecules that are right next

to this fluffy part stay next to that fluffy part.

- [Destin] And so, it's pulling that air with it.

- It depends whether or not you're using

a Lagrangian framework, which centers on here,

or an Eulerian framework that centers on the overall mesh.

- That's what I was thinking.

I was wondering if it was

a Lagrangian or Eulerian framework,

but I wasn't going to say anything.

The first thing we did was create a new tip

for the bull whip and attach it to the whip

and after that we set up the camera system.

The way we're getting this shot

is using the schlieren technique,

and this is what took us so long to coordinate.

Basically, we have a point-light source right here

and that light is coming out, it's spreading out,

it's hitting this mirror, this parabolic mirror,

and as it comes back, what it's doing

is it's converging to this point right here.

You can see there's the light coming through

at the focal point.

And then we've got red and green gels right there

and (whip cracks).

(laughing)

That's scary.

Go watch Derek's video on schlieren,

it's better than this one.

We're just gonna show you

how a whip breaks the sound barrier.

That is unnerving.

After everything was set up, we literally got crackin'.

(whip cracks)

OK, that triggered.

Let's see what it did.

We learned two major things in my buddy's garage.

First a question, though.

What point in the whip extension

do you think the crack happens?

Growing up, I used to play Castlevania a lot,

so for me, it made sense that the crack of the whip

would happen at full extension of the whip

'cause that's what you want to do with the bad guys, right?

You want to keep them as far away from you as possible.

When we set up a high-speed camera

expecting the whip to crack

like it does in Castlevania, the shockwave

would always enter the field of view before the whip did.

(thundering)

Therefore, it was clear that the crack was happening

before the whip was fully extended.

So it's cracking way back there.

- It's cracking before we think.

- I learned something.

I didn't know that. - Yeah.

I didn't know that either.

- It actually happens as the whip unrolls,

not at the end like I thought.

And in order to visualize what's happening,

we switched from the overhand strike

to the sidearm strike.

What you're about to see here are two engineers

that have researched this stuff

and were totally blown away because the experiment worked

and we're starting to see things for the first time

that we totally didn't expect.

OK.

We are getting somewhere.

(laughing)

(whooshing)

(thundering)

(snapping)

The second thing that we learned in the garage

is there may be a mechanism that's causing it

to accelerate just before breaking the sound barrier.

(whip cracks)

Those strands right there are not in tension.

You see that?

- Yeah, they're just, it's chaos.

- [Destin] And then there's this moment--

- [April] Where they all come together.

- [Destin] Where they all come together

and when it starts to pull,

that's when the initial shockwave starts.

- [April] So it's the collapse.

- [Destin] The collapse is when it happens.

- [April] And the drag coefficient's going down.

- The fact that we're seeing a new mechanism

is a really big deal.

So obviously, we have to take this more seriously.

We just figured out how whips work.

We should totally publish this.

- Yeah.

- Whip shockwaves have been studied

from the experimental perspective in Germany in 1998

as well as the theoretical perspective

at the University of Arizona in 2003.

The Ernst-Mach Institute paper, by the way,

freakin' amazing.

There's a dude in it that looks

like a moose wearing bells and a clown suit.

(laughing)

I don't know what's happening.

It's actually a great paper.

You should totally read it.

They talk about wishing they had a faster high-speed camera

so they could see what happens at the tip.

Also, the paper at the University of Arizona,

they try to measure with math

the entire length of the whip as it unrolls.

They try to describe that movement.

But what if we could design one experiment

that would do all of this?

Like all of it at the same time.

It would measure the three-dimensional position

of the wave as it goes down the whip.

It could also measure the tip velocity

as it goes supersonic.

What if we could do that?

And that's exactly what we're about to do.

Under the guidance of my doctoral advisor,

Dr. Kavan Hazeli at the University of Alabama in Huntsville,

we've assembled the team and we're about

to figure this junk out.

We designed the experiment and gathered together

in what's called the atom lab.

It uses an array of cameras to track

anything with reflective tape on it.

The way it works is essentially this.

You have a camera and you have

a little infrared light around it, right?

If you have a piece of reflective tape out there

and you shine the light from the infrared camera

onto the reflective tape, it bounces back to the camera

and it shows up as a really bright dot.

So we simply put reflective tape around the whip

every 250 millimeters down the length of the whip.

We also put reflectors on her arm

so we could better understand

the mechanical input to the whip.

The image from one camera would essentially

be an array of white dots at 500 frames per second,

but if you coupled this data with the data

from other cameras, you can triangulate

each individual segment of the whip

at 500 frames per second giving you

true three-dimensional data.

OK, here we go.

This is the kind of data we can now get from a whip strike.

- [Man] Three, two, one, go.

(whip cracks)

- The footage you're now watching

is 5,000 frames per second.

You can can that the Vicon cameras up on the wall

are taking data at 500 hertz,

which means they're flashing every 10 frames

on the high-speed camera here.

You'll notice that the whip unrolls normally,

very similar to how the paper

from the University of Arizona

described it mathematically.

Let's make a few observations here.

First, there seems to be a wave that moves down on the whip.

As the hand moves forward and then stops,

it transfers momentum into the whip itself.

Then, one segment of the whip,

as it unrolls and straightens out,

seems to transfer all of its momentum

into the next segment, and then the next segment

and so on and so forth.

As indicated by this red line moving along the bottom here,

you can see the velocity of that straightening out

of the whip moves forward.

We can then look at the atom lab data

and measure the input momentum in three dimensions

and use that information as a tool to help us build a model.

So the whips coming up towards the mirror.

(muffled mumbling)

That's awesome.

Is that awesome?

- Yes.

- Another thing to look at

is what's happening on the top of the whip.

The velocity is speeding up.

Most researchers think this has to do

with the conservation of momentum.

The whip is tapered so each smaller section

on the way down has to speed up

to maintain the same amount of momentum.

This is the exact reason we took so much time up front

to measure the mass and dimensional properties

of the whip all the way down.

This is where it gets most interesting for me.

If you look closely at the atom lab data

you'll notice that right at the tip of the whip

the markers seems to disappear

right when the whip accelerates.

This is because the trackers lose the position

of the whip markers when they're traveling their fastest.

Even if the atom lab didn't lose the track,

you can tell that the frame rate of the atom lab

isn't sufficient to determine the acceleration

through the most interesting part of the wave,

which, of course, is the shock formation.

This is exactly why we set up the schlieren camera.

The atom lab gets all of the wave kinematics

on the macro scale and then the phantom can record

the tip velocity and actually capture

the formation of the shockwave.

(light guitar music)

(whooshing)

I'm not gonna explain any of our preliminary conclusions

but at this point we're doing two types of analysis,

obviously how that wave propagates,

but also the tip velocity of the whip.

If you watch closely, it looks like the tip's

getting pulled along behind that shockwave.

This is super complicated and we're still analyzing this.

(whooshing)

(smacking)

What we do know is that the popper isn't necessary.

Dr. Kanistras really wanted us to visualize

the end of the whip with only a knot on it

and just look at how happy he was

when his hypothesis was proven correct.

- There you go.

There you go.

There you go.

- So it's like this.

For the first time in history, we have true X-Y-Z data

from the handle all the way to the tip of the whip

and we can straight up write an equation for whip dynamics