that it's not always the most athletic team that wins in sport, sometimes it involves

the physical manipulation of objects, so sometimes it's the most intelligent team.

So today, on Smarter Every Day, let's take a look at the physics of curling.

[music]

OK before we watch some curlers we need to learn the basics of the sport.

This is the curling sheet and the circles are the house. The goal is to get your team's rock

closest to the button. There's four people on each team. The thrower,

the sweepers and the skip who's in charge. Each team has eight stones

to throw, so each person throws two. They alternate with the other team

so there's a total of 16 stones thrown. The very last one is called the hammer,

which is a major advantage. Do you have any idea how difficult it was to find a

curling stone in Alabama? It is really hard. Anyway, so I know what you're thinking. Curling's like

the caveman sport right? I'm gonna slide this rock on ice and I'm gonna hit another rock

and we're just gonna try to out-rock each other. But oh no, it's way more difficult than that.

In fact there's so many things I had never even considered until

I took a closer look at how this works. For example, the simplest question of them all.

What makes a curling stone curl? OK let's pretend for just a second

that this isn't my coffee table, it's actually a curling sheet.

So we know from watching TV that when a player is back here at the hack, which is where they start,

and he pushes it toward the house where you're at, which is the bullseye on the ice, as he

rotates it or spins it counter clockwise it'll curl in the direction

of that rotation, right? Now my assumption is that has something to do with this,

which is called the running band. You'll see the bottom of the curling stone is concave

but there's this circular frictional interface that interfaces with the ice.

So we should be able to model a circular frictional interface of a moving

sliding object on a rigid surface right? Which is this,

a glass. I'm gonna take this circular object, I'm gonna put it down on the

low friction surface, I'm gonna push it towards you and spin it, and expect

a curl in the direction of rotation. Let's give it a shot.

But I don't see that. Let's try this again. Set this down,

push toward you, spin it, curl. Here we go.

No. It curls in the opposite direction. This is actually what's really

confused scientists for a really long time. That interface of a normal

moving sliding spinning object on a rigid surface, behaves completely

different with normal objects than it does with a curling stone.

There's something magical happening right here on this running band between the stone

and ice. So clearly the next step is to go find us some ice.

Ohhh!

I wet my pants. Let's try again.

Oh man. So the curling stone

goes in the direction of rotation but the cup goes opposite, which I can understand because

as it's decelerating on the table it's trying to tip over, causing more force

on that leading edge of the cup. So when it's spinning it's pushing against

the table with its leading edge. That makes sense to me. So let's go to the curling club

in Milwaukee Wisconson and see if they can teach us something with prepared ice

and skill, most importantly. - We are at the Milwaukee Curling Club. We are located

at the fairgrounds in Ozaukee County. This club is

the oldest continuous curling club in the United States. (Destin) Before a game can be played the ice has

to be properly prepared, which is a science within itself. If a stone

rests on flat ice it creates a lot of contact friction which makes the stones run

slowly. Curlers use an intricate technique called pebbling to decrease the friction

of the stones on the ice. Deionized water that's been purified by reverse

osmosis is sprinkled onto the ice in a very specific way and allowed to freeze.

Here you can see Jay pebbling the ice by swinging a sprinkler nozzle

back and forth. You can't imagine the amount of variables that have to be controlled during this process.

Number of arm swings, how fast he walks, humidity,

the difference in height between the tank and the sprinkler head, the temperature of the water, yeah.

It's pretty crazy. After pebbling they use a blade to nip the top of the pebbles off to

create a smooth uniform running surface. Because there's more pressure on the tops

of the pebbles, there's more friction melting which causes less friction.

Which leads us to sweeping. Before I researched curling I thought the sweepers

were somehow increasing or decreasing the friction on one side of the stone or the other and making it curl.

I was absolutely wrong. The sweepers actually sweep in order to heat

the ice up and make it curl less. If you threw two stones exactly the same

and you didn't sweep one but you swept the other, you would find that the swept one

would go straighter and farther. This is a scanning electron microscope image

of a pebble on the ice. You can see that it's been nipped on the top so it's fresh.

This however is an image of a pebble after it's been swept. You can plainly

see that there's grain boundaries from where it melted when it was swept with a broom.

That thin layer of water that forms acts as a lubrication

barrier, reducing the friction and allowing the stone to travel farther and faster.

So this is where it gets interesting. So we're trying to figure out why the

curling stone curls right? So I look around and I find the international experts

for curling physics. I find the guy in Canada, and I find a team in Sweden

and so I start reading all their papers. Turns out these guys are not even close to

agreeing. It's really interesting. In fact they've never even communicated

with their voices. They only communicate via technical paper. Fascinating.

I called the guys up on the phone and I had like an hour and a half conversation with both

groups, trying to understand what exactly is the mechanism.

Harald Nyberg at Uppsala University in Sweden explained something to me

called the scratch theory. Visualize a stone rotating and moving down the ice.

Now think about the running band and what the scratches that would make

look like as it goes down the sheet. The edges of the running band would make this really

awesome overlapping pattern as it slides down. The Swedish scientists

say that because the rough spots at the rear of the band

have to hop over the scratches created by the leading edge of the running band, this will

induce a force on the rear of the stone making it curl in the direction of rotation.

They claim to prove this by showing images of pebbles

that have been scratched at an angle after the stone slides over them. They also did an experiment by scratching

ice really deeply with sandpaper at two angles and pushing a stone

across the scratches without rotating it.

Pretty convincing right? Not so fast. Dr Mark Shegalski at the University of

Northern British Columbia in Canada once threw a stone with a polished metal running band

which doesn't produce the same type of scratches. He observed that it

curled like a normal stone by throwing it on freshly pebbled ice.

Dr Shegalski believes that the mechanism is something called asymmetric friction melting.

When the stone travels over the ice the friction heats up the ice and melts it,

creating that lubrication barrier that we discussed earlier. He believes that there's

more frictional wetting on the front side than the back due to the rock tending to tip over

just like the cup did in our earlier experiment. Another possibility is that

because the side of the rock that's advancing moves faster relative to the ice than the

retreating side, it could be creating more lubrication. You can visualize this

by looking back at the difference in the contact patterns. This additional

relative motion would create more frictional heat up on top, which would melt more ice.

This water could then be transported forward by the rotation and

lubricate the leading edge of the band more than the rear edge. That forward tipping of the rock

or the water transport theory pushes the rock into the direction of rotation.

Both scientists are convinced that their theory is the dominant mechanism

taking place at the back of the running band. But they agree however that more work still

needs to be done. Personally I don't think either one of the theories can stand up to all of the questions

on its own. I think the ultimate model might be a combination of both of the theories.

Dr Shegelski believes there may even be 1 or 2 other mechanisms at play here that would help

describe the mysterious motion of the curling stone. Who knows,

what I do know however is that the nations that have scientists researching the physics

of curling are the same nations that most often have olympic athletes

on the podiums for curling. Hey I have a huge announcement here

on my way home from work. I'm not sure if you can tell but these videos take a lot of time

and effort. Yeah you can, you're smart. You know what I'm doing here. You know how there are two different

experts for curling right? And I consulted them both to get the best idea.

Well I've done the same thing for something else. Jack Conte from Patreon

and Hank Green, co-founder of Subbable, have created two different platforms that

content creators like me can use to try to generate support for what they do.

Now the idea is if you enjoy and place some kind of value on

Smarter Every Day, that you can voluntarily... You can, I'm not saying

do, but you can voluntarily decide to assist

what I do. Anyway, you can go to either one of these pages, Patreon or Subbable, and there's all

kinds of different perks on there. There's ways to reach out to me, there's

posters, there's all kinds of stuff. Infographics. So Patreon is a

per video model, and Subbable is a per month model. Now I didn't know what

would work best so I contacted both these guys and decided to test it for myself

to see which one works the best for Smarter Every Day. Anyway, you can test them

and see what works best for you. Anyway, I'll leave links to the Patreon and Subbable

Smarter Every Day pages and if you would be willing to support Smarter Every Day, that would be awesome.

It would make my life better because I can streamline my workflow

and probably be a better dad because I might have a little more time. Anyway, I'm Destin.

Thank you for even considering that. Subbable and Patreon, Smarter Every Day.

I'll leave links here and in the video description. Thanks, bye.

[laughing]

You're trying this at your house right? [Glass breaking] Oh

[whispered] crap. I just broke it.

Don't tell on me. In physics there's a principle called the conservation

of momentum, so if momentum is mass times velocity and the curling stones

all have the same mass, you could assume that it's velocity that's conserved.

For the most part, the forward velocity of the stone just before impact is gonna be

equivalent to the forward velocity in the system just after impact.

This works for both the X direction and the Y direction. Since there's no sideways

velocity before impact, the lateral velocity after the impacts has to equal out

to zero. Isn't that cool?

[ Captions by Andrew Jackson ] captionsbyandrew.wordpress.com

Captioning in different languages welcome. Please contact Destin if you can help.