Physicists Still Don’t Know What Puts the Curl in Curling | Hacker News Reader

Physicists Still Don’t Know What Puts the Curl in Curling

newyorker.com

194 pointshoaxcracker87 comments

Smarter Every Day did a YouTube video on this (https://www.youtube.com/watch?v=7CUojMQgDpM) where he talks about the phenomenon and also talks to the exact same guys featured in this New Yorker article. I think he does a much better job than the New Yorker article, too.

Destin also notes that, countries that study curling tend to win olympic medals. He said that in 2014, and it's interesting to note that both Sweden and Canada won medals in curling in both 2014 and 2018. [Edit: Though I believe he did say it after the 2014 games.]

Canada has won medals in men's and women's curling every year since they reintroduced it in 1998. 2018 was actually the first year we didn't, winning only in the new mixed doubles event. The Swedish women have medalled in all but one.

So I wouldn't read too much into this. Countries where curling is popular tend to win Olympic curling medals, and countries where curling is popular also tend to study it.

This was the first year I watched the event. I knew "generally" what curling was but not I am hooked. I loved watching it. Going to see if there is a team close by.

It really is fun to at least try. I've only gone once, but had a blast.

For anyone that lives in a warmer climate, can't ice skate or wants more to watch I'd suggest you checkout lawn bowls (https://en.wikipedia.org/wiki/Bowls). The games are very similar and typically people that like one like the other.

Maybe, that is they add a lawnmower or rake in front of the bocci ball. :-)

Common mistake, but lawn bowling is very different than bocce. Bocce balls are uniform spheres that you can either roll or lob. They travel in a straight line. Lawn bowling balls are oblate spheroids that are weighted on one side so that they will always curve in a certain direction (more for light throws, so you can draw around a guard like in curling). They are always rolled. So it really is a lot more like curling than bocce, though granted there is nothing analogous to sweeping.

Thanks. Didn't know.

It might be the case that countries that invest in the study of curling will also produce better curlers. It might also be the case that a country that is mad about curling will produce good curlers, and have scientists who are interested in studying it.

It's a great video from SmartEveryDay though.

Also ~80% of the world's curlers are in Canada (1M+). They shouldn't be losing to anyone.

The rest of the world is quickly catching up though, and Canadian coaches are moving abroad to coach these teams. South Korea, China, Japan, Switzerland, and Great Britain are all now competitive teams that are coached by Canadians.

And the US mens curling team won in Seoul, featuring 4 guys from Minnesota and 1 from Wisconsin. Although it's a small state, Minnesota is a powerhouse contributor to winter sports like curling and hockey for the US.

Seems the obvious confounder for that correlation is that countries who win olympic medals actually care about curling. Whereas, those countries where there is not enthusiasm for curling, why would a researcher be inspired/motivated to study these phenomena? I doubt the researchers are directly affecting the play of any curler, since the researchers themselves can not understand the curling of the stones anyway.

I love when he says "and DONT kneel on the ice" as it shows someone kneeling on the ice messing it up.

> I think he does a much better job than the New Yorker article, too.

Not shocking at all! Destin is amazing and it seems like the New Yorker article as written by a wannabe comedian or something who was trying to work lame punchlines in everywhere instead of inform us of something.

The top YouTube comment on the video (by "The Beast") is pretty excellent: "This is all science friction to me."

Curling Canada has a video online (https://www.youtube.com/watch?v=50cSDUIDMuM) detailing the steps that a rink needs to go through in order to prepare their ice for a championship game. It's kindof amazing how many steps are involved, and I wonder if that has anything to do with it.

I never new so much work could go into curling ice.

Thanks for share!

This video was really awesome! Thank you for sharing!

That's incredible. I wish they said how many hours it takes to build that.

This paper came out and changed the game a few years ago. It's called scratch theory and the front edge of the stone scratches tracks for the back edge to run along.

There was a sweeping summit this summer to discuss brooms and how to make them scratch the ice less because it caused players to have extremely high stats for tournaments.

Brad Gushue video showing the effect: https://www.youtube.com/watch?v=haEuz42YCdM These brooms have been made illegal in tournament play.

Some Scotties players describing the effect: https://www.youtube.com/watch?v=147vRw0kmAY

The scratch theory paper: https://www.thesalmons.org/lynn/curling/A5.pdf

Investigating a complex physical phenomena that isn't tied to either fundamental research or immediate application = little chance of funding. The article is all "oh gosh, curling is so complex" but the real thing here is the limitations of our knowledge, and the ability to apply knowledge across abstraction boundaries.

One of the professors at the university I went to recently released some research on curling.

https://www.unbc.ca/newsroom/unbc-stories/researchers-discov...

For those of us who don't curl,

"It’s like golf: it’s easy to watch a guy hit a golf ball, and you think, ‘This isn’t very athletic.’ And then you get out there yourself and find that it’s incredibly difficult."

Anyone know how much force is applied to the golf ball on a professional's drive?

Tour pros routinely swing driver at 120-125 mph, and produce ball speeds of 180-190 mph. I’ve seen numbers in the range of 2,000 - 4,000 pounds of force applied to the ball.

If you're #50 in the PGA in terms of average ball speed [0] and drive a regulation ball (46g)[1] at 175 mph (78 m/s), that ball has an energy of 140 J. If it accelerated from 0 to 78 m/s in 0.5 ms [2], it accelerated at 156 km/s^2 for that short time, at a power of 280kW (375 horsepower). The force applied to the ball was therefore 7100N, corroborated here [3].

A driver is 45" standard, but assuming that you don't hold the club at the tip we can call it 42" or 1 meter. You don't rotate the club where you hold it, but rather roughly at your shoulders (I will neglect hip rotation), and the average arm length is 25". Arms are not locked straight so let's call it a little shorter, and add 0.6 meters for the arm length.

You now have a 1.6m moment arm applying a force of 7100N through the ball, requiring 11.4 kN-m of torque, or 8400 lb-ft. Edit: I have discovered that wrist torque is significant, and that your wrist and your arms individually supply about 150 and 400 N-m of torque respectively. [4]

To compare, a high-end sports car might have 500 HP (compared to the golfer's 375 HP), and the Tesla P100D has 920 lb-ft of torque.

Of course, the golfer sustains this output for only half a millisecond, and a lower output during the rest of the swing (around 1 second). A car can do it until the tank or battery is empty.

Also I have neglected hip rotation, differing driver and arm lengths, and energy loss due to deformation. But this is a decent first approximation, and at very least in the correct order of magnitude.

[0] https://www.pgatour.com/stats/stat.02402.html [1] https://en.wikipedia.org/wiki/Golf_ball [2] https://dspace.lboro.ac.uk/dspace-jspui/bitstream/2134/11470... [3] https://hypertextbook.com/facts/2001/EmilyAccamando.shtml [4] http://rspa.royalsocietypublishing.org/content/465/2102/551

The peak power comes from the inertia of the club, not the player. The player puts out a much lower power over a longer period of time. 140j over the windup + swing is probably similar to what someone like a boxer needs to put out during a single punch.

Can't I hold both thoughts.

I don't think it's particularly athletic but I do think it's incredibly difficult.

It depends what we define by athletic. The old greek Ideal of a muscular, and healthy body is not something I associate with the standard pro golfer.

I suspect why the stone curls in the direction of rotation is that the edge on that side is moving more slowly relatively to the ice. It melts the ice less, generating less of a film of water and so experiences more friction.

If an axle with wheels rolls such that the left wheel is on pavement and the right hits grass, it will turn right.

A glass on a bar table top doesn't melt the surface to create a film; more motion means more friction not less. So, opposite.

It probably doesn't matter which edge (front versus back) has more weight on it, but rather which side of that edge (left or right) experiences more friction. I suspect even if you slide the glass up an inclined bar table top so that the weight is on the rear, it will still curl opposite.

This doesn't explain why you get the same amount of curl if the stone rotates twice during its run or twenty times.

The delta of the ice on stone speed between the sides is not dependent on the rotational speed but liner speed in relation to the rotational speed.

You're wrong, and even if you were right that wouldn't argue against GP's observation.

V_cm: velocity of center of mass

V_l: velocity at left edge

V_r: velocity at right edge

R: radius of stone

w: angular speed of stone (CCW is +)

  V_l = V_cm + R*w
  V_r = V_cm - R*w
  delta = 2*R*w

Even if it was dependent on linear speed / rotational speed, the only way to change # of turns / run is to change linear speed / rotational speed.

This seems similar to the Magnus effect:

https://en.m.wikipedia.org/wiki/Magnus_effect

(Spinning a pen fast enough can turn it into a wing, causing it to behave strangely when tossed upward)

The faster the rock spins, the less it will curl. Ideal # of rotations is about 2.5 or so.

Water is complex; surface tension like effects can happen in thin films of it other than at the surface. At greater speeds, these are reduced so the Magnus effect is reduced.

So what allows for directional sweeping to make the rock curl even more than it normally would on its own?

And why does the same directional sweeping technique, except applied in reverse, cause the rock to curl less than it normally would on its own?

And why does applying sweeping with both techniques at the same time counteract the directional movement?

Roughening the surface by sweeping thins the film at many points. Your latter two points are more interesting; flow is unlikely, but directional "surface tension" effects (actually organized Van der Waals forces) are certainly possible, given that water is an asymmetrical molecule (which is why surface tension happens at all.)

As a follow up, what causes negative curl (when the rock curls in the opposite direction of rotation)?

Not something you are bound to see in professional level arenas, but not uncommon in clubs.

In fact, what makes club ice seemingly impossible to make behave like arena ice? There is a serious amount of research by Canadian universities that has gone into trying to make club ice act like arena ice for training purposes and the best we have been able to do is "almost the same".

Shitty ice.

To add: Curling on ice used for other purposes too leads to shitty ice. The ice needs to be flat and level. Zambonis do not lead to level ice. Dual purpose rinks likely also don't shave the ice to ensure level and flatness. Likewise, gouges in the ice from skates causes unpredictability and some might compensate by over pebbling prior to playing. Also poorly/sparsely pebbled I've can lead to unpredictableness.

That goes without saying, but what are the exact mechanics?

To your edit:

> Curling on ice used for other purposes too leads to shitty ice

Certainly skating ice is a whole other beast to curl on, but I'm talking about club ice: Ice that is only used for curling.

The club I curl at is strictly dedicated curling ice and most curlers there take care and pride in the ice and work to keep it clean and in to condition. But we are also open to introducing people and bringing new curlers in. Sometimes people don't wear ideal clothing. They will wear clothes that is linty/fuzzy. Some lint or fuzz gets in the ice and a slow curled stone goes over a piece in the ice which causes a lot of friction b pulling the stone and causing it to start curling the opposite way. That's what I often see. Another thing that sometimes messed with the ice and can cause negative curl is outside conditions. The building isn't perfectly sealed and there are doors to the exterior 3 feet from the outer sheets. If it's above freezing the warm air seeping in can cause the ice near the doors to be frosty or wet (or even colder than normal and harder to melt from some friction if it's extremely cold outside). Depending on conditions. This can mess with the friction between the ice and stone regardless of spin direction.

I'm not talking about picking, but more to your last point. However, I'm not sure that that still explains the exact mechanics at play.

Probably gravity. Shaving the ice in the same pattern every time, or excessive traffic on the edges of the sheet probably cause uneven surfaces over time.

Usually it's picked on something or it lost handle which may cause the rock to maybe start turning in the opposite direction. Or they just misread the call and threw the wrong turn.

I'm referring to when the rock simply curls the wrong way. Consistently, no matter who is throwing it.

It seems to be related to uneven cooling of the ice. It really becomes apparent on the southern-most sheet in the club I frequent when the late-winter sun starts beating down on the southern-facing wall next to it, but that is not the only place I have seen it happen.

However, outside of the high level reasons for it happening, I am interested in the specific mechanics, like were previously described for normal curl, that causes the rock to go the opposite way under these conditions.

As I mentioned in a different comment, it's probably just gravity. The rock slides against the curl because the ice is not level. Uneven cooling could cause the ice to freeze unevenly, or maybe even sublimate. Shaving patterns over time can also play a part.

Of course I'm speculating on the specific underlying cause, but I would be pretty surprised if the mechanic isn't just the rock sliding downhill.

A rock spinning too fast will melt more ice in front of the stones path reducing friction and lubing the ice in front and possibly pulling water over to the inside curl side leading it to fall forward more and catch less on the curl side.

P.S. This is just my speculation, not from physics but from being a curler.

Sure no problem, I curl myself :)

Except inverted. Left spin on a ball makes it go right by Magnus. Left curl makes a stone curl left.

I curl and play pool, so I've come up with reasons for the difference in my head.

The slower turning side creates more friction because it grabs the ice, the faster side glides over rather than grabbing. This is obvious when you intently watch as the curl picks up at the end of the shot.

QED, dunno why a bunch of non-curlers wrote these articles they should just curl.

>The slower turning side creates more friction because it grabs the ice, the faster side glides over rather than grabbing.

Don't both sides turn at the same speed?

Not relative to the direction of travel over the ice.

> The slower turning side creates more friction because it grabs the ice, the faster side glides over rather than grabbing.

For surfaces that do not melt to create a liquid film, this intuition is wrong. If any sliding is taking place, then static friction has been overcome, and kinetic friction is at play. Dry surface kinetic friction doesn't increase with speed: well, the coefficient doesn't but the actual force does!

https://physics.stackexchange.com/questions/48534/does-kinet...

Soccer balls also curl toward the spin. Kicking a ball to impart left spin will make the ball curl to the left in midair.

An example of this could be walking on a slippery surface, your shoes grip until a threshhold is reached where you slip and increased speed seems to reduce the friction. I imagine it like tiny bumps that repel the surface of the shoe so that there is less net contact. Or is that the second hypothesis in the article? The relationship between speed and friction is discussed here, also: https://physics.stackexchange.com/questions/29561/does-frict...

THE SOLUTION IS SIMPLE

I suspect that the reason curling stones curl the way that they do is primarily due to one side of the stone being subjected to “static” friction while the other side experiences “kinetic” friction. This would also explain why the curling doesn’t start to happen until the stone’s linear velocity has sufficiently decreased.

If a stone is moving down the ice, away from the observer, and spinning to the left (that is, counter-clockwise when looking down on it), the right-side edge of the stone is moving in the same direction that the stone travels while the left-side edge of the stone is moving in the OPPOSITE direction that the stone travels. This means that the right edge of the stone is moving faster across the ice than the stone’s linear velocity. And the left edge of the stone is moving slower across the ice than the stone’s linear velocity.

As the stone’s linear velocity decreases, there will come a point when the left edge of the stone is no longer moving relative to the ice, while the right edge of the stone is still moving at 2 times the stone’s linear velocity relative to the ice. This means the left side of the stone is now being subjected to static friction while the right side is only being subjected to kinetic friction. The force opposing the left side of the stone is greater than the force opposing the right side of the stone, causing the stone to pivot around the left edge of the stone, “curling” to the left, until the stone's linear velocity reaches zero.

That does in fact sound like a reasonable hypothesis. But you have to account for why a beer bottle or an upturned glass sent spinning down a table curls in the opposite direction; i.e., why is it different for a curling stone? Also, why won't a stone curl if the ice is not pebbled? Your hypothesis should account for that too.

By the way, I think you got downvoted earlier because you started off with the words "the solution is simple", sounding extremely sure, rather than saying you have a theory.

The beer bottle case is explained in the article: "Take a beer bottle or an upturned glass and send it spinning down a table: if it rotates to the right, clockwise, it will curl to the left; if it rotates to the left, it will curl to the right. That’s because the bottle, as it moves forward, also tips forward slightly, adding weight to the leading edge. More weight means more friction."

I guess a heavy curling stone with a lower center of gravity and on a slippery surface would'nt tilt forward like a bottle.

Came back to this thread to see if someone had written an enlightening rebuttal of GP rather than downvoting him.

I wasn’t too surprised about getting downvoted. I suspected that some might find “the solution is simple” a bit brazen, but I wasn’t sure how old the thread was, and wanted to draw some attention and critique before the thread died. My next line was more honest “I suspect that the reason…”. :-)

As for why a curling stone is different than a beer bottle or upturned glass on a table, that wasn’t the goal of my thinking. I was only trying to explain the curling stone. In any case, while having some similarities, the two scenarios are still pretty different. As laxd points out below, the beer bottle and glass have higher centers of gravity which make the “leaning forward” line of thinking seem more reasonable.

As for the effect of pebbled ice, it seems reasonable that the difference between the static friction coefficient and kinetic friction coefficient is greater for pebbled ice than for smooth ice. And if static friction and kinetic friction are approximately the same for smooth ice, then this effect would not contribute substantially to curling on smooth ice.

This was a fun problem to think about. I had hoped that a smarter physicist would tell me why my hypothesis was bogus. As a friction expert, Nyberg almost certainly entertained this idea at some point. I may send him a message to see what it is that I’m missing.

To investigate the pivot/slide model further why not spin a flat bottomed rock down the ice to see whether it curves?

Reminds me a bit of Feynman's spinning plates.

Feynman on "why is ice slippery?"

https://www.youtube.com/watch?v=MO0r930Sn_8

His explanation, actually first given by Michael Faraday in 1859, was later found to be wrong. According to a 2005 article "Why Is Ice Slippery" published on Physics Today, even when a layman is wearing a pair of skating blades, his/her weight can only lower the melting point of water by about three to four degrees. Yet skating is still possible when the temperature of the ice surface is as low as -30 degrees Celcius. Pressure melting cannot explain why skating blades can make ice surface slippery enough in this scenario.

Physics Confession https://xkcd.com/1867/

Is that true? Do we really not know how ice skates work?

I don't think there's an agreed upon explanation for why ice melts so easily when it's compressed, but I may be misremembering.

https://www.explainxkcd.com/wiki/index.php/1867:_Physics_Con...

Do stones curl in the opposite direction in the southern hemisphere?

Would that be called the curliolis effect?

I believe the coriolous effect doesn't actually affect things on a small scale. The water around a drain changing depending on hemisphere is a myth.

Fear not. I was just kidding. It was too good of a pun to pass up.

...i see what you did there ( ͡° ͜ʖ ͡°)

Science has failed us. If it can't figure out how curling works, there's no way it actually put a man in the moon.

Curling is probably a result in a glitch in the simulator's programming of our universe. Maybe a benign side-effect of some optimization that the great simulator thought we would never encounter...

>>Archery, too, is fringe and unexciting

Lost me there. Archery is infinitely more exciting than many Olympic "sports", curling included.

Clearly you're not from Saskatchewan

sigh it's not a Saskatchewan Ricky, it's a Samsquanch!

Well, I am currently standing in Manitoba. Portage le prairie.

The beauty of archery in Saskatchewan is that you get to go hunting way earlier in the season than you do with a shotgun or rifle. That means that hunting season doesn't interfere with curling season :)

Most Olympic sports are fringe and unexciting. Really, who cares how fast somebody runs/bikes/rows X meters?

Who cares what some author or poet decided to write? Who cares what some scientist found? Who cares what food tastes like? Who cares what article someone posted on Hacker News? Who cares about the bug you fixed? Who cares what software exists?

We can all live without any of this, but why?

Not really comparable, art and code are generally original. Transposing competitive sports to software would be as if everybody competed year after year to see who implemented the best and fastest quicksort in C in the shortest amount of time for instance. It's not usually about expression, it's about perfecting something until you're the best at it.

I don't think there's anything wrong with that although like the parent I can't say that I find that particularly interesting myself.

Oh I don't find sports interesting at all either, though I would argue that people do in fact try those sorts of competitions in coding. My beef with parent is the notion that this stuff is objectively pointless somehow just because _we_ don't find it interesting.

It's almost as if... as if... people like different things and have different opinions, isn't it?

Millions of people around the world care. That's why we have the Olympic Games.

Damn, questioning the utility of pan-european institutions is dangerous around here.

I disagree there. Curling has tons of strategy associated with it (“chess on ice”) I’ve never seen in archery. In fact, curling is kind of like archery but you can knock opponents arrows around the target...

Am I missing something with archery?

> Am I missing something with archery?

It involves weapons, and weapons are exciting?

See also: that cross-country-skiing-with-rifles winter olympics event thingy

Biathlon ;)

edit: (I love the winter games)

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