
Physicists Still Don’t Know What Puts the Curl in Curling - hoaxcracker
https://www.newyorker.com/tech/elements/physicists-still-dont-know-what-puts-the-curl-in-curling
======
da_chicken
Smarter Every Day did a YouTube video on this
([https://www.youtube.com/watch?v=7CUojMQgDpM](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.]

~~~
peeters
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.

~~~
sigzero
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.

~~~
flukus
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](https://en.wikipedia.org/wiki/Bowls)).
The games are very similar and typically people that like one like the other.

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

~~~
peeters
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.

~~~
SubiculumCode
Thanks. Didn't know.

------
CaliforniaKarl
Curling Canada has a video online
([https://www.youtube.com/watch?v=50cSDUIDMuM](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.

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

Thanks for share!

------
jbosh
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](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](https://www.youtube.com/watch?v=147vRw0kmAY)

The scratch theory paper:
[https://www.thesalmons.org/lynn/curling/A5.pdf](https://www.thesalmons.org/lynn/curling/A5.pdf)

------
noelwelsh
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.

------
echlebek
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...](https://www.unbc.ca/newsroom/unbc-stories/researchers-discover-why-
curling-rocks-curl)

------
mcguire
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?

~~~
bkanber
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](https://www.pgatour.com/stats/stat.02402.html)
[1]
[https://en.wikipedia.org/wiki/Golf_ball](https://en.wikipedia.org/wiki/Golf_ball)
[2] [https://dspace.lboro.ac.uk/dspace-
jspui/bitstream/2134/11470...](https://dspace.lboro.ac.uk/dspace-
jspui/bitstream/2134/11470/7/Measurement%20of%20Contact%20Time%20in%20Short%20Duration%20Sports%20Ball%20Impacts.pdf)
[3]
[https://hypertextbook.com/facts/2001/EmilyAccamando.shtml](https://hypertextbook.com/facts/2001/EmilyAccamando.shtml)
[4]
[http://rspa.royalsocietypublishing.org/content/465/2102/551](http://rspa.royalsocietypublishing.org/content/465/2102/551)

~~~
snovv_crash
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.

------
kazinator
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.

~~~
leot
This seems similar to the Magnus effect:

[https://en.m.wikipedia.org/wiki/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)

~~~
elif
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.

~~~
claar
>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?

~~~
BenjiWiebe
Not relative to the direction of travel over the ice.

------
Wile_E_Quixote
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.

~~~
mysterypie
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.

~~~
Wile_E_Quixote
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.

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

------
macintux
Reminds me a bit of Feynman's spinning plates.

------
leeoniya
Feynman on "why is ice slippery?"

[https://www.youtube.com/watch?v=MO0r930Sn_8](https://www.youtube.com/watch?v=MO0r930Sn_8)

~~~
The_suffocated
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.

------
tambourine_man
Physics Confession [https://xkcd.com/1867/](https://xkcd.com/1867/)

~~~
joenot443
Is that true? Do we really not know how ice skates work?

~~~
deelowe
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.

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

Would that be called the curliolis effect?

~~~
jjeaff
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.

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

------
jsizzle
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.

~~~
em3rgent0rdr
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...

------
sandworm101
>>Archery, too, is fringe and unexciting

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

~~~
megaman22
Most Olympic sports are fringe and unexciting. Really, who cares how fast
somebody runs/bikes/rows X meters?

~~~
jdewald
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?

~~~
simias
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.

~~~
jdewald
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.

