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03-23-14 04:36PM
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JB42
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Fun with Physics

Here's a fun video by an American from Alabama of all places.

http://www.youtube.com/watch?v=7CUojMQgDpM

It takes a look at why a curling rock curls the way it does.

The short answer is, "We're not sure".

What we do know is that the curling rock curls the "wrong" way. I.e. Other objects curl in the opposite direction of the rotation.

The Canadian expert's theory says the melting that occurs from the passage of the rock best explains why the rock curls in the direction of the rotation.

The Swedish experts believe that the scratches on the running surface create the force on the rock that causes it to curl in the direction of the rotation.

Every curler can tell you two things. 1) If you sweep, that is melt the ice, you decrease the amount of curl. 2) If you scratch the rocks with sandpaper, also called conditioning the rock, you will increase the amount they curl.

I have neither the natural talent or the training to tell you what these two empirically verified facts tells you about these two different theories.

Any budding physical scientists out there on CZ who want to weigh in? (pun intended:-)

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03-24-14 02:14PM
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I just wrote a short review article about this topic for our curling club newsletter. (And I am a card-carrying, long-time practicing physical scientist.) There is a surprising amount of scientific literature about this subject going back to at least 1924.

The most recent, and credible theories to emerge come from Shegelski's group in Northern BC, and from Nyberg's gorup in Uppsala, Sweden.

Until recently, Shegelski's various hypotheses have held sway for lack of anything better. The main problem with the Shegelski hypothesis is that there is very little supporting experimental evidence, e.g., observation of water films under the running surface, a fundamental problem in predicting the total amount of curl seen for curling stones, and a prediction that curl increases with increasing rotation rate. A general problem of all of the "differential friction" theories is that they do not predict enough curl, and they erroneously predict more curl with more rotation. (And we all know that rocks with excessive rotation run slightly straighter, if anything--the differential friction theories predict a strong dependence of curl on increased rotation, that is 4 rotations should curl significantly more than 2 rotations, and that doesn't pass the sniff test.)

The Nyberg model predicts the proper amount of curl (3 feet or more) from first principles of the hypothesis, and has very little rotation velocity dependence on curl, as observed for real curling rocks. Finally, Nyberg visualized the scratches on the ice after a curling rock is thrown, and can manipulate the ice surface to produce artificial curl according to the theory. Because of the experimental verification of many of the critical aspects of the "scratch" theory, this has to be now the leading theory for explaining curl.

Until something better comes along.

Cheers.

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03-24-14 03:17PM
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Thanks RockDoc.

From what curlers know, i.e. scratching rocks makes them curl more, this makes sense.

Do you have an opinion on the following. Is Shegelski's theory helped, hurt, or not impacted by the fact that sweeping causes the rock to curl less?

On the face of it my thought was that his theory would be hurt. I.e. He is hypothesizing that the melting is causing the curl. Or rather the different way the leading edge and the trailing edge react to the melting caused by the passage of the rock creates the curl.

Shouldn't then more melting make more curl? Or does he hypothesize that it's a 'wash' as it's the differential between the leading and trailing edge and that isn't impacted by the extra melting caused by sweeping? Or, as is not unlikely, am I just thinking about this in the completely wrong way?....lol.....

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03-24-14 03:19PM
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request to RockDoc

Would you be averse to emailing me your newsletter article so we can publish it in our newsletter?

We would of course attribute it.

Cheers.

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03-24-14 04:50PM
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Re: request to RockDoc

quote:
Originally posted by JB42
Would you be averse to emailing me your newsletter article so we can publish it in our newsletter?

We would of course attribute it.

Cheers.



I'll PM you with a link.

Cheers.

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03-24-14 05:14PM
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quote:
Originally posted by JB42
Thanks RockDoc.

From what curlers know, i.e. scratching rocks makes them curl more, this makes sense.

Do you have an opinion on the following. Is Shegelski's theory helped, hurt, or not impacted by the fact that sweeping causes the rock to curl less?

On the face of it my thought was that his theory would be hurt. I.e. He is hypothesizing that the melting is causing the curl. Or rather the different way the leading edge and the trailing edge react to the melting caused by the passage of the rock creates the curl.

Shouldn't then more melting make more curl? Or does he hypothesize that it's a 'wash' as it's the differential between the leading and trailing edge and that isn't impacted by the extra melting caused by sweeping? Or, as is not unlikely, am I just thinking about this in the completely wrong way?....lol.....



Well, I'm not sure either theory works really well to explain sweeping. In Shegelski's model, I think the idea is that sweeping warms the ice making it more slippery, so that the differential friction between the front and rear of the rock is lessened, but as you say, that also woudl mean MORE melting on the front edge of the rock, so...it's not a really clear argument to me. And it's not clear that sweeping actually melts the ice--Canadian and Scottish(?) studies with instrumented brooms show that the ice is heated by sweeping, but not nearly enough to melt it.

In the Nyberg model, sweeping maybe polishes out the little scratches, except...the scratches don't exist yet where you are sweeping! So something else is going on here--maybe another study?

Cheers.

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03-24-14 05:23PM
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So I'm no physicist - let's just get that outta the way to begin with. But I want to chime in based on what I believe to be a common theory based on observation and informal testing (i.e. game play).

The rationale I've always visualised for why rocks curl has to do with variations in friction on different points along the stone's running surface. On a rotating stone travelling down the ice, there is a "leading edge" (or inside, towards the direction of curl) and a "trailing edge" (or outside). I've always visualised that the leading edge has less velocity relative to the direction of travel of the stone than the trailing edge, because it is spinning against the direction of travel (while the trailing edge is spinning with the direction of travel).

Based on this, I always thought it logical that the leading edge would be more susceptible to slowing down from friction than the outside edge, because it has less momentum against the direction of travel of the stone. It would dig in slightly faster, producing the "curl" effect as the rock is pulled gradually in that direction.

And I know that if you curl a glass on a table that the opposite effect appears to happen. But isn't that just because of the much higher friction coefficient? In the case of the glass, the whole glass jolts to a stop fairly quickly, and the rotation of the glass seems to jump the back end in the opposite direction of the spin at the very end of it's motion. I visualize it as if the very front of the glass catches first (as it is the initial/strongest point of resistance), and the momentum of the spinning glass only makes it jump sideways once that edge has almost come to a complete stop. [Also, a glass isn't always hollow underneath - I'm not sure if a rounded bottom produces the same effect.]

On ice, with such low friction and such gradual slowing of the rock, maybe you don't get the same effect (i.e. maybe the front edge of the rock is not the strongest point of friction with the ice surface). Perhaps there's a relationship between the coefficient of friction and amount/speed of rotation that can prove the existence or determine the location of the point of most resistance/friction along the running surface? I hypothesize that the point exists, and would lie along the "leading edge" of the rock, somewhere around the 10 o'clock position. And perhaps this effect is more noticeable with less rotation because the leading edge, travelling at lower speed, is experiencing more friction over a fixed distance?

Again, I'm no physicist and could be completely off base. But I've always visualised it this way and it seems intuitive. I also find it fascinating that such an obviously observable result (i.e. we all know what way it will curl) is so hard to measure or prove. Top curlers have definitely noticed the effect, as noted by the acceptance of "switching" techniques brushing a moving stone, as well as brushing the trailing edge of a rock if you want it to spin out of the house. I know this is different than proving an effect through physics, but it seems like the best players understand it.

I wonder if it is possible to measure the friction experienced on different points of the running surface to prove this hypothesis. To me, friction theories are much more worth exploring than the melting theories, as we know by observation that sweeping melts the ice and we know sweeping reduces curl.

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03-24-14 05:35PM
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A difference in friction between the left side of the rock and the right side of the rock cannot produce a net force to the left or right, but only forward or backwards. (Yeah, I slept through vectors in physics class the first time, too.) This is the fatal flaw in all of the left-right frisction theories which were originally proposed for curling rocks. When physicists started to study curling they immediately realized that there has to be a front-back differential in friction to generate a left-right force. A rotated glass on a table curls the "wrong" way because the friction at the front of the glass is larger than at the rear of the glass. This occurs because as the glass decelerates, it tips forward slightly, putting more pressure on the front rim than the back rim, thus generating the required differential friction and a sideward force. If curling rocks use the same principle, the front surface must experience LESS friction than the rear surface, hence the "melting ice at the front of the rock" theories. The problem is that this theory, even if it were right, cannot predict more than about 1 1/2 feet of curl for a curling rock, and to achieve that you need a very high rate of rotation, maybe 5-10 rotations. A curling rock thrown with the normal 2-3 rotations does not have enough angular momentum to generate 3-4 feet of curl seen with real rocks. That was actually known back in the 1930s, but was repeatedly forgotten in later sutdies.

So the Nyberg scratch model looks better, and it has physical evidence as well as quantitative modeling to support it. It is possible that aspects of the Shegelski model could also be at play but they cannot be the dominant factors.

Cheers.

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03-24-14 05:42PM
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quote:
Originally posted by Unregistered
Top curlers have definitely noticed the effect, as noted by the acceptance of "switching" techniques brushing a moving stone, as well as brushing the trailing edge of a rock if you want it to spin out of the house. I know this is different than proving an effect through physics, but it seems like the best players understand it.



I'm not sure there have been any controlled studies to show that "switching" has the desired effect, but teams clearly believe that it does. It is based on a theory (left-right frictional difference) that we know is not correct. If it works, it's due to something else than the commonly given explanation.

When rocks come to a near stop, the trailing edge of the stone becomes stationary with respect to the ice, and in that case the stone may actually "stick" on that point. Stationary friction (unsticking) may be much higher than dynamic friction, and that can cause the rock to "spin" on to or off of the rings around the stuck point. When rocks spin like this, they don't spin around their center of mass, but around the stuck point on the running band. However, rocks curl during shots long before the trailing edge becomes stationary. (If this actually happens during a shot due to a lazy handle, "picks" result.)

Cheers.

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03-24-14 06:05PM
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I always thought that the rock curled because the side spinning back toward the shooter acted 'slower' than the side spinning toward the house due to it (the side spinning back to the shooter) matching speed with the force applied toward the house. If you need to move a heavy barrel to another location, you tilt and turn the barrel in the direction you are aiming for and the barrel will 'curl' in the direction turned.

If so, sweeping the edge of the stone spinning toward the house would make it curl more, sweeping the edge spinning awtoward the shooter would make it curl less.

Someone is about to tell me that is the opposite of what folks see... and I'll nod and agree, then wonder why everyone is still having this discussion.

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03-24-14 06:49PM
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re Sweeping and the Nyberg model

Does it make sense to say that in the Nyberg model sweeping could be explained as follows.

Sweeping heats the ice, i.e. makes it softer, and so the scratches would be somewhat 'blurred' by sweeping. As an analogy an ice sculpture's cuts would be clearer with harder ice.

So the more the ice is heated the less clear the track is for the rock to react to.

As always I am completely open to the possibility that I am looking at this in entirely the wrong way. My brain does certain things well, curling strategy for example I am a god!....lol....But the physical sciences, "Not so much" as the kids say today:-)

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03-24-14 07:01PM
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quote:
Originally posted by Flat Hat
I always thought that the rock curled because the side spinning back toward the shooter acted 'slower' than the side spinning toward the house due to it (the side spinning back to the shooter) matching speed with the force applied toward the house. If you need to move a heavy barrel to another location, you tilt and turn the barrel in the direction you are aiming for and the barrel will 'curl' in the direction turned.

If so, sweeping the edge of the stone spinning toward the house would make it curl more, sweeping the edge spinning awtoward the shooter would make it curl less.

Someone is about to tell me that is the opposite of what folks see... and I'll nod and agree, then wonder why everyone is still having this discussion.



This idea of left-right motion asymmetry works if the retreating edge of the rock is stationary and "stuck" to the ice. But it's not. Until the moment the rock almost stops, the rock is traveling faster forward than the retreating edge is retreating. And a left-right asymmetry of friction (where the frictional forces are acting forward and backward) cannot induce a left-right force.

To get curl, you need a front-back frictional asymmetry, where the frictional forces are directed in a left-right direction. So, to be honest, I don't understand how "switch-sweeping" actually works in terms of physical principles. Maybe the Swedes will look at that next. (Hint, hint...)

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03-24-14 08:30PM
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quote:
Originally posted by RockDoc


This idea of left-right motion asymmetry works if the retreating edge of the rock is stationary and "stuck" to the ice. But it's not. Until the moment the rock almost stops, the rock is traveling faster forward than the retreating edge is retreating. And a left-right asymmetry of friction (where the frictional forces are acting forward and backward) cannot induce a left-right force.

To get curl, you need a front-back frictional asymmetry, where the frictional forces are directed in a left-right direction. So, to be honest, I don't understand how "switch-sweeping" actually works in terms of physical principles. Maybe the Swedes will look at that next. (Hint, hint...)



Thanks for at least entertaining the idea I would just say that the assumptions about what is needed and what doesn't work don't support the current theories or how rocks really behave very well.

Certainly something makes the glass on a table top curl opposite of the stone on pebbled ice. Perhaps there are things we don't know about the scenario?

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03-25-14 12:04PM
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I do think that the trailing edge (the back retreating quarter of the rock) is 'stuck-ish' to the ice in relation to the speed of the other quadrants.

All of the tests I have done regarding the number of spins on the stone support the idea that spin speed has an optimal range for inducing the most curling. I can find no other theory to account for why rocks with fewer turns float at the mercy of the ice, and rocks with more turns run straight(er) while rocks with 2.5 to 4 turns curl predictably.

Humidity, ice temp, closeness of pebble, and sweeping all affect that minute bit of friction that makes the rock curl. This would be expected if you consider the rock to be slightly more'stuck' to the pebble it lands on at 9 o'clock and which it pulls itself around. It is analogous to paddling a canoe from only one back corner (no j-stroke)

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03-25-14 09:54PM
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Left-right frictional asymmetry will not create a left-right force, but it can create a torque that changes the direction of the rock.

If the side of the rock where the rotation is in opposite direction "sticks" for a fraction of a second ... it will create a pivot point causing the rock to slightly "turnover on itself" changing the direction of the rock slightly into the direction of the rotation.

Think of running forward with your arms outstretched and you grab a pole as you run by. It is going to spin you in a new direction ...

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03-26-14 12:07AM
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quote:
Originally posted by Unregistered
Left-right frictional asymmetry will not create a left-right force, but it can create a torque that changes the direction of the rock.

If the side of the rock where the rotation is in opposite direction "sticks" for a fraction of a second ... it will create a pivot point causing the rock to slightly "turnover on itself" changing the direction of the rock slightly into the direction of the rotation.

Think of running forward with your arms outstretched and you grab a pole as you run by. It is going to spin you in a new direction ...



I prefer your analogy.

I suspect that the to speed matching of that trailing side causes the rock to be heavier on that side and so not be 'as' slippery, hence the 'stick'.

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03-26-14 12:53AM
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No amount of "sticking" at 9 or 3 o'clock can cause left-right motion. Only differential friction at 12 or 6 o'clock can cause left-right motion of the stone. The force vectors from left-tight friction act forwards and backwards, not side to side.That aspect of the physics has been recognized for more than 60 years.

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03-26-14 09:12AM
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Re: re Sweeping and the Nyberg model

quote:
Originally posted by JB42
Does it make sense to say that in the Nyberg model sweeping could be explained as follows.

Sweeping heats the ice, i.e. makes it softer, and so the scratches would be somewhat 'blurred' by sweeping. As an analogy an ice sculpture's cuts would be clearer with harder ice.

So the more the ice is heated the less clear the track is for the rock to react to.

As always I am completely open to the possibility that I am looking at this in entirely the wrong way. My brain does certain things well, curling strategy for example I am a god!....lol....But the physical sciences, "Not so much" as the kids say today:-)




I don't know how much validity there is to that.... typically warmer ice will curl more than colder ice.

If anything, the softer ice might make it easier for the scratches to develop.

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03-26-14 09:32AM
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quote:
Originally posted by RockDoc


This idea of left-right motion asymmetry works if the retreating edge of the rock is stationary and "stuck" to the ice. But it's not. Until the moment the rock almost stops, the rock is traveling faster forward than the retreating edge is retreating. And a left-right asymmetry of friction (where the frictional forces are acting forward and backward) cannot induce a left-right force.

To get curl, you need a front-back frictional asymmetry, where the frictional forces are directed in a left-right direction. So, to be honest, I don't understand how "switch-sweeping" actually works in terms of physical principles. Maybe the Swedes will look at that next. (Hint, hint...)




Another thing to keep in mind, when they were doing the testing while developing the EQ heads, they took an infrared camera out on the ice to measure temperature changes.

While it's true the down-stroke generates far more pressure on the broomhead, when you actually look at the heating pattern on the ice with two sweepers going at it, I don't think it matters at all which direction the closest sweeper is sweeping in.

In short, the "switch-sweeping" idea I think is a bunch of nonsense. There's nothing to back it up scientifically.

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03-26-14 10:30AM
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quote:
Originally posted by duct_tape
In short, the "switch-sweeping" idea I think is a bunch of nonsense. There's nothing to back it up scientifically.


That's what I'm thinking, too. Especially since the theory of switch-sweeping is based on a faulty theory of how a rock curls (that is, a left-right frictional asymmetry, which we know is physically impossible--those darn vectors again). That and the fact there are no controlled studies of this phenomenon to demonstrate that it works...only anecdotal "evidence" from curlers.

Just like in any endeavor, curling has its hoary (and often incorrect) truths. Reminds me of theories of cooking prior to the scientific revolution in that field. There were many, many ideas about the chemistry and physics of cooking that were passed on as truth, except they weren't.

To quote Mark Twain:

"It ain't what you don't know that gets you into trouble. It's what you know for sure that just ain't so."

Cheers.

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03-26-14 10:32AM
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quote:
Originally posted by RockDoc


To quote Mark Twain:

"It ain't what you don't know that gets you into trouble. It's what you know for sure that just ain't so."

Cheers.




So true!

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03-26-14 10:33AM
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Re: Re: re Sweeping and the Nyberg model

quote:
Originally posted by duct_tape



I don't know how much validity there is to that.... typically warmer ice will curl more than colder ice.

If anything, the softer ice might make it easier for the scratches to develop.



I believe sweeping makes the pebble tops have less friction through warming. The temp of the ice itself does affect the curl of the rock, but warmer ice (0c vs -5c) is not the same as the 'warming' of ice from sweeping.

It also doesn't seem likely the ice itself is melted at all or that there is a film of water between the ice and the stone. The temps from sweeping are never that high, nor is the heat from friction between the rock and ice. The rock is the same temp as the ice.

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03-26-14 10:42AM
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RockDoc
Swing Artist

 

Registered: Apr 2005
Location:
Posts: 399

Here's another sweeping conundrum for which I don't have any scientific answer (although I think this one is scientifically testable):

Is it more effective to sweep with (a "performance-style" brush) the long axis of the broom perpendicular to the path of the stone (so as to get closer) or with the broom head parallel to the path of the stone (so as to sweep a larger area of ice in front of the stone)? Or does it matter at all which way you do it?

When I ask competitive curlers, most but not all say that sweeping with the broom head parallel to the path of the stone is better. I was taught to sweep as close to the stone as possible (orienting the brush perpendicular to the stone path)--in come cases by high-performance coaches.

For now, I'm sweeping close to the rock with silvered-mylar modified broom heads. Seems to carry a rock OK.

Discussion on!

Cheers.

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03-26-14 10:42AM
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Flat Hat
Harvey Hacksmasher

 

Registered: Mar 2014
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quote:
Originally posted by RockDoc
No amount of "sticking" at 9 or 3 o'clock can cause left-right motion. Only differential friction at 12 or 6 o'clock can cause left-right motion of the stone. The force vectors from left-tight friction act forwards and backwards, not side to side.That aspect of the physics has been recognized for more than 60 years.


quote:
Only differential friction at 12 or 6 o'clock can cause left-right motion of the stone


I assume this is a principle of physics that has been proven over and over again?

The line 'bumblebees can't fly' comes to mind, since we can see them fly (and rocks curl) without the physics to back it up. Probably why nobody agrees, there are multiple studies trying to understand it, and why the current physics can't explain it.

Maybe the affect of rotation speed is not being properly accounted for?

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03-26-14 10:44AM
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duct_tape
Super Rockchucker

 

Registered: Jan 2004
Location: Toronto, Ontario
Posts: 1441

Re: Re: Re: re Sweeping and the Nyberg model

quote:
Originally posted by Flat Hat


I believe sweeping makes the pebble tops have less friction through warming. The temp of the ice itself does affect the curl of the rock, but warmer ice (0c vs -5c) is not the same as the 'warming' of ice from sweeping.

It also doesn't seem likely the ice itself is melted at all or that there is a film of water between the ice and the stone. The temps from sweeping are never that high, nor is the heat from friction between the rock and ice. The rock is the same temp as the ice.




You are correct that sweeping will not melt the ice, the infrared shows at best they're only raising the temperature 2-3 degrees Fahrenheit.

However, regardless of if the ice is 27F from sweeping, or 27F naturally, all the rock is going to care about is what temperature the ice is at as it's interacting with it.

However, there is small film of water under the stone.... in fact, there's a small film of water under anything that's on the ice. The melting is not generated by heat, it's generated by pressure. Without that film of water to lubricate things ice could not be slippery.

__________________
"Ladies and gentlemen, take my advice, pull down your pants and slide on the ice"

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