Why is Ice So Slippery?

[RinkRat II]

<rimshot> 

 We Need This   



Paul B

[kingoffleece]

As a northeast kid growing up playing hockey from a wee age outside I can confirm that it's very difficult to skate when it's way cold out-like well below zero.  Feels like the skates are in mud.

[Nic in Western NYS]

As a northeast kid growing up playing hockey from a wee age outside I can confirm that it's very difficult to skate when it's way cold out-like well below zero.  Feels like the skates are in mud.

Great evidence there!   So you've got temp as a likely variable. To get at whether pressure is another variable, go for a skate when it's cold enough for the skates to feel like mud (-5F or whatever).  Then when the skates feel like mud, strap on a backpack loaded with 50 lbs of books, bricks, whatever.  Then see if it doesn't feel like mud.  Is the physics right with this little experiment?  The idea is to have the same 'contact patch' but apply more pressure through it, see if that can form the lubricating liquid layer on top of the ice. Better yet, have a team of JV hockey players do it and have it be their science project. That way you'll get multiple data points.   

[sturgeon]

As a northeast kid growing up playing hockey from a wee age outside I can confirm that it's very difficult to skate when it's way cold out-like well below zero.  Feels like the skates are in mud.

You could be right, but I grew up in a remote mining town on a small island in NW Ont. When we were kids we spent a lot of time skating on the lake (the only indoor ice was the curling rink, and skating there was frowned upon). I don't remember "mud', even at -20.

[Scud]

Life on earth exists in part because water is more dense than ice. It is one of the four unique things about our planet. The others? We have an extremely large and relatively close moon, leading to substantial tides; we have an atmosphere that is neither too thin nor too dense; and we have a world that constantly rotates at just the right speed.

The combination of cold water sinking (but frozen water rising) and substantial tides means our oceans and larger bodies of fresh water are constantly turning over. This brings nutrients to the surface instead of allowing them to lay on the bottom with no oxygen or sunlight to break them down into useful components. Even further, the movement of the water gets accelerated by the rotation of the earth, creating huge currents. Finally, add in the floating ice at the poles (which conveniently reflects a substantial amount of light) and we have an extension of the warm and cold currents in the ocean into the thinner 'ocean' of air, carrying currents of heat, cold and moisture across the continents.

Eventually, this all leads to the ideal conditions for the rise of life, and the eventual and inevitable creation of Moto Guzzis and roads on which to ride them (Moto Guzzi content!).

Brilliant history of the earth. A good contextual opening. Skip a few billions years of irrelevant bits - and get straight to the point at the end. Moto Guzzi - the pinnacle of evolved life on earth.

[kingoffleece]

I'm in Buffalo.  Snowmobiled up your way plenty of times.  We don't get near as cold as you do but we have had that deep sub zero (F) a few times.
I remember some outdoor rinks in Halliburton WAY back when where it was unreal cold.  Games seemed in slow motion but that was a few (dozen) years ago!

[redrider]

" but there's a lot of pressure under a skate blade, like thousands of PSI"

Only if an elephant is wearing them.

[Chuck in Indiana]

From the google box:
According to the frequently cited — if incorrect — explanation of why ice is slippery under an ice skate, the pressure exerted along the blade lowers the melting temperature of the top layer of ice, the ice melts and the blade glides on a thin layer of water that refreezes to ice as soon as the blade passes.

"People will still say that when you ask them," Dr. Rosenberg said. "Textbooks are full of it."

But the explanation fails, he said, because the pressure-melting effect is small. A 150-pound person standing on ice wearing a pair of ice skates exerts a pressure of only 50 pounds per square inch on the ice. (A typical blade edge, which is not razor sharp, is about one-eighth of an inch wide and about 12 inches long, yielding a surface area of 1.5 square inches each or 3 square inches for two blades.) That amount of pressure lowers the melting temperature only a small amount, from 32 degrees to 31.97 degrees. Yet ice skaters can easily slip and fall at temperatures much colder.

The pressure-melting explanation also fails to explain why someone wearing flat-bottom shoes, with a much greater surface area that exerts even less pressure on the ice, can also slip on ice.

[Triple Jim]

A 150-pound person standing on ice wearing a pair of ice skates exerts a pressure of only 50 pounds per square inch on the ice. (A typical blade edge, which is not razor sharp, is about one-eighth of an inch wide and about 12 inches long, yielding a surface area of 1.5 square inches each or 3 square inches for two blades.)

That's written by someone who doesn't skate.  Ice skate blades are not straight.  They have quite a bit of rocker, so the length of contact along  the blade is quite short.  They're also ground with a concave cross section so they contact the ice at the edges first.  Racing skates are often not ground concave, but are very thin compared to figure and hockey skates.

[Chuck in Indiana]

That's written by someone who doesn't skate.  Ice skate blades are not straight.  They have quite a bit of rocker, so the length of contact along  the blade is quite short.  They're also ground with a concave cross section so they contact the ice at the edges first.  Racing skates are often not ground concave, but are very thin compared to figure and hockey skates.

Oh, I agree
comma but
The pressure-melting explanation also fails to explain why someone wearing flat-bottom shoes, with a much greater surface area that exerts even less pressure on the ice, can also slip on ice.

[bobdar]

Look at this:  https://en.wikipedia.org/wiki/Pressure_melting_point

The effect of pressure on the melting point is negligible until about 50 mPA or 7,250 psi.  Even on one skate (12" long X 1/8" wide), the skater would have to weigh 4800 lbs.  So it ain't the pressure. 

[johnr]

but wait... ANY pressure must produce SOME heat.... getting confused here....

[oldbike54]

 OK Prescott , you opened this bag of worms , where are you now ?

 We may have more than one factor involved here . The worst spill I ever took was on glaze ice , wearing lug sole work boots . Doubt if 160 lbs and relatively soft soled boots caused any heat , as it happened as soon as I stepped onto the ice . Whammo , ouch , damn , what just happened 

 Dusty

[sturgeon]

I'm in Buffalo.  Snowmobiled up your way plenty of times.  We don't get near as cold as you do but we have had that deep sub zero (F) a few times.
I remember some outdoor rinks in Halliburton WAY back when where it was unreal cold.  Games seemed in slow motion but that was a few (dozen) years ago!

Dunno if you were replying to my post, but Haliburton is positively tropical compared to where I was 

[Triple Jim]

Look at this:  https://en.wikipedia.org/wiki/Pressure_melting_point

The effect of pressure on the melting point is negligible until about 50 mPA or 7,250 psi.  Even on one skate (12" long X 1/8" wide), the skater would have to weigh 4800 lbs.  So it ain't the pressure.

I think you missed my post above.

[Chuck in Indiana]

I think you missed my post above.

I think you missed my post above..

The pressure-melting explanation also fails to explain why someone wearing flat-bottom shoes, with a much greater surface area that exerts even less pressure on the ice, can also slip on ice.

[johnr]

OK Prescott , you opened this bag of worms , where are you now ?

 We may have more than one factor involved here . The worst spill I ever took was on glaze ice , wearing lug sole work boots . Doubt if 160 lbs and relatively soft soled boots caused any heat , as it happened as soon as I stepped onto the ice . Whammo , ouch , damn , what just happened 

 Dusty

That does pose a question Dusty.  I agree, Ice seems to be slippery anyway when you walk (or drive) on it, though to be fair, not nearly as slippery as when wearing a set of ice skates.  Could it be that ice tends through it's nature to have quite a bit of free water about? It can't just be smoothness or the same would apply to glass.

[oldbike54]

That does pose a question Dusty.  I agree, Ice seems to be slippery anyway when you walk (or drive) on it, though to be fair, not nearly as slippery as when wearing a set of ice skates.  Could it be that ice tends through it's nature to have quite a bit of free water about? It can't just be smoothness or the same would apply to glass.

 John , in my case it was 5 degrees F , and had been well below freezing for several days .

 Dusty

[Chuck in Indiana]

That does pose a question Dusty.  I agree, Ice seems to be slippery anyway when you walk (or drive) on it, though to be fair, not nearly as slippery as when wearing a set of ice skates.  Could it be that ice tends through it's nature to have quite a bit of free water about? It can't just be smoothness or the same would apply to glass.

Ice is a slippery subject.  I've read stories of "free electrons" on the surface as well as different classifications of ice according to temperature and pressure.
In cases like this, I tend to see and observe. "Oh, fell on my ass. That must be slick for whatever reason.." 

[lucian]

While on the subject of frozen water I thought I would share this cool trick. My sister once told me to try throwing a bucket of warm water into the air outside when the temp is 15 below 0 f or less. Well I did and it about scared the crap out of me when it made a loud bang as it flash froze and then hit the ground like shattered plate glass. Didn't expect such a violent episode. Try it sometime, it's amazing to see and hear.

[Triple Jim]

I think you missed my post above..

I didn't miss it, Chuck.  My assumption is that one or two things may be happening with shoes on ice.  First, the ice surface isn't perfect, so there may be high pressures in very small areas, and second, the friction theory mentioned above may be more relevant with shoes.  But shoes don't glide anywhere near as well as skate blades, so I don't really see a conflict.

[johnr]

Now that I think about it, ice is always wet.  It must be that.

[jas67]

" but there's a lot of pressure under a skate blade, like thousands of PSI"

Only if an elephant is wearing them.

Actually the above statement is true.    The bottom of an ice skate blade has a concave "hollow", so, only the edges actually touch the ice.



These edges also give a nice strong lateral grip on the ice, which is what makes tight turns possible.

[oldbike54]

 Great , now I keep seeing visions of Apollo OH NO  Maybe he knows the answer 

 Dusty

[bobdar]

Now that I think about it, ice is always wet.  It must be that.

That could be, and here's a couple of possibilities:  Not too sure about this one, but humidity in the air could condense on the ice, then freeze, but there would always be a film of liquid in transition.  Then there's the boundary layer problem, where the outermost layer of molecules don't have enough partners to crystalize, so those molecules are "free". 

[oldbike54]

 Free the molecules !

 Dusty

[kingoffleece]

-29C was plenty cold enough for me.
And, we've had the sleds 100 miles North of Timmons.  It's lovely up there-but WOW was it cold.

[kirby1923]

All materials that rub together produce/have a measurable  coefficient of friction.
I suspect that as far as ice is concerned the denser  the material contacting ice has a profound effect on this coefficient and coupled with shape and contact of skates on ice gives a relatively low number compared to other materials or shapes. (and probably changes with temps +/-)

A molecular thing?

We in winter aviation ops deal with this coefficient regularly.

So why slippery?  cause like so many other things in this world... it is!

:-)

Am ice, therefore am slippery!

[redrider]

That whispy 98 pound skate queen is still only 98 pounds of downforce on whatever surface area the blade has. Add some velocity and things change. Angular momentum any one? All we need now is some cabin fever riddled Guzzi folks to spoon on some studded ice tires and tow the volunteer skater across the lake. Do some turns while filming the event and have a temp sensor on the blade. It's winter(not here), we need some entertainment.

[sturgeon]

-29C was plenty cold enough for me.
And, we've had the sleds 100 miles North of Timmons.  It's lovely up there-but WOW was it cold.

Oh yeah. My elderly cousin lives in Timmins and she's always envious of me being in southern Ontario now. I'm not far from you in Kitchener-Waterloo. Right now it's about -6 here, -23 in Timmins, and -29 in Red Lake where I was born and raised.