Friction on Ice: Understanding Coefficient of Friction

[re1s]

Hi I was wondering if anyone could point me towards some resources (I've looked a lot online!), or give me some guidance on this subject.

Just considering ice for simplicity:

The low friction on ice is due to a layer of water created by the ice melting due to friction/pressure. Surely these will change a lot due to the weight and velocity, however some online resources quote a coefficient of friction!

Is friction on ice approximately proportional to the normal reaction force, or will the factors I mentioned effect this a significant amount?

Thanks ( and hi everyone :) !)

 

[Curl]

its not true that the low coefficient is because the ice melts.

the coefficient is low because of the crystal structure of the ice and how it behaves at the surface

 

[JaredJames]

A quick Google of "why is ice slippery" returns loads of results. One of the best I noted was this one:

http://www.buzzle.com/editorials/8-30-2004-58620.asp

Too much to quote so I think it's best you read the whole thing.

Seems fairly simple.

Curl, do you have something to back that up? Everything I just read points more to re1s' explanation (with a few tweaks).

From the above:

Ice is slippery not because it’s smooth but because its melting point drops when pressure is increased.

Let’s see what happens when we bring a sled or skate. On skates, we bring the whole weight of our body to bear down on a very small area, of but a few square millimeters. Due to this the skater exerts tremendous pressure on the ice. Under strong pressure, ice melts at a lower temperature.

 

[russ_watters]

Curl, do you have something to back that up? Everything I just read points more to re1s' explanation (with a few tweaks).

From the above:

Ice is slippery not because it’s smooth but because its melting point drops when pressure is increased.

Let’s see what happens when we bring a sled or skate. On skates, we bring the whole weight of our body to bear down on a very small area, of but a few square millimeters. Due to this the skater exerts tremendous pressure on the ice. Under strong pressure, ice melts at a lower temperature.

Clearly, that explanation may work for some things but can't possibly work for others. While the pressure on an ice skate blade could be 1000 psi, the pressure under a toboggan would be a good two orders of magnitude lower.

So that explanation is insufficient because it doesn't deal with all the scenarios we deal with on an everyday basis. Even a car doesn't put all that much pressure under its tires (35 psi). For those, the likely explanation is simply that ice is smooth.

 

[re1s]

Thanks guys.

Yeah that was one of the resources I found - however my my question is this:

Surely the frictional coefficient would change when applying a greater force? Meaning it's not a linear function like Ff = -u * Fn. A greater normal force should also mean greater pressure, which means lower melting point and thus a lower frictional coefficient?

u = u(Fn), and won't be constant?

 

[re1s]

So either ice is slippy because it is smooth, or because it has a layer of water?

 

[JaredJames]

Clearly, that explanation may work for some things but can't possibly work for others. While the pressure on an ice skate blade could be 1000 psi, the pressure under a toboggan would be a good two orders of magnitude lower.

So that explanation is insufficient because it doesn't deal with all the scenarios we deal with on an everyday basis. Even a car doesn't put all that much pressure under its tires (35 psi). For those, the likely explanation is simply that ice is smooth.

Sorry, missed a bit I thought was covered there.

http://lptms.u-psud.fr/membres/trizac/Ens/L3FIP/Ice.pdf

Within that PDF, it discusses a layer of water present on ice and the causes of "ice being slippery" even when standing still. A good read.

Consider this an elaboration.

its not true that the low coefficient is because the ice melts.

the coefficient is low because of the crystal structure of the ice and how it behaves at the surface

Unless anyone can provide sources otherwise, I'm taking this as the reason for the 'slipperiness' and I see no reason not to. Without meaning to sound nasty, my statement regarding sources stands (heck it's your guidelines), especially seeing as it doesn't appear as straight forward as most would think.

 

[AlephZero]

Clearly, that explanation may work for some things but can't possibly work for others. While the pressure on an ice skate blade could be 1000 psi, the pressure under a toboggan would be a good two orders of magnitude lower.

So that explanation is insufficient because it doesn't deal with all the scenarios we deal with on an everyday basis. Even a car doesn't put all that much pressure under its tires (35 psi). For those, the likely explanation is simply that ice is smooth.

If there is any relative motion, the work done be the friction force generates heat which produces a thin film of water. That is the main issue with car tires, where the engine or brakes are powerful enough to start the slipping. The water film then reduces the dynamic coefficient of friction to almost zero and even if the tire pumps the water away, the next later of ice instantly melts because the tires are spinning.

The same probably applies to skis and toboggans, over most of the contact area behind the front part.

This probably explains why a very small amount of sand or grit gives a very big improvement in grip. The small amount of contact with the sand is enough to keep the tire rolling not slipping, which stops the water film being created, and most of the friction force to move the car is then the static or rolling friction on the ice.