2

This is a common phenomenon and most of the people have encountered this while moving on ice:falling down! Why does this occur?

According to me, it is due to lack of static friction. But why will it be absent here? What is the cause? I have heard that there is a thin layer of water above the ice . But why,unlike other solids, is this thin layer present? Will it not cool down to ice?

  • See Rosenberg, "Why is ice slippery?" Physics Today 58.12 (2005): 50. – David Hammen Oct 20 '14 at 02:04
  • That people are falling has absolutely nothing to do with the absence of friction. They are falling for the very same reason as on surfaces with friction: by shifting the plumb line of their center of mass outside of their base of support. We do this all the time while walking, which is really a series of controlled falls. What happens on ice is that the horizontal force that our brain expects from surfaces with friction is missing. The correct response to "falling" is therefor NOT to push ones foot backwards to gain upward and forward momentum but to retract it until stability is restored. – CuriousOne Oct 20 '14 at 02:18

2 Answers2

1

Actually, the surface of the ice is frozen, until you step on it, then it melts. When pressure on the ice is increased, it can melt if it is not too cold. This is why ice skates work so well on the ice. They create a lot of pressure; creating a water layer between the ice and blade. This water layer decreases the friction so the blade slides easily.

From Chemwiki:

... one is able to melt ice simply by applying pressure and not by adding heat. enter image description here

LDC3
  • 3,817
  • 1
    This is the standard story, but it is so very wrong. – David Hammen Oct 20 '14 at 01:50
  • 1
    @DavidHammen The phase diagram lends credibility to it in that when pressure increases at the melting point, it becomes a liquid (near B). So if the ice is very near the melting point, what I stated is credible. What is your explanation? – LDC3 Oct 20 '14 at 01:55
  • 1
    No, it doesn't. Pressure explains why an ice skater with extremely thin blades can skate on ice at -3.5 C or warmer. People can skate on ice or ski on snow at -35 C. – David Hammen Oct 20 '14 at 02:06
  • 1
    @DavidHammen You haven't prove to me that what I said is wrong. Everything I said is credible. You didn't answer my question. What is your explanation? – LDC3 Oct 20 '14 at 02:40
  • 1
    That people can skate on ice below -22 C immediately falsifies the pressure melting concept. You can apply all the pressure you want and ice at -22 C will not melt. It will merely turn into ice III at about 210 MPa, then ice V at about 350 Mpa, then ice VI at 630 MPa, then ice VIII at about 2 MPa, and so on. It will never melt, at least not per the phase diagram. I picked -22 C on purpose so I could just barely skate by the ice Ih / ice III / water triple point. – David Hammen Oct 20 '14 at 03:05
  • 1
    @DavidHammen OK, the ice won't melt if it is very cold. This still doesn't explain why the friction is barely there. – LDC3 Oct 20 '14 at 03:07
  • @LDC3: David Hamen has a point when he says that the usual thermodynamic argument doesn't cut it. That doesn't mean there is no lubricating layer. It only means that one can not derive it from an equilibrium state function. – CuriousOne Oct 20 '14 at 06:03
0

Walking on ice can can be hazardous if you're wearing a nice pair of dress shoes with slick soles. The backward-sloping phase transition between water and ice cannot explain this phenomenon, and it certainly cannot explain why a hockey puck slides so easily on ice. Nor can frictional heating.

That ice is slippery is instead a boundary layer effect. That the surface of ice was somehow wetted was first proposed by Michael Faraday, but scientists forgot that hypothesis for a while. Scientists didn't have the tools to study this phenomenon until the 1960s, and it remains an area of active research to this day.

The lack of ice molecules above means the water molecules near the boundary layer can't quite organize themselves into a nice crystalline structure. Instead, those boundary layer molecules form a quasi-liquid layer at the ice/air boundary. The thickness of this layer is quite temperature sensitive. This means ice and snow stop being slippery when the temperature gets too cold. Pressure obviously plays a role, too; your dress shoes would make rather lousy ice skates. How pressure plays a role is still a bit up in the air.

The Physics Today article on the subject by Robert Rosenberg, "Why is ice slippery?" Physics Today 58.12 (2005): 50, is highly readable and very interesting. Read it for more info. And don't be afraid to follow the links in my google scholar search.

David Hammen
  • 41,359