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B Ice skating mechanism

  1. Nov 16, 2016 #1
    As far as I know, ice skating works because high pressure lowers ice melting point. A thin layer of water forms between the blades and the ice which greatly reduces the friction.

    Just now, I am reading a thermodynamic book. For the phase boundary between ice and water, dP/dT ~ 135bar/K. Assuming a man with weight 600N, blade area ~ 2.5mm x 20 cm x 2 ~ 10^-4 m^2. The pressure is around 60 bar. So it only changes the melting point by about 0.44°C.

    The book claimed that this is not a significant change. The ice won't melt unless the temperature is already very close to 0°C.

    If the book is right, what is the real mechanism for ice skating?
    If the book is wrong, which means the mechanism is true, does it mean that if the ice is, say -1°C, we can't glide on the ice?
     
  2. jcsd
  3. Nov 16, 2016 #2
    Yes when you apply pressure the temperature changes only a little, and yes IF the only reason for the melting was because the temperature changed this would only work near freezing. I think your text is trying to make the point that adiabatic temperature rise is NOT the mechanism that makes ice skates work.

    However you shouldn't equate dP/dT with "changing the melting point". In fact your premise is that the melting point didn't change at all in temperature (still 0 C).

    Now go look at a phase diagram of water. Look at the slope of the melting point as a function of temperature vs pressure. It has a distinct negative slope. Even if the pressure did not cause any temperature rise, even if dP/dT was zero, changing the pressure would still cross the melting point because the temperature at which water melts drops with increasing pressure. With pressure the temperature doesn't change (much). Instead the point in temperature at which water melts changes a lot.
     
  4. Nov 16, 2016 #3
    I only thought about the phase change. Do you mean that high pressure on the ice at an instant will change the temperature at that point? I am sorry that I don't quite understand the last paragraph
     
  5. Nov 17, 2016 #4
    I'm afraid I misunderstood your post. I'm wrong. Please ignore my previous comment.
     
  6. Nov 17, 2016 #5
    So I did some reading and there are several additional factors

    The blade is concave so you ride on two edges increasing the pressure. This helps explain it up to degrees instead of half a degree, but still falls short.

    There is friction causing heating. Ok, that's fine when you are moving, but the ice is slippery when you are standing still.

    In fact the ice is slippery when you walk on it with flat shoes. The water must already be there. This brings us to the actual answer. The surface does not have the same properties as the bulk, and down to -20C or so ice will have a layer of water on top
     
  7. Nov 17, 2016 #6
    First, not all blades are concave. All speedskates in the world are flat across the bottom of the blade with the edges making exact 90-degree angles.

    And long blades like speedskates are narrower than hockey for figure skates.

    Also, speedskates are rockered all the way from the toe to the heel - there is no flat blade - so the actual point of max pressure is very high (and constantly changing depending on the angle of the blade). When water fails to freeze under the blade (as it sometimes does) it can be heard as a squeaking (just like boots in snow) and the glide turns to traction.

    AND, when really good speedskaters are skating you can hear tiny pops from the blades (it's how you know you're getting good - ask any speedskating coach). The popping can only be caused by the ice re-freezing after having been melted by pressure. It does it in an instant and seems to emanate from slightly behind the major pressure point (thus my belief that it's an instant re-freezing that causes it).

    And lastly, if the ice is very cold (slow ice we call it) then friction between the blade and the ice will conspire to melt the ice.

    If you examine the 2mm wide track a speedskate makes on the ice you can see water crystals (frost) having formed across it's width (you need very good eyes or a lens to see this).

    All from experience...
     
  8. Nov 17, 2016 #7
    To conclude, there may be two reasons apart from the change of melting point.
    1. Ice melt due to friction.
    2. Generally speaking, the ice rink is already very close to the melting point of ice.

    Am I right?
     
  9. Nov 17, 2016 #8
    From what I've read apparently the most important thing is there is a slippery surface layer of poorly bound molecules even at 1 atm pressure and down to -20C. The surface molecules are not in the same environment as bulk molecules. One side is not bound to other molecules. With less constraint the bonds that they do have are also different with different bond lengths and strengths. The disturbed bonds don't end with the first layer of molecules. The difference in the first layer means there is a different environment for the second layer etc. so you have to go several layers deep before you get to molecules that have the bulk spacing and bonds. That surface region has different bonding than the bulk and different thermodynamics. Apparently the bonds are weak and can be broken and reformed easily in a liquid like way if you can call such a thin layer a liquid.

    All the other stuff happens too (pressure melting, friction, adiabatic heating), but apparently the starting point for answering why is ice slippery is "because the surface layer is slippery".

    I should probably stop talking having only said foolish things before I say something really stupid. However, I never do. Sheer speculation here, but I would guess that the liquid solid line is also different in this near surface layer so maybe pressure melting happens more easily than the bulk phase diagram would suggest. I mean since the bonding is already more water like, perhaps it takes less effort to drive the water transition deeper into the surface layer. Since the bonding changes from water like to solid over a transition region, maybe the main thing that changes with temperature is the depth of the water layer created with pressure.
     
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