Solid substances get “harder” when pressure is put on them. If we were try to use ice skates on concrete, we would fall on our face. Why is it then that we slip and can skate on ice, even when it is 0 degrees?
Because ice is actually less dense than water. When you put pressure on it, a thin film melts to water, allowing you to slide.
Shoot. I’m getting a vague nasty feeling that Cecil has spoken on this and disagrees with me.
Anyway, that’s what I’ve heard all my life.
I’ve heard it all my life too, and don’t believe it. Well OK, I agree that you can melt ice by pressure - simple method is to get an icicle, lay it between two blocks, and hang a weight from it with a thread. The pressure of the thread melts the ice and the thread will eat its way through the ice.
But that takes time! I’m sure the heat created by the friction is much more significant. Skate blades are rough, right? (Haven’t used one in years…)
For complicated reasons I don’t fully understand ice has a thin layer of liquid water on it’s surface when the ice is warm enough. This layer allows your skate to slide over the ice with very little friction. At lower temperatures this layer disappears and ice is no longer slippery. I’m not sure the temperature when this happens, but I think it’s somewhere around -10 F or colder.
Smeghead:
I’ve seen that myth in textbooks and I believed it too. The problem is that the pressure between the skate and the ice is much too small to cause the ice to liquify. I think you’re right about Cecil talking about this, but I can’t find the column online.
Surely it’s friction that causes the ice to melt, leaving a thin film of water to slide around on.
Dr Lao is correct. The pressure from the skate is not enough to actually melt the ice. And even if it were, how would light weight objects like a puck melt the ice? Yet pucks slide on ice just fine.
The surface molecules of ice aren’t fully incorporated into the crystaline structure of the ice, so they have somewhat more freedom of movement. This makes them act somewhat like a liquid and so lubricates whatever is sliding on it. This is called surface melting.
The colder the ice, the fewer layers of surface melt molecules there are.
You want skate blades to be relatively smooth . . . if they had razor-sharp blades, they’d cut through the ice instead of resting on top of them. You run your finger straight down an ice skate blade and you get maybe a thin layer of stuff on your finger.
Ice is, in general, fairly smooth (so long as it ian’t bitch-ass cold). When you have a lot of people skating on it, it gets a thin (or sometimes not so thin) layer of water on it. Water is also smooth. This is why stuff slides on ice so well: very little friction.
Ice is cold :-). I was hoping to get an answer that I somewhat recalled having to do with a very few substances P-V-T curve. I believe that water and maybe ammonia had a negative characteristic curve which caused them to move from solid to liquid under pressure ?
Ice does, in fact, change states from solid to liquid under pressure. This process is called regelation. This makes the ice slippery (thin film of water on hard smooth surface). This allows, among other things, ice-skates to slide, glaciers to move, and thread to cut through an ice cube. Simply search for “physiscs” + “regelation” and you’ll find a host of further information.
Thank you, and Goodnight.