Ice skates are one of the core equipment of ice sports, speed skating, figure skating, and ice hockey can’t be played without ice skates. Anyone who understands ice skating knows that the blades of ice skates are relatively sharp, even said to be more dangerous.
For example. In the women’s 3,000-meter relay race at the 2010 Winter Olympics in Vancouver, the Chinese team won the gold medal, and during the team members’ forgetful celebrations and hugs with their coaches, Wang Meng inadvertently gave her teammates a broken face, and her teammate Zhang Hui’s face was cut, and she received ten stitches afterward.
If you go to the Internet and search for “ice skate cuts”, you will find that cases of ice skate injuries happen all the time on the field of play and even on the training ground. So the question is, if it’s dangerous, why are ice skates made so sharp? Simply put, ice skates are designed to be thin, first and foremost, for grip when stomping the ice to start and accelerate. After all, the blade is embedded in the ice and provides a reaction force perpendicular to the blade when stomping. Other than that, the most common explanation is that increasing the pressure against the ice surface lowers the melting point of the ice, causing the ice at the point of contact to melt into a film of water and thus reducing friction.
- Dry friction and wet friction
Dry friction is the friction between solids and solids, wet friction is the friction between solids and liquids or gases, the latter coefficient of friction is much smaller than the former under the same pressure. For example, in order to reduce the friction between machines, industry usually adds lubricant, which is to take advantage of the characteristics of wet friction is smaller. For example, a floor that has just been swept by a wet mop is very slippery because of a similar principle.
The surface of ice, which is smoother than the surface of most objects (it is said that there is a very thin film of water on the surface), is not enough for ice skating, and its coefficient of friction would be further reduced if a layer of water could be sprinkled on the surface of the ice. But now there is a problem, if you do sprinkle water on a cold ice surface, the water will also solidify very quickly. What should we do then? You can’t just sprinkle lubricant on the ice, can you?
This is where the second point comes in.
- The melting point of ice and pressure
For crystals such as ice, the volume becomes smaller when it melts, and at higher pressures, water tends to become a smaller liquid, in other words, its melting point decreases. This can be answered from the three-phase diagram of water.
The reason why ice skates are made sharp is to increase the pressure on the ice surface by reducing the force area, so that the ice at the original contact point will melt quickly, thus playing a lubricating role. The melting point of the ice surface that has been cut will rise due to the disappearance of pressure, and the melted water will quickly freeze again. This is the refreezing phenomenon.
- Make snowballs
Have you ever wondered why the originally soft snow can be turned into a hard snowball by just squeezing it?
Snow is made of tiny ice crystals. When it is squeezed from outside, its melting point drops and it melts. After it is formed into a snowball, there is no more pressure on it. The melted part freezes again, and the remaining unmelted part is frozen into shape, so the snowball can maintain its shape after being squeezed. When rolling a snowball, the gravity of the snowball itself is also used to achieve the process of “melting” → “solidification” of the snow.
