Tomorrow evening, the 2022 Beijing Winter Olympics will open!
There are 109 events in this Winter Olympic Games
Split ice and snow
Like yesterday took down the opening curling
Physical and intellectual brilliance[1]
Beautiful figure skating
Flying like a dragon[2]
and the Traditional Advantage of the Chinese Team short track speed skating
Wang Meng: You are here to fight for the second[3]
It's all ice.
Why can Wang Meng step on an ice knife and gallop?
How can I skate faster and more steadily?
Official spit, the most deadly[4]
There are three types of ice knives we usually say, including speed skating knives, synchronized ice knives and hockey ice knives, which, as the name suggests, are used for speed skating (including short track speed skating), figure skating and ice hockey. According to the different needs of different sports, different types of ice blades have different shapes, weights and even structures. Specifically, the speed skating ice knife has the narrowest blade, only about 1.4mm, compared to the widest pattern ice knife blade blade is 3.5mm thick, which can support athletes to make a variety of complex movements on the ice surface steadily.
I can't count how many times the feather knot has been turned[5]
The blade of the ice blade can be embedded in the ice surface, avoiding unwanted sliding in the direction of the vertical blade. Very little friction can be maintained in the direction of the ice blade. With the help of ice knives, athletes can glide freely on the ice without moving for an ice street dance.
this! It's hip-hop[6]
So why is there so little friction between the ice knife and the ice surface? A common theory is that the very small contact area between the ice knife and the ice surface creates a very large pressure, which causes the melting point of the ice to decrease, forming a film of water on the surface. It is this water film that reduces the friction between the ice knife and the ice surface. However, in a tweet two years ago (Ice: I've felt a little stressed lately| serious play), we asked the soul: How much does the pressure applied by the ice knife drop the melting point? Can it really melt the ice?
In the phase diagram of water, the BD segment of the solid-liquid dividing line describes the trend of ice melting points with pressure changes. The slope of the BD segment less than zero means that the melting point gradually decreases as the pressure increases.
Phase diagram of water[7]
Let's assume an athlete weighs 70 kilograms and look at how much the melting point of the ice can be lowered by the pressure he exerts on the ice when he stands on the ice with an ice blade. According to the calculation of speed skating ice skaters with the smallest contact area between the blade and the ice surface, in general, the front end of the blade is 8 to 9 cm beyond the toe, and the rear end is 5 to 6 cm beyond the heel [8]. Take the length of the blade of the ice knife is 50 cm, and the average width is calculated as 1.4 mm. When standing on one foot, the athlete exerts a pressure of about 10 Pa on the ice surface through an ice knife, which is 10 standard atmospheres. Substitute the Craberon equation that describes the phase transition
The result is that the melting point of the ice at this time is 273.08K, which is minus 0.07 °C. In the various competitions of the Olympic Games, the ice surface temperature is generally -5 °C ~ -7 °C. Obviously, relying solely on pressure will not melt the ice.
Some readers may think that although the friction on the ice is small, it is not completely absent after all. Is it possible that the heat generated by friction melts the surface of the ice into a film of water, which in turn reduces the friction on the ice surface? If that's what you think, congratulations on making the same conjectures as physicists back then.
In 1939, two scientists at the University of Cambridge hypothesized that the heat generated by the rapid friction between the ice knife and the ice surface during skating may be the main reason for the melting of the ice surface to form a water film.[9] Unfortunately, it was later proved that the thickness of the water film caused by frictional heat production will be significantly less than 20 μm, and the roughness of the ice surface itself is already tens to hundreds of microns. In other words, the water film generated by friction cannot even fill the "small puddle" on the surface of the ice layer, and it cannot provide effective lubrication.
With the development of experimental technology, after 1960, scientists gradually became able to observe the microstructure of the ice surface with enough precision. When the temperature is not too low, the surface of the ice layer in contact with the air does not mutate from the regular arrangement of water molecules into air, but there is a transition layer [10,11]. The interior of the transition layer has a gradually changing structure, from crystalline ice with neatly arranged molecules, to ice-water mixtures, to complete liquid water films. The miniature ice particles in the ice-water mixture are connected by weak forces such as van der Waals force and hydrogen bonds, forming a porous structure that can store liquid water, like gel beads. The closer to the air, the more disordered the arrangement of water molecules in the transition layer, the more liquid water there is, the weaker the connection between solid ice crystals, and even the emergence of freely moving miniature ballasts and ice particles in the water film.
Schematic of transition layers on ice surfaces[12]
In the latest theory[12], miniature ballasts and ice particles in the water film act as bearing balls, transforming the macroscopic sliding friction of skating into microscopic rolling friction. The pressure generated by the ice knife acts on the semi-molten ice water mixture in the transition layer, which releases liquid water like a pinched gel bead. This water increases the thickness of the original water film, makes the lubrication effect better, and further reduces the coefficient of friction on the ice surface. In this way, the ice knife can reduce the resistance through the transition layer on the surface of the ice without direct contact with the crystalline ice layer, and achieve smooth glide.
In speed skating competitions, sometimes a one-centimeter advantage will determine the final outcome of the game. Athletes have long since pinned their hopes for drag-reducing lubrication entirely on the nature of the ice surface itself. An excellent pair of ice knives is like the Ruyi golden hoop stick in Sun Wukong's hand, which is the technical guarantee for athletes to win the competition.
The ice knife used on the field now is no longer like we think, just a simple piece of iron. Its side shape and blade design are rich in mechanical principles. In the case of short-track speed skating knives, the side is a complex curve. Depending on the preferences of the athletes, this curve can consist of one or more arcs with a large radius of curvature. The curvature of these arcs can differ from each other, but transition smoothly at junctions. The specific design of the arc shape is closely related to the habits and movements of athletes, and there is no unified design standard and theoretical analysis. According to some existing numerical simulations[13], the cycloidal design with a radius of curvature of 15 to 20 m may be conducive to reducing the friction between the ice surface and the ice knife during specific force.
The arc shape of the front, middle and rear sections of the ice cut is different[14]
The blade of the ice knife is more different from the usual imagination, it is a grooved wide blade. The profile of the blade is an arc with a hollow in the middle and protruding downwards on both sides, forming a side edge of two sharp angles. The radius of curvature of a circular groove face is a relatively important design parameter. In the case of a certain width of the knife edge, the radius of curvature determines the depth of the depression in the center of the groove, which determines the sharpness of the two side edges.
Schematic diagram of slot-edged ice blade[15]
When sliding in straights, the advantages of slot-edged ice knives over ordinary single-edged ice knives are not particularly obvious. However, when passing through corners, the athlete needs the ice knife to provide a horizontal centripetal force, at which point the single-edged ice knife appears to be out of reach.
In high school physics, we learned the formula for centripetal force
In the case of the radius and mass of the turning trajectory are basically unchanged, if you want to pass the curve at high speed, you must have a huge centripetal force. Compared to single-edged ice knives that embed the ice surface at a 90° angle, the slot-edged ice knife can be embedded in the ice surface at a sharper angle because it only contacts the ice surface with the sharp angle on the side of the grooved edge. Under the same positive pressure of the split back, the groove-edged ice blade is embedded in the ice surface at a greater depth, providing a larger force area than a single-edged ice knife. Since the pressure between the ice knife and the ice surface is basically constant when the athlete pedals the ice, the larger force area can withstand more force. At the same time, while the inclination angle θ of the athlete's body remains unchanged, the side blade of the slot-edged ice blade is embedded in the ice surface, and the surface is more vertical than that of the single-edged ice blade. According to the principle of vector decomposition of forces, the horizontal force of the combined force is greater. These factors ensure that slot-edged ice blades can provide a greater centripetal force horizontally than single-edged ice blades. Therefore, the use of slotted edge ice knives allows athletes to complete the transition between the two straights more smoothly without slowing down when cornering.
Comparison of the force of a single-edged ice knife (left) and a slot-edged ice knife (right) when turning and pedaling ice[16]
Modern ice knives are designed to allow people to skate more steadily, faster and safer on ice. With the blessing of modern technology, small partners may wish to do a good job of safety protection, follow the footsteps of the ice piers of the Winter Olympics, and walk on the ice rink together
Wu "Tiger" take-off ~[17]
bibliography
[1] Sina Weibo @ People's Daily
[2] https://b23.tv/ZvteYbh
[3] https://b23.tv/2TPU2j5
[4] https://b23.tv/xeh7rXR
[5] https://b23.tv/SCudXQg
[6] https://b23.tv/4ln3qOD
[7]http://philschatz.com/chemistry-book/contents/m51080.html
Li Menghan. Research on the influence parameters of ice knife drag reduction based on water-based lubrication theory[D].North China University of Technology, 2021.
[9] Bowden, F. P.; Hughes, T. P. Proc. Roy. Soc. Lond. A 1939, 172 (949), 280.
[10] Rosenberg, R. Physics Today 2005, 58 (12), 50.
[11] Orem, M.W. J. Colloid Interface Sci. 1969, 31 (2), 278.
Wang Guangzhen,Wang Wenliang,Gao Xin,Zhang Shuyong. Causes of extremely low coefficient of friction on ice surface: Research progress and model analysis[J].University Chemistry,2019,34(01):33-38.
ZHOU Ji. Parametric design and friction characteristic analysis of speed skating ice knife[D].Jilin University,2016.
Liu Jincheng,Hu Lanping. Effect of microstructure changes of ice knife grinding on the performance of ice knife[J].Ice and Snow Sports, 1999(3):72-75.
Dou Zhaoliang,Liu Fengbin,Wang Jiadao. Research on drag reduction and efficient driving technology of speed skating ice knife[J].Ice and Snow Sports,2018,40(04):13-18.
Liu Feng,Guan Ruhua. A Theoretical Study on the Use of Slot Blade Ice Knives for Short-track Speed Skating[J].Journal of Physical Education,2013,20(03):108-111.
[17] https://b23.tv/RTIOG8M
Cover image source: B station @ Bilibili screening Ji
Memes come from the web
EDIT: Hidden Idiot