Abstract
Figure skaters graciously glide over ice and perform intricate aerobatics, speed skaters achieve incredible velocities over ice, and thousands of persons each year stumble in ice as they first try out this practice for leisure. This paper reviews the scientific concepts and laws that make ice skating possible, as well as the factors that influence performance and ice skates design.
Introduction
Ice skating, gliding along an ice surface using keel-like runners known as ice skates [1], is a sport practiced by many solely for recreational purposes, while others engage in it as an inherent part of other sports, such as figure skating, ice-hockey, bandy and others. Natural bodies of frozen water such as lakes and ponds, as well as indoor and outdoor man-prepared especial surfaces serve as ground to practice sports on ice. It is unsurprising that ice skating has become such a popular activity, after all, what is there not to enjoy about sliding on ice using special skates? Ice skating has become a symbol that represents the winter season, and some rinks are even internationally recognized for being popular holiday tourist attractions, such as the Rockefeller Center in New York City.
However, how many ice skaters stop to analyze the physics behind this activity? Is it possible that famous figure ice skaters or hockey players marvel on the science that allows them to so gracefully glide on ice while performing advanced maneuvers? Probably just a few, but this paper will analyze the engineering behind this popular sport.
How it works
The first concept that must be introduced when explaining how ice skating works is friction. Friction is defined as the force that resists the sliding or rolling of one solid object over another [2], thus effectively reducing their energy of motion; it is nearly independent on the area of contact between both solids, and it is proportional to the force pressing them together.
Ice surface friction is naturally low, given that it is covered by a thin layer of water that gives it its slippery quality. It was previously thought that the pressure created by ice skates over the surface produces a liquid film over the ice, a phenomenon known as pressure melting, a hypothesis supported by an engineer named John Joly in 1886 [3]. However, the experiments he conducted were inconclusive and this hypothesis was later debunked as it was proven that the pressure exerted by skates over ice is not nearly enough to melt it, and it is now accepted that except in temperatures near the melting point, pressure melting is not a significant factor.
An alternative was suggested to be frictional heating, which sustains that the friction created by the movement of ice skates over the ice surface produces enough heat to melt it, supported by the fact that blades leave warm tracks on the surface. However, this theory cannot explain the reason for which ice is slippery even when someone simply stands on it, therefore it was also discarded.
So, how is it that a liquid layer exists over ice even in temperatures below zero? Experiments conducted by Michael Faraday in 1850, regarding the freezing together of two ice cubes when they come into contact, a phenomenon known as regelation, were the first concrete steps towards answering this question, but were ignored by the scientific community for over a century [4]. Eventually, further investigations on the subject were conducted and concluded that “in most crystalline materials each surface molecule has an identical binding energy, while in the surface of ice, molecules are bound by different forces” [5], therefore atoms in the surface of ice can move freely and the interaction among them generates a thin liquid film over the ice.
This thin layer of water over the ice is what allows for skaters to glide over it so effortlessly, as the friction between the blades and the ice is nearly zero and skaters can impulse themselves forward by pushing down their weight with one leg, which exerts a perpendicular force from the ice, and using the other leg to glide forward by taking advantage of this force. This movement is repeated alternating legs. A diagram that illustrates the basics of ice skating is presented in Figure 1. When angle α is increased, the skater can push the ice with greater force, thus increasing acceleration and skating faster [6]. The maximum speed that a skater can achieve is directly proportional to the speed with which he can move his feet, as skating speed is determined by the relative velocity of his feet to the ice.
Figure 1. - Basic Ice Skating Motion
The existence of this force is explained by Newton’s Third Law, which states that for every action there is an equal and opposite reaction [7], which is the reason why when skaters push off the ice with their blades, they apply a force to the surface which in response creates an equal but opposite force that propels the skater and allows him to glide or jump.
A common mistake made by beginners is standing too upright when skating, when on the contrary, the proper way for a skater to stand is with bended knees and chest slightly pushed forward to assume a squatting position. This way balanced is improved by counterbalancing the torque caused by the horizontal component of the F force, while also allowing for greater acceleration and speed.
Applications
The physics behind ice skating can be used to further analyze variations of maneuvers used in this sport, as movement is not always realized in such a straight forward manner. For instance, those who practice short track speed skating can often be seen leaning significantly towards the ice while making turns, with their skates no longer upholding a perpendicular position relative to the surface, as shown in Figure 2. When performing these maneuvers, they no longer accelerate using the force previously explained, but rather by taking advantage of the momentum gained on the straight trajectory. Momentum is a quality that describes an object resistance to stopping [8], the heavier and faster an object is moving, the more difficult it is to slow it down.
Figure 2. - Short Track Speed Skating Turning Position
Similarly, when figure skaters spin, they draw their arms and a leg inward which reduces rotational inertia, but since angular momentum is conserved, the velocity of the speed increases to compensate. Conversely, when the arms and one leg are extended outward, the velocity of the spin is reduced [9].
On a different note, understanding the basic physics concepts that define ice skating aids in the design of ice skates, which play a major role in this sport. Original skates were made from animal bones, but over the years significant modifications have been made based on experimental evidence for ice friction. Observations concluded that the fastest skates are those that allow for longest strides as this allows for greater locomotor efficiency. Today, ice skates are designed for different applications, being divided into three major categories: hockey skates, figure skates and speed skates. These vary in blade length and width to fit their application requirements. For instance, as previously mentioned, the fastest skates are those that allow for longer strides and thus, speed skating skated have the longest blades, of about 46 cm [10]. Figure skates have the widest blade with a hollow bottom, designed for spinning and hockey skates have higher boots and have extra layers of leather in the toe area to protect the player from other players’ blades.
These are the basic concepts required to understand the physics behind ice skating, as well as the factors that influence skater’s stability, speed, and advanced stunts.
Looking into the future
In its origins, ice skating was used as a means of transportation utilized in Finland for survival purposes, as they had to mobilize through frozen water bodies in order to hunt for food or perform daily activities, and this practice proved to be much more energy efficient than walking around all the lakes and ponds to get to their destinations [11]. Today, the most popular purpose for practicing ice skating is simply leisure, while others consider it a base for sports such as fen skating, figure skating, kite ice skating, speed skating and tour skating.
New technologies are being developed in order to improve the way Olympic caliber athletes train, by accelerating their ability to learn and perfect complex spins and jumps. A study worth mentioning is currently being developed by Jim Richards at the University of Delaware, in which through the use of motion capture technology on ice skaters’ suits, 3D models are developed and allow for the study of different movements through computer simulation. This way, skaters can analyze the effects of the use of different techniques without actually performing the stunts themselves, which has the potential of reducing failed attempts that may result in joint injury [12]. The end goal is to eventually make this technology affordable and thus widespread, in the form of light sensors coupled to a smartphone application.
Conclusion
Ice skating, whether performed for sport or leisure, is an activity that relies on basic physical principles. The low friction between ice and the blades of ice skates, which is guaranteed by the thin film of water present in the surface of ice, allows for skaters to almost effortlessly glide over the surface of what can be natural bodies of frozen water, or artificially developed indoor or outdoor ice rinks. Understanding of basic concepts of physics are crucial for skaters to improve their performance, as these define variables such as speed and stability. These concepts are also important in the design of ice skates and their adaptation to the different applications of ice skating.
References
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[2] The Editors of Encyclopedia Britannica. «Encyclopedia Britannica.» 21 April 2014. Friction. Web. 01 April 2016.
[3] Rosenberg, Robert. «Why Is Ice Slippery?» Physics Today (2005): 50-54.
[4] Formenti, Federico. «A review on the physics of ice surface friction and the development of ice skating.» Sports Medicine (2014): 276-293.
[5] Watkins, M, et. al. «Large variation of vacancy formation energies in the surface of crystalline ice.» Nature Materials (2011): 794-798.
[6] Real World Physics. The Physics of Ice Skating. s.f. Web. 01 April 2016.
[7] The Physics Classroom. «Newton's Third Law.» s.f. The Physics Classroom Website. Web. 01 April 2016.
[8] Moskowitz, Clara. «The Physics of Figure Skating.» 16 February 2010. Live Science. Web. 01 April 2016.
[9] Hokin, Samuel. «The Physics of Everyday Stuff: Figure Skating Spins.» s.f. B sharp. Web. 01 April 2016.
[10] Sorokanich, Robert. "How Olympic Ice Skates Are Designed for Speed, Spins, and Swerves." n.d. Gizmodo. Web. 01 April 2016.
[11] National Geographic. «Bone Ice Skates Invented by Ancient Finns.» s.f. National Geographic Website. Web. 01 April 2016.
[12] CNN. The 3-D technology that is helping ice skaters. 28 June 2013. Web
