Page 18 - Science Focus (issue 15)

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Ice-Skating is a common leisure activity that usually takes place at large ce-Skating is a common leisure activity that usually takes place at large
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shopping malls. I believe many of you must have skated before. However, have you opping malls. I believe many of you must have skated before. However, have you
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ever wondered why we can move freely on ice with a pair of ice skates? What are the ver wondered why we can move freely on ice with a pair of ice skates? What are the
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mechanisms and theories behind? Let us explore!echanisms and theories behind? Let us explore!

Extraordinary Science:

T he Mechanism of

科學不一樣： 探討溜冰的奧秘

You may have heard of “solid-liquid phase The description above sounds reasonable,
equilibrium” when you studied thermodynamics based on what we’ve learned from school.
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at school . As shown in Figure 1, you may notice However, investigations by many physicists revealed
from the phase diagram that the freezing point (i.e. that the mechanism of skating was much more
the melting point; the red line in Figure 1) of water complicated, which could be explained in more
decreases as the pressure increases. As we skate, than one way.
the contact area between the blade and the ice
surface is very small, and the body weight would According to the article, “Pressure Melting
concentrate on that contact surface. This exerts a and Ice Skating”, published by a geophysicist,
great pressure to the ice surface (greater than the Samuel Colbeck in 1995 in the American Journal
standard atmospheric pressure, i.e. 1 atm, or 101 of Physics, the decrease in friction could not be
325 Pa in SI unit), and allows the ice in the ice rink due to high pressure-induced reduction of the
to melt, even when the temperature is lower than freezing point. He explained that at 0 °C, for every
0 °C. In this case, the friction between us and the additional pascal of pressure exerted, it would only
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ice becomes especially small, akin to having a thin cause a decrease of 7.37 x 10 °C in the melting
layer of water as lubricant. We can therefore skate point of water, due to the pressure-melting effect.
freely on ice. Therefore, if we are skating on a -1 °C ice surface
and we want to lower the melting point to -1 °C, by
Nevertheless, if it’s true, why wouldn’t all the ice calculation, a pressure of 1.4 x 10 Pa (around 140
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in the ice rink turn into water when there were a lot atm) is necessary. Assuming that the contact area
of skaters? This is because the pressure exerts on the between the blades and the ice surface is 10 cm ,
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ice surface would return to normal after the blade to exert the equivalent pressure , the weight of the
passes. As a result, the freezing point would return to skater must reach 1400 kg. We clearly know that
0 °C, which is the one under standard atmospheric it’s impossible, and that’s why the melting caused
pressure again. The water membrane freezes again by pressure change is not a convincing reason to
and the ice rink won’t become a pool of water. explain why we can skate freely on ice.