I was watching a hockey skating competition. The best skaters are able to accelerate quickly and turn without losing much speed; however, most people have similar top speed around 20-25 mph. What is the bottleneck on top speed in skating? It must be the skate friction somehow because speed skating, which uses different skate, is much faster than ice hockey skating in terms of top speed.
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1$\begingroup$ IDK, even despite the fact that I have skated on a speed-skating oval, but if they're really going 25 miles per hour, the wind resistance will not be trivial. $\endgroup$– Solomon SlowCommented Sep 30, 2019 at 20:02
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7$\begingroup$ Ice skating world records for 500m and 1000m are about 55 km/h (35mph) average, top speeds are even higher. Ice hockey trades top speed for agility $\endgroup$– MSaltersCommented Oct 1, 2019 at 7:47
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3$\begingroup$ Take a look at the top speed for ice boats (sail-driven). $\endgroup$– Carl WitthoftCommented Oct 1, 2019 at 13:49
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4 Answers
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The two big factors are air resistance and skate edge.
Air resistance increases as the square of speed. At 25mph, for example, about 90% of a cyclist's effort goes into overcoming air resistance. Speed-skater's slippery clothing makes a big difference at high speeds.
A skater propels themselves forward by pushing side-to-side, not forwards-backwards, with their skates at an angle to the direction of travel. At higher speeds, your skates need to make a smaller angle relative to your forward motion, and this means you need a longer, straighter edge on the skates to be able to grip. It's a bit like a lever (small movement one end can make big movement the other), or perhaps a sailing ship (angle the sails right and you can go faster than the wind is blowing).
Hockey skates are curved for manoeuvrability and you'll run out of edge pretty quickly. Speed skates have very long, sharp straight edges, so they can grip and produce useful forward force even at very high speeds.
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1$\begingroup$ Oh, interesting, I'd not come across them. Thanks! $\endgroup$– jcupittCommented Oct 1, 2019 at 9:15
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2$\begingroup$ Well, then skate in a vacuum. Only partly joking - if we could build a flexible spacesuit.... $\endgroup$ Commented Oct 1, 2019 at 13:51
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4$\begingroup$ "Air resistance increases as the square of speed." The power needed to overcome it is cubic in speed. $\endgroup$ Commented Oct 1, 2019 at 20:29
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1$\begingroup$ The second difference between hockey skates and speed skates is, that the later have a blade that can turn around a pivoting joint as the athlete stretches their foot. This allows for a longer contact with the ice, allowing more momentum to be exchanged within a single stroke, thus making the skating more efficient. $\endgroup$ Commented Oct 2, 2019 at 4:19
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Two important things come into play:
1) Air resistance and friction with the ice. Kinetic friction is not completely linear, but it won't increase with speed the way air resistance does. Air resistance will be a significant force at 25 mph.
2) Leg Speed. To exert a propulsive force on the ice, when you go to accelerate you must be moving your foot at more than 25 mph since that is how fast the ice is moving relative to your body to begin with. This is, incidentally, also the major limiting factor for runners, and the reason why people can get to much higher speeds on bikes.
As Nuclear Wang points out in the comments, bikes don't completely skirt this limitation - they are limited by tire speed which is limited by leg speed. But the mechanical advantage of the pedal system allows the tire speed >> leg speed
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$\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$– tpg2114Commented Oct 1, 2019 at 23:19
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6$\begingroup$ "To exert a propulsive force on the ice, when you go to accelerate you must be moving your foot at more than 25 mph" I think this is wrong. In the frame of reference of the speed skater, their feet move comparatively slowly, and move sideways. $\endgroup$ Commented Oct 2, 2019 at 10:51
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As with most limiting factors with regards to speed they are usable input power, and power dissipation.
Input Power
Human legs can provide power by pushing with a certain speed, higher force and higher speed increase the amount of power available. Human muscles cannot apply the same force at all speeds but rather as the speed increases the force that the muscles can apply decreases. This decrease isn't exactly an inverse relationship, so the amount of power available depends on the speed of the action with a maximum somewhere in the middle.
Maximizing Power
Anyone familiar with bike gears understands that by changing your gears you can change how effective you are at pushing the bike forwards. What a rider is doing by changing gears is changing the speed at which their feet must move in order to produce thrust. This can be used to tune that speed to match the speed at which the rider produces maximum power.
A similar mechanism can be used in the case of ice skating: the angle of the skates. The angle of the skate determines the ratio between the skaters forward velocity and the skater's foot's velocity. Just like with the bike, this can be used to tune the speed to match maximum power output.
This is very different from running for example, where the runner has no choice but to move their legs at the speed they are running.
Power dissipation
In all of these cases you can keep accelerating until your power output is exceeded by your power dissipation. In the case of ice skating there's air drag and skate drag.
The skate drag for ice hockey skates is higher than for speed skates due to their shorter/more curved blades1. This results in a higher effective coefficient of friction. Interestingly, this coefficient of friction comes into play with determining the ideal angle of skates. If the skater chooses a shallower angle so they can move their legs slower and apply more force, then some of that additional force is eaten up as additional power dissipation from the increased friction. So it turns out that the skater should actually choose a slightly wider angle than what would allow them maximum power output as that slight bit of additional power would be eaten away by additional frictional losses.
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In ice hockey skating, you want maximum maneuverability while in speed skating you want the fastest speed. To maneuver on skates requires a coefficient of friction higher than that of skating straight ahead. I don't know what the skates look like but I'd think the sides of the blades would have high coefficient while the bottoms would have lower.
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$\begingroup$ The most important effect is that for speed skating the blades are not rigidly attached. The front is hinged and the rear spring-loaded. $\endgroup$– MSaltersCommented Oct 1, 2019 at 7:51
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2$\begingroup$ @MSalters: is that the most important effect? The use of clap skates in competition is relatively recent, and I'm pretty sure speed skaters where faster than hockey skaters even before the introduction of clap skates. $\endgroup$ Commented Oct 1, 2019 at 13:17
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5$\begingroup$ @RoelSchroeven is correct - the longer blade allows for more force to be applied without losing lateral grab on the ice, as well as reducing the pressure (same mass, larger interface). Hockey blades are short, and rounded, so that one can turn without having to raise the skate - turning is almost impossible on a single speed skating blade. $\endgroup$ Commented Oct 1, 2019 at 13:52
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$\begingroup$ Jimh, the physics of friction between the blade and the ice is very very complicated - involving special behavior of the top layer of the crystalline ice/ $\endgroup$ Commented Oct 1, 2019 at 13:54
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1$\begingroup$ @CarlWitthoft Actual speed skate blades are also rounded. Albeit with a much larger radius, normally around 21 meter. Ice hockey skates are rounded with a radius of about 3,9 meter. $\endgroup$– WilgertCommented Oct 1, 2019 at 16:01