Category: Science

An experiment no student ever did

Bev1 Comment
The title page of Newton’s Opticks. Courtesy of Wikimedia Commons.

I used to teach at Shimer College, and one of the courses I taught frequently was Natural Sciences 3: The Nature of Light. As part of the final exam, I asked the students to select one of the Queries from Isaac Newton’s* Opticks and design an experiment to answer it. Extra credit was available for actually doing the experiment. One of the more popular choices was Query 6,

Do not black Bodies conceive heat more easily from Light than those of other Colours do, by reason that the Light falling on them is not reflected outwards, but enters the Bodies, and is often reflected and refracted within them, until it be stifled and lost?

Newton 1952, pp. 339-340

This is illustrated by leaves on an outdoor ice rink.

Here’s a leaf frozen onto the surface of my backyard rink. It’s not black, but it’s dark enough to illustrate the principle.

It’s a sunny day with a temperature right around freezing. I can skate on the ice just fine, but look what’s under the leaf:

It’s a little pool of water in a hole shaped just like the leaf! Clearly the leaf is causing the ice to melt. The sun’s rays must have heated it up, just like Newton said. What’s left to do is figure out what’s going on inside the leaf that makes it heat up. I’ll leave that as an exercise for the reader. Meanwhile, here’s a montage showing how the ice melts around a darker leaf over a few warm, sunny days.

As the ice melts, a puddle forms on top of the leaf.

This explains why it’s a bad idea to use black plastic sheeting or paint hockey lines on your backyard ice rink: they’ll make the ice melt! Of course, if it’s cold enough, the ice will be fine.

*Isaac Newton was born on Christmas Day, 1642. Happy Birthday!

References

Isaac Newton. 1952 [1704]. Opticks. New York: Dover Publications.

Squeaking skates

Bev1 Comment
It’s a bit chilly out today. Image from weather.gov.

Did you know that skates squeak when it’s cold enough?

Listen for the squeaking.

I think the squeaking may be related to surface melting. This unsolved physics problem has been attracting a lot of interest because it affects many materials, not just ice. Robert Rosenberg summarizes it in his 2005 article “Why is Ice Slippery?“.

The most common explanation of how skating work is the “pressure-melting” hypothesis: skating is possible because skates melt a thin layer of water on top of the ice to glide on. There are some problems with this explanation. For one thing, if pressure-melting is really behind how skating works, it should stop working at about -3.5 degrees Celsius (25.7 degrees Fahrenheit). It doesn’t (Rosenberg, 50).

In fact, there’s always a thin layer of liquid (or liquid-like) water on top of ice. Michael Faraday figured this out in 1859 when he noticed that when you hold two ice cubes together, they freeze into one. Rosenberg describes a more recent experiment in which researchers measured the force required to pull two ice spheres apart. The colder it got, the less force was required, which means the film gets thicker as the temperature gets higher.

It’s possible that the recent low temperatures thinned the film on the surface of the ice enough to make my skates squeak. When a door squeaks, you put oil in the hinges to lubricate them. Perhaps there wasn’t enough oil (in the form of a liquid-like layer) to keep my skates from squeaking like there usually is.

I’ve observed other skates squeaking at higher temperatures. The layer thickness needed probably depends on several factors: how sharp the blades are, how they’re shaped (including the radius of hollow and the rocker), and the weight of the skater.

References

Michael Faraday. 1859. “Note on regelation.” Proceedings of the Royal Society of London 10:440–450.

Robert Rosenberg. 2005. “Why is ice slippery?” Physics Today 58.12:50–55.

Figures & science

Bev

The nineteenth century was when the system of compulsory figures was being formalized and governing bodies were created. Vandervell & Witham’s A System of Figure-Skating was the first `scientific’ treatise on figure skating. At the time, most figure skaters were rich  men: they had the disposable income to spend their  time — all day, on days when the weather was suitable — skating and thinking about skating. This was also an age when science was getting done. Rich men, perhaps even the same rich men, were also spending their time discovering the secrets of the universe.

There are a lot of similarities between skating figures and doing science. In both, one must be patient and meticulous. Every minor error must be examined and corrected. It can take many days, or years, to get a minor point right. And at the end, one has results that may not seem impressive to the uninitiated.

Was it a coincidence that these two activities involved the same people? Or was science a fundamental shaper of figure skating?

References

Henry Eugene Vandervell & T. Maxwell Witham. 1880. A System of Figure Skating: Being the Theory and Practice of the Art as Developed in England, with a Glance at its Origin and History. 3rd ed. London: Horace Cox.