www.whyville.net Aug 5, 1999 Weekly Issue

How Do Ice Skaters Get Spinning So Fast? Part 3

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By Dr. Leila Gonzalez
(and friends)

Note: This is a reprint of an article that originally appeared in the LA Times on March 25, 1999.

For the last two weeks we have been exploring the question: “How do ice skaters get spinning so fast?” I asked you to watch figure skaters and send me a description of what they do as they start to spin faster. Many of you observed that the speed of the spin seems to be related to the position of the skaters' arms; the closer the arms are to the body, the faster they spin, and the further away, the slower they go. So skaters control the speed of their spin by changing the positions of their arms (and also their legs), just as in the simulation in Whyville!

      So, that answers the question “How do skaters control their spin?” but not the question “Why does this work?” I was very happy to see that several of you sent in your own ideas about why this works. Some of you mentioned that it is easier to spin something that is compact and several others said this could be due to the effect of wind resistance. You said that when the arms were out, the wind resistance probably increases, and the skater then slows down. But does this really explain why the spin increases when the arms are close to the body? Here’s an experiment we designed that removes wind resistance as a variable. If we look at two identical things (since things that are the same shape and texture will each have the same amount of wind resistance), and we see a difference in how they behave while spinning (or rolling in this case), there must be something else going on. I suspect that the conservation of angular momentum has some effect (just like my physics friend Alicia hinted last week).

1. Connect to the Web. This week in the Whyville Spin Lab, you can look directly at the effects of resistance on spinning figure skaters and all the other new objects we have left in the lab.
2. Watch March 25th on CBS, “People of the Century: Great Minds, Great Discoveries.” Take a look at great thinkers and the methods they used that led to their discoveries. And on Sunday, March 28th, watch The Learning Channel’s show about the perils of space travel, called “Blast Off: True Stories From The Final Frontier.” Think about how conservation of momentum affects everything, including rockets, planets, and astronauts walking in space. Then write me!
3. Try this: Get 2 identical cans of fruit cocktail. Put one of them in the freezer for 24 hours. Then, you will need a long, wide board (wide enough for the two cans to roll down side by side without falling off) and a book (to raise one end of the board). Once your can of fruit cocktail is frozen, line both cans (frozen and unfrozen) up at the top of the board, and let them go at EXACTLY the same time. Try this several times and write down whether one can rolls faster down the board than the other. Because they are the same shape, any difference you see in the cans’ rolling speed cannot be due to differences in wind resistance. Can you figure out what is going on? (Next week we will finally tell you what we think!)
4. Next week, after I have read your observations from last week (send them in!), we will talk about what the conservation of momentum has to do with figure skaters changing the speed of their spins. (Do you already know? Tell me!)


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