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Tops and gyros

Frequently I begin my column by delving into my childhood for pleasant memories about some activity or other that just happens to relate to my topic.

Not this time.

This week, my topic is tops and gyroscopes, and the fact is that as a kid I never saw the point of them at all. Remember those little wooden tops you wrapped a string around and then flung at the ground? I couldn’t have cared less. So they spun around and around for a while, then they fell over. Big deal. (All right, I admit I was never able to make one of the darn things work. Want to make something of it?)

Gyroscopes were even more boring: a metal wheel spins inside a metal cage. Whoopee! Are we having fun yet?

Ah, but now, as a mature, sophisticated adult with a science column to write, I can appreciate the finer qualities of the two toys.

The interesting thing any spinning object is that, once it starts spinning, it resists being tipped. You can feel this very clearly with a toy gyroscope: it feels as if some invisible force is holding it in place.

This invisible force is called “angular momentum.” Regular momentum is the tendency of a moving object to keep moving in the same direction until some other force acts on it. Angular momentum is similar: the tendency of a spinning object to keep spinning in the same position until acted on by another force.

Angular momentum is what keeps a top balanced on its sharp, pointed base as long as its spinning. As the spin runs out, so does the angular momentum, and eventually it falls over. Before that happens, the top exhibits something else, which you and I would probably call wobbling but science has labelled “precession”: the top of the top starts to go around in a little circle.

Angular momentum is “conserved,” which is best demonstrated by a figure skater, spinning slowly with her arms outstretched, who pulls them in and suddenly starts spinning much faster. It takes more energy to spin an object with a large radius, such as a skater with her arms outstretched, than an object with a small radius — the skater with her arms pulled in. The skater establishes a certain amount of angular momentum when she starts spinning; when she pulls her arms in, she no longer needs as much energy to spin, but she still has all the momentum she started with and it has to go somewhere, so her spin speeds up.

Angular momentum makes gyroscopes very useful devices. They’re usually mounted in a framework called a gimbal, which can then be mounted in other gimbals rotating in up to three different directions at right angles to each other. Because the gyroscope, assuming you can keep it spinning (usually done electrically), will maintain its original orientation, by measuring the way the gimbals move around it you can determine, for example, whether an airplane is level (indicated by the artificial horizon on the control panel) or which direction it’s pointing (indicated by the gyrocompass).

Really large gyroscopes can be used not only as instruments, but as stabilizers. Large spinning masses help to keep cruise ships from rolling, for example, preventing the Love Boat from becoming the Barf Boat.

An even better example of a gyroscope-stabilized vehicle is the bicycle. As you probably recall from your own childhood (which is good, because it means I don’t have to bore you with mine), the biggest problem with learning to ride a bicycle is getting going. Once the wheels are spinning, they become stabilizing gyroscopes, resisting tipping and helping to keep the bicycle upright.

On a much, MUCH larger scale, the Earth itself is a giant gyroscope. It’s even got a slight wobble, which accounts for the fact that the exact times when spring and autumn officially start change from year to year, a phenomenon called the “precession of the equinoxes.” And, of course, the whole solar system is also spinning, exhibiting conservation of angular momentum so that changes in one part affect others — for example, the Earth’s spin is slowing because of friction from the ocean’s tides, and to conserve angular momentum, the moon is slowly receding.

At this point, I’m supposed to say something like, “Had I only known these fascinating facts as a child, tops and gyroscopes would have been my favorite toys.”

Sorry. I still think tops and gyros are boring as toys — but they’re fascinating science!

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