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Spinning Tops!

Justin Gallagher

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spinning-top2-02.jpg

The spinning top, a toy found across many of the world's cultures and even among ancient archaeological ruins, lays bare some profound physical principles. The first is the conservation of angular momentum, the law that dictates that, in the absence of external influences, something spinning must keep spinning. Because a top balances upon a tiny point, it experiences a minimal amount of friction with the surface below it, and thus continues spinning for a delightfully long time, demonstrating the law.

But as friction eventually slows the top, it becomes unstable and starts to wobble, leading to the demonstration of another principle, called "precession." When the top wobbles, its axis of rotation the invisible line running vertically through its center tips sideways, making an angle with the table.

This angle allows the force of gravity to exert a "torque" on the top, putting additional spin on it, and this causes it to swing (or precess) outward in an arc, still spinning as it does so. In an effort to conserve its total angular momentum, the top precesses faster the slower it spins; this explains why tops typically lurch outward just as friction brings their spinning to a stop.



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