Thursday, September 20, 2012

How Playing the Viola (or any stringed instrument) is Like Playing with a Yo-Yo

I really enjoy listening to the How Stuff Works podcast. I always learn something I never knew before (or even really thought about), like how a yo-yo works. In the podcast I listened to yesterday, the banter concerned the way the spinning motion of a yo-yo gives it gyroscopic stability:
1. The yo-yo is held up in the air, giving it the potential to fall to the ground.

2. The yo-yo has string wound around it, giving it the potential to spin as it unwinds.

When the yo-yo is released, both forms of potential energy change to kinetic energy. The yo-yo spool falls straight to the ground, which builds a certain amount of linear momentum (momentum in a straight line). At the same time, the string unwinds, and the spool spins, which builds angular momentum (momentum of rotation).

When the yo-yo reaches the end of the string, it can't fall any farther. But, because it has a good deal of angular momentum, it will keep spinning.

The spinning motion gives the yo-yo gyroscopic stability. A spinning object resists changes to its axis of rotation because an applied force moves along with the object itself. If you push on a point at the top of a spinning wheel, for example, that point moves around to the front of the wheel while it is still feeling the force you applied. As the point of force keeps moving, it ends up applying force on opposite ends of the wheel -- the force balances itself out. This phenomenon keeps a yo-yo's axis perpendicular to the string, as long as the yo-yo is spinning fast enough.
I do not have the vocabulary or knowledge of physics to explain why sometimes it feels like pulling and pushing the bow perpendicularly across a string feels like using a yo-yo, but the above explanation does a good deal to illuminate the feelings of opposing forces that seem to be at work while I'm doing it. It is a helpful image to use in order to try to keep the string vibrating (or to minimize the gap in vibration) between bow changes.

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