How To Build a Short Foucault Pendulum 79
KentuckyFC writes "Set a pendulum in motion and you'll inevitably give it an ellipsoidal motion, which naturally tends to precess. That's bad news if you want to build a Foucault Pendulum, a bob attached to a long wire swinging in a vertical plane that appears to rotate as the Earth spins beneath it. The natural precession always tends to swamp the rotation due to the Earth's motion. There is a solution, however: the behavior of the ellipsoidal motion is inversely proportional to the pendulum's length. So the traditional answer has been to use a very long pendulum (Foucalt's original in Paris is 67 meters long). Now scientists at Carnegie Mellon University have another solution (abstract). They've created a motor that drives a pendulum in a way that always cancels out the precession. That means the effect of Earth's rotation can be seen on much shorter pendulums such as the 3-meter pendulum on which they've tested their motor. That's just the start though. They say there is no limit to how short the new generation of Foucault Pendulums can be, and even talk about the possibility of tabletop devices."
Inevitably? (Score:5, Insightful)
What if I pull the pendulum using a string, tie the string to a fixed object, wait for the pendulum to stop moving, then cut the string?
Or any of a hundred other methods; that's just the first that came to mind.
I'd be more concerned about vibrations, friction effects, poor suspension system, etc. that affect the precession of a small pendulum after it starts swinging. Fortunately this device seems to counteract those forces as well.
Re:Inevitably? (Score:3, Insightful)
Set a pendulum in motion and you'll inevitably give it an ellipsoidal motion, which naturally tends to precess.
I suspect the point is that the summary is somewhat poor, in that it's incomplete or just plain wrong. Why would an elliptical motion given to a pendulum tend to precess, even in the absence of a rotating reference frame? A "natural" precession (termed so in the summary) doesn't come about unless the axial symmetry of the Hamiltonian is broken. Even for a pendulum in which torsional modes and swinging modes are coupled, or for one in which a ratchet allows for elliptical motion in one angular direction, etc., there is no precession.
Re:Inevitably? (Score:2, Insightful)
A: That apple is red.
B: Really, that apple is absorbing and reflecting light in such a way that
the frequencies of the radiation bouncing off of the apple and striking your retina are causing you to perceive that the apple is red.
B: Hey, where did you go?
People use language with varying degrees of precision. Try to learn to deal with it.
Re:Ummmm (Score:3, Insightful)
And they couldn't even put it in a vacuum chamber? :)