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How To Build a Short Foucault Pendulum
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."
A new generation of executive ball-clickers is born.
That sounds like a feat of engineering, not science.
At any rate, I'm sure the pendulum clock industry is ecstatic. Can I get a Ph.D. for building a motor to flip sand-filled hourglasses over?
A slashdotter who didn't build his own computer is like a Jedi who didn't build his own lightsaber.
(maybe slightly OT)
As well as a physics experiment, Foucault's Pendulum is a fantastic book by Umberto Eco.
It's in the same subject area as the Da Vinci Code before that opportunist lightweight Dan Brown ever put pen to paper, and it has far more depth and erudition. There's even some code (BASIC) in it...
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.
How can I believe you when you tell me what I don't want to hear?
When I was a kid I used to dream about having a tabletop Foucalt pendulum. My friends told me I was mad, and my parents tried to discourage me from thinking about it. "There's no such thing as a small Foucalt pendulum, Zouden! Maybe one day we'll take you to visit the one in Paris, but you'll never get to have your own one." Now, finally, my prayers have been answered.
"A week in the lab saves an hour in the library"
I live near the equator, you insensitive clods!
You don't, and as scientific proof of the Earth's rotation, this is obviously completely useless. But if you trust the motor, this is a fun way to see what a Foucault pendulum does, without the expense and inconvenience of needing a full-sized model.
It's a toy, but it's an educational one. It's not like we need to do experiments with it (although I guess you could try to measure changes in the Earth's rate of rotation or something).
SMAH used to have a Foucalt Pendulum in their main entrance lobby right in front of the original Star Spangled Banner. Damn if that wasn't one of my favorite things in Washington. Sadly, it was removed after being modded -1 off topic.
You don't, and as scientific proof of the Earth's rotation, this is obviously completely useless. But if you trust the motor, this is a fun way to see what a Foucault pendulum does, without the expense and inconvenience of needing a full-sized model.
True enough, but if you ever want to show it to others, there will be skeptics.
I once saw a full-size Foucault pendulum at a science museum. If you stood and watched it for a few minutes you could see the precession (there were markings on a ring around the pendulum, so it was easy to see where it swung before). I overheard some other patrons asking if it was powered, why it didn't come to a stop, etc. The museum guide explained that it was not powered and how it worked, and mentioned that because of air resistance they used an electromagnetic ring to give it a tiny "push" with each swing to keep it going. He also explained that because the magnet was circular, it would always push the pendulum directly back the way it came rather than from side to side.
Several onlookers remained convinced that it was a trick and the electromagnet was causing the precession. And remember, these are people standing in a science museum, looking at an exhibit so massive it required the entire building to be designed around it, whose entire point was to show this effect.
Now imagine if there had been a motor attached to it, designed to "compensate for ellipsoidal motion"....
A large Foucault pendulum will run just fine for quite a while with no motor or pushing device attached. The motor in the science museum was just so they didn't have to send someone out to give it a shove every shift/day/week.
Naturally, if you really want it to be convincing, you have to not only not power it, but you also have to start it many times and observe that it ALWAYS goes around the same way, at the same rate.
Unfortunately scientifically rigorous demonstrations are often too complicated and time consuming even for science museums.
This is very cute. The pendulum is powered, weakly, by a coil centered under the pendulum's rest point pushing against a permanent magnet in the bob. This is symmetrical; it pushes radially away from the rest position. So there's no active control over the direction of swing.
The new insight is that if the pushing pulse is delayed to the right point in the cycle, the applied force dampens, rather than increases, the tendency for the oscillation to become ellipical. The optimal time for the pushing pulse has been worked out. It's a neat little result.
And they couldn't even put it in a vacuum chamber? :)