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# How To Build a Short Foucault Pendulum

#### kdawson posted more than 5 years ago | from the watching-the-earth-rotate dept.

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."

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#### Anonymous Coward 1 minute ago

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### And suddenly... (5, Funny)

#### detox.method() (1413497) | more than 5 years ago | (#26844455)

A new generation of executive ball-clickers is born.

### Re:And suddenly... (2, Funny)

#### foobsr (693224) | more than 5 years ago | (#26844503)

A new generation of executive ball-clickers is born.

Yep. Though I am wondering if they can get their balls to the right place after having fucked up finances and spelling.

CC.

### Re:And suddenly... (0, Offtopic)

#### Smidge207 (1278042) | more than 5 years ago | (#26845469)

I thought you typed that a new generation of ball-lickers was born. Then I was all naw...

=Smidge=

### Not feasible since... (1)

#### wsanders (114993) | more than 5 years ago | (#26845601)

... Sharper Image went out of business.

However, there is a huge market for "executive ball-busters".

### Re:And suddenly... (0)

#### Anonymous Coward | more than 5 years ago | (#26845707)

If you could sell one executive ball-clicker to every executive ball-licker I've ever met, you would be very wealthy indeed.

### Re:And suddenly... (1)

#### impaledsunset (1337701) | more than 5 years ago | (#26849089)

Given that they are already altering the natural motion of the pendulum with a motor, couldn't they simply attach a motor that makes the pendulum move the way that is needed to demonstrate the rotation of the Earth to construct even shorter one? It would be cheating, but... after all, all that matters is to convince the ones watching the experiment, right? :D

Swingin' baby!

### My next Christmas Present from my Wife (0)

#### Anonymous Coward | more than 5 years ago | (#26844489)

Next useless item available for purchase on Think Geek?

### Wow.. Quite the BIG assumption in the summary (0)

#### Anonymous Coward | more than 5 years ago | (#26844545)

You insensitive clod.. I'm off-world on a non-rotating frame of reference... my pendulums don't precess in ellipsoidal motion, you dirty-minded person, you.

### Re:Wow.. Quite the BIG assumption in the summary (0)

#### Anonymous Coward | more than 5 years ago | (#26847005)

Off-world? Do you have pendulums on Uranus? And is there a video?

### Re:Wow.. Quite the BIG assumption in the summary (1)

#### Roger W Moore (538166) | more than 5 years ago | (#26848257)

Since you have a pendulum you must be in an accelerating reference frame otherwise the pendulum will not swing and hence not be a pendulum. This being the case then your pendulum will still precess in an ellipsoidal motion caused by the effect that this article is aiming to isolate you from!

### created a motor to compensate? (4, Funny)

#### Lord Ender (156273) | more than 5 years ago | (#26844559)

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?

### Re:created a motor to compensate? (1)

#### 0xdeadbeef (28836) | more than 5 years ago | (#26845403)

That sounds like a feat of engineering, not science.

Who let you out of the concent? Don't you have a naval to gaze at?

### Re:created a motor to compensate? (4, Funny)

#### gardyloo (512791) | more than 5 years ago | (#26847261)

Don't you have a naval to gaze at?

Some people just can't keep their ship together.

### Re:created a motor to compensate? (2, Funny)

#### Usquebaugh (230216) | more than 5 years ago | (#26847415)

That's far to subtle a pun for /. readers to fathom.

### It's not so deep (1)

#### marcus (1916) | more than 5 years ago | (#26849113)

Choose another unit.

### You obviously haven't... (1)

#### fractoid (1076465) | more than 5 years ago | (#26853399)

...been doing this furlong.

### Great book too (4, Interesting)

#### pzs (857406) | more than 5 years ago | (#26844607)

(maybe slightly OT)

As well as a physics experiment, Foucault's Pendulum [wikipedia.org] 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...

### Re:Great book too (-1, Troll)

#### Anonymous Coward | more than 5 years ago | (#26846855)

That's super awesome how you insert personal attacks about someone in a total irrelevant way. Thanks for generally contributing to the decline of us all!

### Re:Great book too (0)

#### Anonymous Coward | more than 5 years ago | (#26848263)

Hey now, if you're going to flame the GP, do it for a legitimate reason. Like endorsing a book with BASIC code in it. *shudder* What are you trying to do, give us brain damage [catb.org]?

(Kidding.)

### Re:Great book too (1)

#### mako1138 (837520) | more than 5 years ago | (#26848355)

You know, the book was pretty superficial.

### Re:Great book too (1)

#### TeknoHog (164938) | more than 5 years ago | (#26848669)

The Illuminatus! Trilogy is another book in the same thread, even more fantastic IMHO, and predates FP by more than a decade.

### Re:Great book too (1)

#### Mr. Slippery (47854) | more than 5 years ago | (#26850831)

The Illuminatus! Trilogy is another book in the same thread

I don't think Illuminatus! [wikipedia.org] can be described as "in the same fnord" as any other work.

### Inevitably? (4, Insightful)

#### pushing-robot (1037830) | more than 5 years ago | (#26844635)

Set a pendulum in motion and you'll inevitably give it an ellipsoidal motion, which naturally tends to precess.

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? (1)

#### Hatta (162192) | more than 5 years ago | (#26845451)

I think the whole point of the article was that "vibrations, friction effects, poor suspension, etc" cause the precession, even if you start the pendulum with a completely linear force.

### Re:Inevitably? (1)

#### pushing-robot (1037830) | more than 5 years ago | (#26846009)

Yeah, TFA explains the issues pretty well. I was disputing the summary's poor description of the problem.

### Simpler Solution (1)

#### Roger W Moore (538166) | more than 5 years ago | (#26848653)

Of course there is a far simpler solution to demonstrate the same effect: build a large turntable and put your short pendulum on that. Since the rotation is far faster than the Earth's the short pendulum will show precession when you sit on the turntable and stays stationary as the table turns if you are not on it.

The big advantage to this approach is that you can put a video camera into both frames which really shows the difference. I've got a couple of videos of this which I made for my lectures which I should probably to upload to YouTube.

### Re:Inevitably? (0)

#### Anonymous Coward | more than 5 years ago | (#26845541)

Actually the traditional method of starting one of these was to tie the pendulum with a silk thread, and burn through it.

### Re:Inevitably? (1)

#### Usquebaugh (230216) | more than 5 years ago | (#26847433)

So you end up with the weight on the floor?

### Re:Inevitably? (1)

#### camperdave (969942) | more than 5 years ago | (#26845695)

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?

The ellipsoidal motion does not come from how you release the pendulum. A pendulum will naturally swing in a plane. That plane remains stationary relative to space. The Earth, however, is rotating on its axis. As it rotates, it drags the fulcrum out of the plane. This causes a sideways force on the pendulum bob, which forces it to follow the ellipsoidal path.

### Re:Inevitably? (1)

#### pushing-robot (1037830) | more than 5 years ago | (#26845937)

Thanks for not even reading the summary.

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 Foucalt 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

### Re:Inevitably? (2, Insightful)

#### gardyloo (512791) | more than 5 years ago | (#26847055)

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? (2, Insightful)

#### maxume (22995) | more than 5 years ago | (#26847963)

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.

### Good lord (4, Funny)

#### Zouden (232738) | more than 5 years ago | (#26844647)

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.

### Re:Good lord (0)

#### Anonymous Coward | more than 5 years ago | (#26845215)

You must have had an interesting childhood...

### Re:Good lord (0)

#### Anonymous Coward | more than 5 years ago | (#26845895)

Please tell me you were being sarcastic...

### Re:Good lord (2, Funny)

#### A. B3ttik (1344591) | more than 5 years ago | (#26846345)

Please tell me you were being sarcastic...

Obviously. I remember all of the other kids on my block had Foucalt Pendulums but my parents wouldn't let me get one.

### Re:Good lord (1)

#### mako1138 (837520) | more than 5 years ago | (#26848405)

Androids have parents?

### Re:Good lord (1)

#### paulgrant (592593) | more than 5 years ago | (#26852481)

score: +10 funny ;)

### Not wanting to be a spelling Nazi, but... (0)

#### Anonymous Coward | more than 5 years ago | (#26844699)

Foucault. Not 'Foucalt'.

Foucault. With a 'u' between the 'a' and the 'l'.

### Wrong summary (1)

#### barberousse (1432239) | more than 5 years ago | (#26844717)

Please correct the summary. It is a Foucault pendulum not Foucalt.

### Re:Wrong summary (0)

#### Anonymous Coward | more than 5 years ago | (#26844789)

Just to piss you off I fixed the title but not the summary.
-kdawson

### One option (1)

#### clickclickdrone (964164) | more than 5 years ago | (#26844831)

Take one banker, tie rope to neck, swing. Prod occassionally.

### Re:One option (1)

#### zappepcs (820751) | more than 5 years ago | (#26845191)

That would be a default pendulum, though many of us would like to see a defendant pendulum. Can you imagine a huge executive clacking ball thing made from the heads of wallstreet CEOs?

### Re:One option (1)

#### Briareos (21163) | more than 5 years ago | (#26847269)

I prefer this [notfunnycartoons.com], actually - there's no sweeter justice than a set of balls crushing a convicted banker's head.

Just kidding, of course...

np: Shweta Javeri - Heart Of Darkness (A Monstrous Psychedelic Bubble Vol. 1 (Disc 2))

### Re:One option (1)

#### Captain Splendid (673276) | more than 5 years ago | (#26848407)

Take one banker, tie rope to neck, swing. Prod occassionally.

Damn hippie. Here's how you do it:

-Take one banker, string him by his feet.
-Charge the local kids a buck or two for a few swings.
-Replace with fresh bankers as old ones get too messy.
-Profit!

Bonus points if you spend your time lecturing same bankers on how you're repackaging worthless assets (bankers) and selling them on to clueless investors (the kids).

### Equator. (4, Funny)

#### TheLink (130905) | more than 5 years ago | (#26844935)

I live near the equator, you insensitive clods!

### Re:Equator. (2, Informative)

#### greg_barton (5551) | more than 5 years ago | (#26846435)

Only on /. can you see jokes like this. Awesome.

### Re:Equator. (1)

#### TeknoHog (164938) | more than 5 years ago | (#26848763)

So, which way does your toilet flush?

### Re:Equator. (0)

#### Anonymous Coward | more than 5 years ago | (#26849699)

Down. We tried it the other way but it got awfully messy.

### Re:Equator. (0)

#### Anonymous Coward | more than 5 years ago | (#26850525)

So, which way does your toilet flush?

Down.

### Ummmm (1)

#### Drunken_Piper (1232578) | more than 5 years ago | (#26845277)

How do we know the motor isn't causing the precession?

### Re:Ummmm (2, Informative)

#### GleeBot (1301227) | more than 5 years ago | (#26845485)

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).

### Re:Ummmm (4, Interesting)

#### LargeMythicalReptile (531143) | more than 5 years ago | (#26846259)

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"....

### Re:Ummmm (2, Interesting)

#### ceoyoyo (59147) | more than 5 years ago | (#26847421)

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.

### Re:Ummmm (2, Insightful)

#### rcw-home (122017) | more than 5 years ago | (#26851781)

because of air resistance...

...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

And they couldn't even put it in a vacuum chamber? :)

### Re:Ummmm (0)

#### Anonymous Coward | more than 5 years ago | (#26861819)

(there were markings on a ring around the pendulum, so it was easy to see where it swung before)

At the one in San Francisco, they set up pegs a few inches high around the circumference of the circle on the floor. As the pendulum precesses, it comes closer and closer to the next peg until a point on the bottom of the weight knocks the peg over. It would be interesting to get a speeded-up video of the scene, where people gradually fill the railing around the circumference of the circle. They start at the location of the next peg to fall, then fill in around the railing in both directions. As soon as the peg falls, the railing is abandoned and gradually fills up with new spectators waiting to see the next peg fall.

### Shooting birds with cannons (not at!)... (1, Funny)

#### Anonymous Coward | more than 5 years ago | (#26845509)

That's the most stupid thing I read in the last time... why do you need a FC in the first place if you put a motor into it...

It's like... showing that birds can fly by stuffing them into a cannon and fireing them upwards....

### Defeats the purpose? (1)

#### josath (460165) | more than 5 years ago | (#26845667)

Doesn't using a motor to modify the swinging of the pendulum defeat the whole purpose? I thought the point was to have it seem like it was magically rotating...having a motor modify the swing, takes away from that magic.

### Re:Defeats the purpose? (1)

#### SnarfQuest (469614) | more than 5 years ago | (#26845771)

It works like retrophrenology (diskworld reference). If you adjust the motor properly, you can change the length of the day. Brings a new meaning to daylight savings time.

### The Smithsonian Museum of American History (2, Informative)

#### niro5 (1081199) | more than 5 years ago | (#26845897)

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.

### Re:The Smithsonian Museum of American History (1)

#### koogydelbbog (451219) | more than 5 years ago | (#26847021)

Griffith Observatory in LA has one.

http://www.griffithobservatory.org/exhibits/brotunda.html [griffithobservatory.org]

that's probably a bit too long a drive though.

### Re:The Smithsonian Museum of American History (1)

#### Chabo (880571) | more than 5 years ago | (#26847715)

The Boston Museum of Science has one -- they have it set up to knock over small rods to mark the time.

Still one of the most memorable things I ever saw at that museum. That was so COOL!

### Re:The Smithsonian Museum of American History (1)

#### arb phd slp (1144717) | more than 5 years ago | (#26849325)

Boston! I knew I'd seen one of these when I was a kid, but I couldn't remember where! That was the one. Thanks.

Goes to show you how much of an impression a demo like that is on a science-nerdy kid. I don't remember much else about that trip (I went to a Chuck E Cheese for the first time that day I recall), but I have never forgotten that pendulum showing the rotation of the earth.

### +1 Hell yes (1)

#### Overzeetop (214511) | more than 5 years ago | (#26847867)

Absolutely the only reason for going when I was young. Otherwise, meet me in the air an space museum.

### Re:+1 Hell yes (0)

#### Anonymous Coward | more than 5 years ago | (#26849401)

Are these really kinda rare? USF here in Tampa has one and I assumed (my mistake) most every collage would have one. Oh well...

### 3 meters long -- not special. (0)

#### Anonymous Coward | more than 5 years ago | (#26845983)

Short Foucault Pendulum with a length below 3 meters are nothing special, e.g. physics department in Wuppertal. 3 cm would be special. Btw, very long pendulums have problems on there own as well -- like standing waves on the pendulum.

### Solution in search of a problem (1)

#### Locke2005 (849178) | more than 5 years ago | (#26846473)

Is there really a lot of demand for a table-top device that measures the rotational velocity of earth? Wouldn't it be easier to just google it [google.com]?

### Re:Solution in search of a problem (1)

#### SpacePunk (17960) | more than 5 years ago | (#26851213)

It's basic science that the same experiments be done over, and over, and over again. Why do you hate basic science?

... anyone?

### Re:And this is important because... (1)

#### BattleApple (956701) | more than 5 years ago | (#26849109)

Maybe it's more interesting than important. Does everything have to be important? Besides, it could lead to other discoveries that no one has thought of yet.

### My school did this a long time ago (1)

#### nasor (690345) | more than 5 years ago | (#26847527)

When my school wanted to build a Foucault pendulum that was less than 1 story high to decorate a new building, they installed a gadget at the top that automatically dampens the precession. This was over ten years ago.

### Try try agin (1)

#### SpinningCone (1278698) | more than 5 years ago | (#26847993)

People said i was daft to build a short Foucault pendulum. but i built it anyway... it started to precess .. so i built another one. that started to precess. so i built a third one. That one started to precess then fell over and burst into flames. but the FOURTH one worked great. and that's what your getting my lad the shortest Foucault pendulum in the world!

### Re:Try try agin (1)

#### Mr Thinly Sliced (73041) | more than 5 years ago | (#26855421)

But Father, I just want to - SING!

### It's a neat little result. (3, Interesting)

#### Animats (122034) | more than 5 years ago | (#26848621)

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.

### That's all well and good... (0)

#### Anonymous Coward | more than 5 years ago | (#26848771)

But when the Templi Resurgentes Equites Synarchici start kidnapping Carnegie-Mellon's staff to use in an occult ritual, don't say you weren't warned!

Umberto Eco FTW!

### This proves once again... (0)

#### Anonymous Coward | more than 5 years ago | (#26850013)

That it isn't the size of your *pendulum* that matters, It's the motion.

### Lost Pendulum (0)

#### Anonymous Coward | more than 5 years ago | (#26851955)

So maybe it will be possible to locate the island with a home setup instead of going to that creepy church. :)

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