# Foucault pendulum movement.

**URL:** <https://boards.straightdope.com/t/foucault-pendulum-movement/725032>\
**Category:** Factual Questions\
**Created:** [July 13, 2015, 10:01am UTC](https://boards.straightdope.com/t/foucault-pendulum-movement/725032 "2015-07-13T10:01:49Z")\
**Posts on this page:** 5\
**Page:** 3

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**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\
**Post date:** [July 17, 2015, 2:44pm UTC](https://boards.straightdope.com/t/foucault-pendulum-movement/725032/41 "2015-07-17T14:44:51Z")

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> [@CalMeacham](#):
>
> The one at the Franklin Institute had a small-scale model showing the pendulum cable anchored in a sphere resting atop three other spheres. But you don’t need anything as complex as that. As I mentioned upthread, my high school physics teacher set up a Foucault pendulum in our classroom using an ordinary anchor point in the ceiling, with a universal joint shortly below it to allow the pendulum to swing freely in any direction.
> 
> [Universal Joint - Yahoo Image Search Results](https://images.search.yahoo.com/search/images;_ylt=A0LEVyQY7KhVBmcAwK9XNyoA;_ylu=X3oDMTEyMWhjYzF1BGNvbG8DYmYxBHBvcwMyBHZ0aWQDQjA1NzZfMQRzZWMDc2M-?p=Universal+Joint&fr=yfp-t-252)

Actually, a universal joint wouldn’t work.  
What you were probably thinking of was a "[snap swivel"](http://mactackle.com/secureshopping/images/swivel_crane_interlock_snap.jpg)

But, all of these have very significant friction - using one would cause the pendulum to be “slow,” I would think.

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**Author:** ![CalMeacham](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/calmeacham/32/35_2.png) [@CalMeacham](https://boards.straightdope.com/u/CalMeacham)\
**Post date:** [July 17, 2015, 3:21pm UTC](https://boards.straightdope.com/t/foucault-pendulum-movement/725032/42 "2015-07-17T15:21:36Z")

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> [@beowulff](#):
>
> Actually, a universal joint wouldn’t work.  
> What you were probably thinking of was a "[snap swivel"](http://mactackle.com/secureshopping/images/swivel_crane_interlock_snap.jpg)
> 
> But, all of these have very significant friction - using one would cause the pendulum to be “slow,” I would think.

No, he used a universal joint. It worked very well.

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**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\
**Post date:** [July 17, 2015, 5:19pm UTC](https://boards.straightdope.com/t/foucault-pendulum-movement/725032/43 "2015-07-17T17:19:10Z")

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> [@CalMeacham](#):
>
> Why do you need a motor? Those big pendulums in the museums have enough momentum to keep going on their own all day without an extra power source. And my high school physics example only had to run for an hour.

The only reason for a motor is if it’s going to run continuously. If the museum staff resets the thing in motion every morning before opening then you’re probably right that inertia will carry it through the 12-ish hours the exhibit is viewable. That does however raise a requirement to be able to reliably and repeatably launch the thing every day using semi-skilled staff. How that’s done is left as an exercise for the reader.

Also see my link here [The Nat |](http://www.sdnhm.org/about-us/our-museum/history/museum-lore/foucault-pendulum/) for a museum’s description of their large-scale pendulum exhibit which does have a motor.

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<div class="post-metadata">

**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\
**Post date:** [July 17, 2015, 5:51pm UTC](https://boards.straightdope.com/t/foucault-pendulum-movement/725032/44 "2015-07-17T17:51:44Z")

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> [@CalMeacham](#):
>
> No, he used a universal joint. It worked very well.

I don’t see how - a universal joint does not allow rotation between the two shafts - only angular movement.

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<div class="post-metadata">

**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\
**Post date:** [July 17, 2015, 5:56pm UTC](https://boards.straightdope.com/t/foucault-pendulum-movement/725032/45 "2015-07-17T17:56:10Z")

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Much later add: This link [Foucault Pendulum](http://www.animations.physics.unsw.edu.au/jw/foucault_pendulum.html) originally from post# 20 by **rsa** shows vids of a more-or-less museum scale pendulum which has lost about 1/3rd of its swing amplitude in an hour of operation. So it seems implausible an unpowered pendulum could last the full 8+ hours a museum would typically be open.

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