# The Fulcout's (sp?) Pendulum **URL:** \ **Category:** Factual Questions\ **Created:** [November 22, 2002, 4:10am UTC](https://boards.straightdope.com/t/the-fulcouts-sp-pendulum/138095 "2002-11-22T04:10:47Z")\ **Posts on this page:** 11\ **Page:** 1
**Author:** ![Jinx](https://avatars.discourse-cdn.com/v4/letter/j/c6cbf5/32.png) [@Jinx](https://boards.straightdope.com/u/Jinx)\ **Post date:** [November 22, 2002, 4:10am UTC](https://boards.straightdope.com/t/the-fulcouts-sp-pendulum/138095/1 "2002-11-22T04:10:47Z")
What’s the deal with the pendulums in museums? Supposedly, they have some special mount, perhaps a ball joint, which allegedly “allows the earth to rotate below it”. This is the classic explantion as to how it can knock down pins. But, if this explanation were true, wouldn’t the museum have to be on the pole? I say it’s rotating ABOUT the pole - like practically everything else. So, how do the pins get knocked down? Is it something to do with the Coriolus (sp?) Force, perhaps? What’s the real deal here? - Jinx ---
**Author:** ![samclem](https://avatars.discourse-cdn.com/v4/letter/s/a9a28c/32.png) [@samclem](https://boards.straightdope.com/u/samclem)\ **Post date:** [November 22, 2002, 4:36am UTC](https://boards.straightdope.com/t/the-fulcouts-sp-pendulum/138095/2 "2002-11-22T04:36:58Z")
[http://www.si.edu/resource/faq/nmah/pendulum.htm](http://www.si.edu/resource/faq/nmah/pendulum.htm) ---
**Author:** ![RiverRunner](https://avatars.discourse-cdn.com/v4/letter/r/977dab/32.png) [@RiverRunner](https://boards.straightdope.com/u/RiverRunner)\ **Post date:** [November 22, 2002, 4:47pm UTC](https://boards.straightdope.com/t/the-fulcouts-sp-pendulum/138095/3 "2002-11-22T16:47:30Z")
On a trip to a museum one time when I was in high school, my classmates and my math teacher were convinced the point of Foucalt’s pendulum was that the earth’s motion _started the pendulum swinging_. I couldn’t convince them otherwise. These were very bright people, but they pretty much threw a mental shoe on that one. RR ---
**Author:** ![RealityChuck](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/realitychuck/32/195_2.png) [@RealityChuck](https://boards.straightdope.com/u/RealityChuck)\ **Post date:** [November 22, 2002, 4:58pm UTC](https://boards.straightdope.com/t/the-fulcouts-sp-pendulum/138095/4 "2002-11-22T16:58:33Z")
The link is a good one, but it leaves out the main reason why this is proof: A Foucault pendulum moves in a straight line back and forth (this is a characterist of a pendulum allowed to move solely due to gravity and with no outside forces). Since it moves in a straight line, any apparent change in its swing is due to the floor moving beneath it. ---
**Author:** ![3waygeek](https://avatars.discourse-cdn.com/v4/letter/3/e95f7d/32.png) [@3waygeek](https://boards.straightdope.com/u/3waygeek)\ **Post date:** [November 22, 2002, 5:18pm UTC](https://boards.straightdope.com/t/the-fulcouts-sp-pendulum/138095/5 "2002-11-22T17:18:54Z")
> [@](#): > > \*Originally posted by RealityChuck \* > \*\*The link is a good one, but it leaves out the main reason why this is proof: > > A Foucault pendulum moves in a straight line back and forth (this is a characterist of a pendulum allowed to move solely due to gravity and with no outside forces). Since it moves in a straight line, any apparent change in its swing is due to the floor moving beneath it. \*\* Which was subtly pointed out in last week’s episode of [Junkyard Wars](http://tlc.discovery.com/fansites/junkyard/episode/season_09.html) – the one with the blind navigation vehicles. One of the teams used a pendulum as a backup navigation device. ---
**Author:** ![panamajack](https://avatars.discourse-cdn.com/v4/letter/p/47e85d/32.png) [@panamajack](https://boards.straightdope.com/u/panamajack)\ **Post date:** [November 22, 2002, 6:20pm UTC](https://boards.straightdope.com/t/the-fulcouts-sp-pendulum/138095/6 "2002-11-22T18:20:21Z")
And speaking of throwing a mental shoe … from the Smithsonian site linked above (emphatic link added) : > [@](#): > > As you move off the north pole down to a more southerly point like Washington, for example, the earth not only rotates under the pendulum, but it carries Washington, the building, and the pendulum, in a [great circle](http://www.its.bldrdoc.gov/fs-1037/dir-017/_2490.htm) about its axis. This goes to another point of the OP – the period it takes a pendulum to travel a full circle is more than 24 hours if you’re not on a pole. It’s related to latitude, and if Washington actually _were_ at the equator, you wouldn’t see the effect. (In fairness, the SI’s page did cover that.) ---
**Author:** ![ftg](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ftg/32/2801_2.png) [@ftg](https://boards.straightdope.com/u/ftg)\ **Post date:** [November 22, 2002, 7:11pm UTC](https://boards.straightdope.com/t/the-fulcouts-sp-pendulum/138095/7 "2002-11-22T19:11:10Z")
The formula for the period of the pendulum is simple: 1/sin(degrees latitude) days. ---
**Author:** ![Hello\_Again](https://avatars.discourse-cdn.com/v4/letter/h/85e7bf/32.png) [@Hello\_Again](https://boards.straightdope.com/u/Hello_Again)\ **Post date:** [November 22, 2002, 8:34pm UTC](https://boards.straightdope.com/t/the-fulcouts-sp-pendulum/138095/8 "2002-11-22T20:34:05Z")
but the pendulum always swings west! \ ---
**Author:** ![bryanmcc](https://avatars.discourse-cdn.com/v4/letter/b/eada6e/32.png) [@bryanmcc](https://boards.straightdope.com/u/bryanmcc)\ **Post date:** [November 22, 2002, 11:09pm UTC](https://boards.straightdope.com/t/the-fulcouts-sp-pendulum/138095/9 "2002-11-22T23:09:37Z")
> [@](#): > > \*Originally posted by panamajack \* > \*\*And speaking of throwing a mental shoe … from the Smithsonian site linked above (emphatic link added) : > > > [@](#): > > > > As you move off the north pole down to a more southerly point like Washington, for example, the earth not only rotates under the pendulum, but it carries Washington, the building, and the pendulum, in a great circle about its axis. > > \*\* Correct me if I’m wrong, but I thought a great circle had it’s center at the center of the earth. If I’m not mistaken, the rotation of the earth caries Washington, the building, and the pendulum in a circle equal to the latitude that Washington is situated at. Please point out my mistake – I’d hate to have to correct the Smithsonian. 🙂 ---
**Author:** ![Xema](https://avatars.discourse-cdn.com/v4/letter/x/9de053/32.png) [@Xema](https://boards.straightdope.com/u/Xema)\ **Post date:** [November 22, 2002, 11:20pm UTC](https://boards.straightdope.com/t/the-fulcouts-sp-pendulum/138095/10 "2002-11-22T23:20:12Z")
> [@](#): > > \*Originally posted by bryanmcc \* > Correct me if I’m wrong, but I thought a great circle had it’s center at the center of the earth. If I’m not mistaken, the rotation of the earth caries Washington, the building, and the pendulum in a circle equal to the latitude that Washington is situated at. You’re right, but I think they can wriggle out of this by pointing out that they put “great circle” in lowercase. If they’d said “Great Circle” you’d have them dead to rights. As it is, they could claim that they’re simply talking about a pretty big circle. ---
**Author:** ![panamajack](https://avatars.discourse-cdn.com/v4/letter/p/47e85d/32.png) [@panamajack](https://boards.straightdope.com/u/panamajack)\ **Post date:** [November 22, 2002, 11:45pm UTC](https://boards.straightdope.com/t/the-fulcouts-sp-pendulum/138095/11 "2002-11-22T23:45:53Z")
That was my thought exactly, **bryanmcc**. **Xema** , they could try and make the claim, but not only is ‘great circle’ a term that doesn’t require capitalization, it’s extremely confusing to use the same words to refer to something that’s close to it, but not it. I suppose your point is that they _could_ weasel out of it, but honestly, it’s hard not to see it as a mistake. And I think they should change it to improve clarity. (I’ll be sending a suggestion once I get to my home e-mail.)