
Foucault pendulum - bottle2
https://en.wikipedia.org/wiki/Foucault_pendulum
======
lurquer
I found it very difficult to understand WHY Foucaults pendulum behaves the way
it does.

I could picture what was happening with the pendulum at the Norh Pole... earth
rotates under the pivot point every 24 hours. And, I understand what happened
st the Equator... no effect on the pendulum as nothing is rotating beneath it.

But, at points in between, it was hard to understand why the pendulum might
only rotate 80% or so depending on how far north it was. (Easy to derive
mathematically, but hard to truly intuit what was going on.)

As I doubt I'm the only one, allow me to provide the thought expirement that
make it click for me; specifically, why the pendulum turns some fraction of a
complete rotation while the Earth -- at any point -- makes a full circuit
every 24 hours.

Put a pendulum in your car. If you drive straight, the pendulum won't be
affected. If you turn, of course, the pendulum will turn a bit. Easy to
visualize so far. Now, let's stop the Earths rotation for a moment. Let's get
in the car and with the Pendulum and drive due East from L.A. All the way to
N. Carolina (or wherever due east would end up.) Across the Atlantic. Across
Southern Europe, Asia, Pacific, and back to L.A.

Now I've been driving straight this entire time, so would the pendulum have
moved?

Well, as it runs out, I HAVEN'T been driving 'straight'. The entire time, I
would necessarily had to have been veering a little bit to the left to keep me
in my due East path. If I was truly driving straight the entire time, I would
have made a Great Circle and dipped down into Africa at some point.

In any case, as my desktop pendulum moves around the globe every 24 hours, it
isn't traveling in a straight line... just as a car transporting it along its
path would have to be curving a bit to stay on course.

This was the 'ah-ha' moment that allowed to understand the gradual increase in
the pendulums rotation as you move north (or south) from the Great Circle of
the equator.

Maybe that is common-sense for everyone else. If so, disregard. ;)

~~~
gregfjohnson
Here's the intuition that clicked for me, similar to the above but with a
different twist at the end.

Imagine a large flat parking lot, and drive around the parking lot in a
circle. Say you have a small pendulum swinging freely, suspended in your car.
If you drive the full 360 degrees, the pendulum will appear from the reference
frame of the inside of the car to have rotated a full 360 degrees. (Let's say
that you are driving fast enough to render the rotation of the earth, the
latitude of the parking lot, etc. negligible.)

Whatever fraction of the circle you drive, the pendulum will appear to rotate
that same fraction as viewed from inside the car.

Now, here is the fun part. Imagine that you are driving at some fixed latitude
on a large sphere. Create a big cone that is just tangent to the sphere on
your path. What happens if you drive the entire distance and return to your
starting point?

Cut the cone in a vertical straight line starting at its apex, and then
flatten out the cone. It will form a flat circle, but with a pie slice
removed.

If you had driven the perimeter of this partial circle in the parking lot, you
would see that the pendulum only rotated part-way around. The same thing will
happen if you drive the full distance around the sphere at the corresponding
latitude.

~~~
lurquer
I had heard it explained explained that way as well. Made sense then and now.

Not to digress, but IF the earth was stationary and everything rotated around
it, can you think of a mechanism to explain the pendulums behavior? (Could be
ether, or some medieval conception of ptolemic shells, you name it... but it
has to be consistent and sort of reasonable sounding.)

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sandworm101
The Foucault Pendulum is on my list of three dead-simple machines that prove
science is real.

(1) Using a soup can and some string to measure radiation:
[https://en.wikipedia.org/wiki/Kearny_fallout_meter](https://en.wikipedia.org/wiki/Kearny_fallout_meter)

(2) Measuring the force of gravity, and thereby weighing the earth, using a
tuna can and some other string.
[http://www.fourmilab.ch/gravitation/foobar/](http://www.fourmilab.ch/gravitation/foobar/)

(3) Using a string and a bowling ball prove the earth is round without going
outside.
[https://en.wikipedia.org/wiki/Foucault_pendulum](https://en.wikipedia.org/wiki/Foucault_pendulum)

~~~
erikig
I didn't know about Cresson_Kearny [1] until this but he was pretty great. He
reminds me of a cold war Macgyver-like survivalist with a number of
interesting gadgets [2]

[1]
[https://en.wikipedia.org/wiki/Cresson_Kearny](https://en.wikipedia.org/wiki/Cresson_Kearny)

[2]
[https://en.wikipedia.org/wiki/Kearny_air_pump](https://en.wikipedia.org/wiki/Kearny_air_pump)

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mdturnerphys
The one at BYU has signs around it saying something like "Warning! 10,000
ohms! Do not touch!" I guess the thinking is that those who don't know what
ohms are will think it's dangerous and those that do will have enough respect
for physics to not disturb the pendulum.

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spookybones
Relatively interesting:
[https://en.wikipedia.org/wiki/Allais_effect](https://en.wikipedia.org/wiki/Allais_effect)

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aidenn0
TIL the smithsonian removed it's Foucault pendulum in 1998 (right about when I
moved away from the DC area). It was about 15 meters long and quite
impressive; they had little pins setup that it would knock down, so you could
look at when you arrived in the museum and see the difference when you left.

~~~
redis_mlc
The Oregon Convention Center has one, watched it every O'Reilly Open Source
Conference that was in Portland.

[https://www.youtube.com/watch?v=-p7P0DQvp0g](https://www.youtube.com/watch?v=-p7P0DQvp0g)

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NegativeLatency
Find one near you!
[https://en.wikipedia.org/wiki/List_of_Foucault_pendulums](https://en.wikipedia.org/wiki/List_of_Foucault_pendulums)

~~~
pengaru
I'm surprised to see Fermi National Accelerator Laboratory listed still. When
I last visited Wilson Hall which must have been nearly a decade ago now, the
pendulum had been removed. If memory serves it happened after the cable
snapped from wear and they decided to just get rid of it.

This [0] post claims they removed it in 2010.

It was kind of mesmerizing to watch back in the day, Wilson Hall is a tall
building with an open center, it made for a great Foucault Pendulum.

[0] [https://www.vofoundation.org/blog/cabinet-physics-riding-
alo...](https://www.vofoundation.org/blog/cabinet-physics-riding-along-
foucaults-pendulum/)

------
frenchie4111
I've always wondered, are there bots that just post these interesting
wikipedia articles periodically? It seems like twice a week or so a random
wikipedia article is trending.

~~~
rubatuga
I posted my WiFi Coax article because it confused me at first.

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lihaciudaniel
No. No! This isn't named after the philosopher Foucault

~~~
decasteve
It’s named after the book by Umberto Eco, which is a pen name Dan Brown
sometimes uses for his more esoteric and intelligent writing.

~~~
meatsock
Umberto Eco is not Dan Brown

------
empath75
> The pendulum was introduced in 1851 and was the first experiment to give
> simple, direct evidence of the earth's rotation.

The existence of the day-night cycle should have sufficed.

~~~
lidHanteyk
In addition to sibling comments, we should note that not all observations are
experiments. There is a distinct difference between experiment and mere
observation: In an experiment, we are carrying out a repeatable recipe in
order to try to falsify a hypothesis. We are not merely theory-crafting,
working to explain what we see, but we are trying to disprove what we have
theorized.

~~~
redis_mlc
> In an experiment, we are carrying out a repeatable recipe in order to try to
> falsify a hypothesis. We are not merely theory-crafting, working to explain
> what we see, but we are trying to disprove what we have theorized.

Actually, no. Not sure where you got the idea that an experiment has to
falsify a hypothesis.

