# 2201: Foucault Pendulum

 Foucault Pendulum Title text: Trust me, you don't want to get on the wrong side of the paramilitary enforcement arm of the International Earth Rotation and Reference Systems Service.

## Explanation

Black Hat is attending what appears to be a guided tour of a museum with a Foucault pendulum. Megan is explaining to him, Cueball and Ponytail about the device which demonstrates the rotation of the Earth.

Black Hat, being himself, immediately sees an opportunity to cause chaos and seizes it with both hands, quite literally — he grabs the pendulum, which causes the others to shout after him to stop. At first this would seemingly be for fear of ruining the delicate demonstration. However, in the final panel, the news anchor Blondie reveals to us that by arresting the motion of the pendulum, Black Hat has somehow stopped the rotation of the Earth. However, it was only briefly, since the local geophysicists managed to wrestle him down, and it must be assumed that they then quickly restarted the pendulum and thus the Earth's rotation.

This of course is blatantly impossible, since the Foucault pendulum's motion is tied to the Earth's rotation, not the other way around (at least in any significant way, see below). A Foucault pendulum is a regular pendulum that swings from a bearing that allows rotation in any direction, like your shoulder joint instead of your elbow, as a demonstration that the Earth is rotating beneath it. If the Earth were stationary, the pendulum's plane of oscillation would not change relative to its immediate surroundings, but the Earth is not stationary, so the pendulum's plane of oscillation will appear to rotate over the course of a day, although in reality it is the Earth that rotates. The low-friction bearing doesn't allow the rotation of the Earth to affect the motion of the pendulum, so it tends to stay aligned with its original inertial reference frame rather than with its surroundings, which rotate with the Earth. A Foucault pendulum located at one of the poles will take a full day to "move" one full round. At the equator there is no movement, and in between it will take longer than 24 hours (24 hours divided by the sine of the latitude).

The fact that the Earth's rotation does not influence the motion of the pendulum does NOT mean that other forces can't affect it - for example, someone running up and manually repositioning the pendulum. Of course, the apparent rotation of the pendulum's plane relative to the Earth is an effect of the planet's motion, rather than the cause of it. Thus, stopping a Foucault pendulum manually does not entail pausing the rotation of the Earth.

If it were somehow possible for a Foucault pendulum to control Earth's rotation (see above), Black Hat would probably not want to alter the momentum of the pendulum if he were not at one of the Earth's poles. That is assuming he was told that it was related to Earth's rotation and assuming that he would prefer to preserve his own life over creating chaos (unless he has some means to prevent his being slammed into a nearby wall at the speed of sound). This is because, if the rotation of the Earth were to be stopped for even very short amounts of time (a few seconds), it would cause everything on Earth that wasn't bolted/fastened to the ground to move eastward compared to the now stationary ground. Objects near the Equator would suddenly be moving at a speed of 300-360 meters per second, likely causing the death of most lifeforms on Earth beneath a certain latitude almost instantaneously. Those close enough to the poles may survive, though. Also this will cause massive windstorms, tsunamis, volcanic and tectonic events on a scale not previously observed on Earth. This would likely cause a mass extinction event and wipe out most of humanity in the initial events (which would eventually lead to our total extinction). It is possible that Black Hat's grabbing the pendulum would cause a gradual slowing prior to stopping, minimizing the issue (though this doesn't seem to be the case), but the results would still be catastrophic, as the aforementioned events are still likely to occur (specifically the tsunamis and volcanic events). However, as mentioned above, if this pendulum were located at the South Pole, then Black Hat and other people around him would not be affected immediately, and he could both do it, survive and be stopped again. The question is whether there would be any more news stories to cover this, given what would happen to the rest of the world! If there was no one to readjust the pendulum's rotation, then certain events would happen after the initial damage (see this video [1]). Randall previously covered this scenario in detail in his what if? book, see XKCD's Creator Explains What Would Happen If Earth Stopped Spinning.

To be completely correct, the angular momentum of the Earth+pendulum system is constant, so that when Black Hat "stops" the pendulum's rotation with respect to the Earth, he actually transfers to the pendulum some of Earth's angular momentum, thereby slightly slowing Earth's rotation. But the order of magnitude of that effect is (at most) in proportion to the pendulum-to-Earth mass ratio. Earth's mass being ~6 x 1024kg, the effect for any practical pendulum would be beyond the 20th decimal place and would therefore go totally unnoticed.

The title text mentions the International Earth Rotation and Reference Systems Service, an organization providing standards for global time and reference frames; this organization would have a very rough day after the fiasco with Black Hat. The title text refers to a (probably) fictional paramilitary enforcement arm of this organization and the foolishness of tangling with such a force. If Foucault pendulums were somehow able to influence the rotation of the Earth in any measurable way, the IERS enforcers would probably strictly control their installation and monitor their use (and misuse). Black Hat is likely in for a rough day as well. It seems likely that some on this team are geophysicists, since they were the ones who took Black Hat down.

## Transcript

[Megan is addressing Cueball, Black Hat and Ponytail as they are looking a pendulum, which consists of a large swinging sphere with a spike beneath it hanging on a string coming down from above with seven small markers under it. Megan is pointing at it while it is swinging towards them, as indicated with lines behind the sphere and the string.]
Megan: This Foucault pendulum demonstrates Earth's rotation. It stays in a fixed plane while the Earth rotates under it.
[Close-up on Black Hat holding a hand to his chin.]
Black Hat: Hmm, really.
Black Hat: So that means...
[Zoom out to Black Hat running to the pendulum grabbing it with both hands. He knocks over two of the seven markers that can now be seen to be small pegs standing up on he floor beneath the pendulum. A line goes from his hand on the pendulum up to a sound bubble. Two people off-panel yell at him.]
Character off-panel 1: Hey!
Character off-panel 2: Stop him!
Sound:Grab
[Blondie as a news anchor uses her held out hand to indicate Black Hat who is shown on a screen next to her. There is a caption in two lines above it. The first line in normal text, the second line in white in a black rectangle. There are further unreadable text below the picture.]
Blondie: The Earth's rotation was briefly halted today until geophysicists wrestled the intruder to the ground...
Breaking
news

# Discussion

Why are we assuming there are any side effects like these? The pendulum stops the Earth from rotating -- couldn't this include the entire planet, including the atmosphere and loose objects such as humans? There probably would be some volcanoes from the sudden loss of centrifugal force, though. 162.158.39.23 13:52, 13 September 2019 (UTC)

I take it the pendulum is somewhere deep underground, which would shield Black Hat from the cataclysmic side effects? Besides, they have several backup pendulums that while not enough to maintain rotation are sufficient to slow the half enough to preserve life.108.162.212.149 20:11, 11 September 2019 (UTC)

I fear that the use of multiple pendulums to smooth out the catastrophy of stoping the Earth's rotation would probably just cause bits of the Earth to keep going and other bits to stop. Hey! That's plate techtonics! Obviously there are subtly dampened/purturbed pendula in secret (masonic?) temples all across the world, making all that happen! Someone likely pushed the one in Atlantis too far, one fateful day...
(BTW, the unsaid catastrophe element reminds me of a classic short story)162.158.154.31 15:29, 12 September 2019 (UTC)
These effects will of course not work on Black Hat or any other that are at the Eart Rotation controlling Pendulum at The South Pole observatory. There the Earth stopping will not have any effect :-) --Kynde (talk) 10:53, 13 September 2019 (UTC)
Well, maybe a bit of nanodiziness..? 162.158.158.209 16:17, 16 September 2019 (UTC)

If the pendulum could really affect Earth's rotation, Black Hat wouldn't need to stop the pendulum entirely; he'd only need to prevent its plane of oscillation from rotating. Another thought: if the pendulum and Earth's rotation were really bidirectionally linked somehow, there would probably be nothing Black Hat could do to alter the pendulum's plane of oscillation -- any more than he could alter the rotation of the Earth with just one human being's strength. That last thought doesn't seem to be the case within this story, though, or else the final frame's news report wouldn't have happened. Trueflint (talk) 20:46, 11 September 2019 (UTC)

Who says it's the energy from Black Hat's grab specifically? Maybe the magic pendulum just tells a device in the Earth whether or not the planet should be spinning, based on the current state of the pendulum. 172.68.46.167 08:15, 12 September 2019 (UTC)

Why does the description description Megan as a "professor?" She could just as easily be a teacher, a docent, a scientifically-interested parent, or just a random bystander. 11 September 2019

Agree this is more like a tour guide if anything. I will correct this. --Kynde (talk) 10:51, 13 September 2019 (UTC)

The sentence "It stays in a fixed plane while the Earth rotates under it." and the correspoding text in explanation are wrong. It would be true only if the pendulum was located on one of Earth's poles. Elsewhere, the plane in which the pendulum moves would still rotate with respect to its surrondings, but slower than on the pole. The rotation speed is proportional to the sine of latitude. At the equator, the plane would stay fixed with respect to its surroundings. See Wikipedia.--162.158.93.207 23:45, 11 September 2019 (UTC)

As a Physics teacher, I strongly support this. The fact that a Foucault's pendulum is keeping its oscillation plane constant with respect to an absolute reference frame is a common misconception, it should be mentionned as frequently as possible.--108.162.229.48 09:17, 12 September 2019 (UTC)
The reason this pendulum works on Earth rotation is of course that it is the one placed on the South pole! That must be obvious (apart from this being a silly, though very funny, comic). :-D --Kynde (talk) 10:51, 13 September 2019 (UTC)

The Foucault Pendulum in this comic strongly resembles the one in the Franklin Institute in Philadelphia (both in shape and the way it is knocking over the pegs). Perhaps this should also be noted in the trivia section?162.158.126.46 05:16, 12 September 2019 (UTC)

Then list *all* the ones it strongly resembles. Do you think Philadelphia’s is the only one with pegs? I think that is the usual presentation. 172.69.70.107 10:01, 12 September 2019 (UTC)
A Google search reveals that nearly all of them have a weight that looks like the one in this comic, and many of them have some sort of pegs to demonstrate the circular motion over the course of a day. To put the location of any of them in the explanation (as it is now) is probably not appropriate. If there is a significant one somewhere in the world (largest, oldest, etc), then maybe we could mention that specific one. Ianrbibtitlht (talk) 14:56, 12 September 2019 (UTC)
But this is the one that determines the Earth Rotation (and is placed at a pole, see above ;-) Also the one in a small museum here in Aarhus Denmark has the same setup --Kynde (talk) 10:51, 13 September 2019 (UTC)

Do we have anything about resonance transfer being proportional to the difference in mass? 172.68.189.19 16:20, 12 September 2019 (UTC)

I don't think there's any indication that this comic takes place during a physics lecture. It's more likely that it takes place in a science museum, and this is a museum tour guide explaining the Foucault Pendulum to museum visitors. Ianrbibtitlht (talk) 00:16, 13 September 2019 (UTC)

Agree and will change this now. --Kynde (talk) 10:51, 13 September 2019 (UTC)

I don't know if it's still the case, but back in the 90s some Earth orientation work was very hush-hush military business. It turns out that really detailed models of how the Earth moves are important for targeting long-range missiles. Dfeuer (talk) 02:35, 13 September 2019 (UTC)

Are you suggesting that has some relevance to this comic about the Foucault Pendulum? Ianrbibtitlht (talk) 03:07, 13 September 2019 (UTC)
I'm suggesting it could relate to the notion of paramilitary forces coming for someone who's messing around with Earth orientation. Dfeuer (talk) 12:50, 13 September 2019 (UTC)

You really do not want to mess with IERRSS (IRS for short). Also, the pendulum is in an inertial reference frame to what? Can it be used to detect earths rotation around the sun as well, and the solar systems rotation around the galactic core Torax (talk) 10:16, 13 September 2019 (UTC)

This is one of my new favorites. Love this idea, and of course Black Hat's reaction to the possibility for mayhem.  :-) --Kynde (talk) 10:51, 13 September 2019 (UTC)