Main Page
Welcome to the explain xkcd wiki! We already have 12 comic explanations!
(But there are still 2912 to go. Come and add yours!)
Latest comic
| Pendulum Types |
 Title text: The creepy fingers that grow from a vibrating cornstarch-water mix can be modeled as a chain of inverted vertical pendulums (DOI:10.1039/c4sm00265b) and are believed to be the fingers of Maxwell's Demon trying to push through into our universe. |
Explanation
|
This explanation may be incomplete or incorrect: Created by CREEPING TENDRILS OF STARCH - Please change this comment when editing this page. Do NOT delete this tag too soon. |
This comic shows and describes several pendulums. The first three are actual physics models, while the last one is made up for the comic.
The simple pendulum consists of a joint, rod, and weight, and when released, it swings in a regular fashion. These are used commonly in pendulum clocks to keep time.
The double pendulum consists of 2 joints, 2 rods, and a weight, and when released, it swings in a chaotic fashion. Interestingly, this follows by the mathematical definition of chaotic, being that small changes result in vast differences. This pendulum is thus nearly unpredictable, and due to this chaotic nature, real life applications are very limited.
The inverted pendulum consists of a simple pendulum that is placed upside down, with some apparatus underneath vibrating it vertically to keep it upwards. If left unpowered (or improperly controlled from positional feedback) it will fall, hence the "unstable" part. (The comic, however, appears to depict Kapitza's pendulum, a powered version that does not rely upon monitoring and feedback-control.)
The nightmare pendulum appears to be an inverted double pendulum, with an additional uninverted pendulum swinging within its much more substantial weight (which is also adorned with archaic/mystical symbols). The comic claims that this pendulum summons Maxwell's Demon, and jokingly implies that Maxwell’s Demon is an actual entity, and not a thought experiment.
The title text continues this joke explicitly, by referencing a real paper titled Vibro-levitation and inverted pendulum: parametric resonance in vibrating droplets and soft materials and implying that the paper ties the "creepy fingers" to Maxwell's (real) Demon. The paper only actually suggests that the phenomenon is related to inverted pendulum dynamics
Transcript
- [Four types of pendulums are shown in a single panel. Each has a bullet list below the depiction.]
- [Label:]
- Simple pendulum
- [A basic pendulum consisting of a joint, rod, and weight swinging in a regular arc]
- Periodic
- Stable
- Useful for timekeeping
- [Label:]
- Double pendulum
- [A pendulum consisting of 2 joints, 2 rods, and a weight swinging in a more loopy arc]
- Aperiodic
- Chaotic
- Moderately cursed
- [Label:]
- Inverted pendulum
- [An upside-down basic pendulum with some apparatus underneath vibrating up and down]
- Finely balanced
- Unstable
- Becomes stable when vibrated
- [Label:]
- Nightmare pendulum
- [An inverted double pendulum, with an additional uninverted pendulum swinging within a large weight adorned with archaic/mystical symbols]
- Forbidden
- Unphysical
- Summons Maxwell's Demon
New here?
Feel free to sign up for an account and contribute to the explain xkcd wiki! We need explanations for comics, characters, themes, memes and everything in between. If it is referenced in an xkcd web comic, it should be here.
- If you're new to wikis like this, take a look at these help pages describing how to navigate the wiki, and how to edit pages.
- Discussion about various parts of the wiki is going on at Explain XKCD:Community portal. Share your 2¢!
- List of all comics contains a complete table of all xkcd comics so far and the corresponding explanations. The red links (like this) are missing explanations. Feel free to help out by creating them!
Rules
Don't be a jerk. There are a lot of comics that don't have set in stone explanations, feel free to put multiple interpretations in the wiki page for each comic.
If you want to talk about a specific comic, use its discussion page.
Please only submit material directly related to—and helping everyone better understand—xkcd... and of course only submit material that can legally be posted (and freely edited.) Off-topic or other inappropriate content is subject to removal or modification at admin discretion, and users posting such are at risk of being blocked.
If you need assistance from an admin, feel free to leave a message on their personal discussion page. The list of admins is here.
Logo
Explain xkcd logo courtesy of User:Alek2407.
