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| Momentum Demonstration |
 Title text: "I'm doing really well on the Hertzsprung-Russell academic diagram, which has spectral class on one axis and Rate My Professors rating on the other." |
Explanation
This is a spin on a classic lecture demonstration of the conservation of angular momentum, whereby someone holding dumbbells with arms extended on a slowly rotating swivel chair brings them closer in to their body, and (by conservation of angular momentum) rotates faster. The expectation is that they then hold the dumbbells further out again to slow down.
In this comic, Beret Guy performs the demonstration accordingly beside Cueball and Hairbun. However, he eventually spins up fast enough to exceed his breakup rotation rate, as an astronomical object would, like a star being formed out of a collapsing spherical cloud of gas. In such astrophysical situations, exciting things happen to that spherical cloud, such as not being a sphere any more (these things tend to form disks instead). Another feature of such accretion disks is that the central protostar emits jets along the axis of rotation, which carry angular momentum away and permit the object to continue rotating close to its breakup velocity instead of disintegrating by rotating too fast to stay gravitationally bound. The particles in these jets are very fast-moving, which means both that they're hot and that they have a lot of momentum themselves. This is most likely one of the strange powers of Beret Guy. The upward-going jet pierces the ceiling, causing chunks of it to fall, and the downward-going one both lights the floor on fire and also, presumably via conservation of (linear) momentum, propels Beret Guy upwards -- though it's unclear why it should do this since the downward-going jet should have the same momentum as the upward-going one, and their effects on Beret Guy should cancel out.
The Hertzsprung–Russell diagram shows the relationship between the stars' absolute magnitudes or luminosities and their effective temperatures (colors). The title text suggests there is a (fictional) Hertzsprung–Russell academic diagram showing the relationship between professor's effective temperature and their rating on the Rate My Professors site. What it would mean to 'do well' on this diagram is unclear, since the two factors have little relationship to start with. In the real world, most professors would not be meaningfully differentiated in their temperature[citation needed]. However, the axial jets shown in the comic imply a much higher effective temperature, especially given the flames visible below Beret Guy's chair.
Transcript
- [Beret Guy is spinning on a chair, holding two weights away from him. Hairbun and Cueball stand watching him.]
- Beret Guy: See, when I hold the weights out, I rotate slowly.
- [Beret Guy holds the weights closely, and spins faster. Cueball has his hand up as if thinking.]
- Beret Guy: But if I pull them in, I spin faster, due to conservation of angular momentum.
- [Beret Guy is spinning much faster, and the chair is moving upwards, beams blasting out. Hairbun and Cueball are backing away.]
- Beret Guy: Now, to help shed momentum, I form an axial jet!
- [Beret Guy's chair continues moving upwards, and fire and smoke are spreading around his jet. Hairbun and Cueball run away.]
- Hairbun: AAAA
- Cueball: AAAAA
This Week's Explanation for Improvement
As part of the Explanation Improvement Project, each week the community selects one older, stagnant, or otherwise neglected explanation to expand and polish together. This week's pick is comic 430: Every Damn Morning, you can help us by editing its explanation!
To get involved with the project, you can also nominate an explanation that you think should be improved, or vote on current nominations!
| Every Damn Morning |
 Title text: There was something about a cup and a sword and a tree and a green hill... |
Explanation
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Welcome to This Week's Explanation for Improvement! Anyone is encouraged to help explain comic 430: Every Damn Morning; click here to get started! If you are new to wiki editing, you can read our editing FAQ for guidance. Here are some suggestions for improving this explanation: .svg){kind=icon} Explain the various occurrences in the dream! Could they be references to other comics?
.svg){kind=icon} Why do humans forget their dreams so quickly?
.svg){kind=icon} Improve the #Transcript, it's not descriptive enough.
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Cueball has had a wondrous and striking dream (seemingly involving various action-packed escapades with Megan and/or on board a Zeppelin), including, among other things, a swordfight and a nuclear detonation, but is unable to recount it to Megan, as he has forgotten it by the time he reaches her at the table downstairs, indicated by the dream fading and losing detail.
The title of the strip suggests that this is a regular occurrence: he wakes up with vivid memories of elaborate and fascinating dreamscapes, but is unable to tell anyone about it, because the memories fade so quickly, he's lost almost everything before he gets the chance. The title text is a reference to C. S. Lewis's novel The Voyage of the Dawn Treader, in which Lucy Pevensie reads a story that “refreshes her spirit” – “That is the loveliest story I’ve ever read or ever shall read in my whole life. Oh, I wish I could have gone on reading it for ten years.” – but afterwards can only remember that it had something to do with "a cup and a sword and a tree and a green hill." A cup and a sword are also present near the center of the dream, with a large mushroom cloud in the background that could be misinterpreted upon recollection as a tree; these items possibly serving as another reference and/or providing further context to the title text.
Most people have trouble remembering the majority of their dreams, or only remember the most significant or recent parts. A contributor to this phenomenon is that the ability to store long-term memories is largely switched off during REM sleep, meaning that experiences within dreams typically only last from a few minutes to seconds in memory. Furthermore, levels of norepinephrine, a neurotransmitter critical for attention and memory formation, also drop to near zero during REM sleep. Without this chemical signal, the brain cannot encode experiences normally. There are also theories that the brain naturally discards memories from dreams so that people do not confuse those mental fabrications with real-life events.
The various parts of the dream are common in xkcd. Zeppelins were seen before in 73: Zeppelin and 288: Elevator, and continue to be a recurring theme.
Another effort to remember dreams was made in 269: TCMP.
Transcript
- [In the background, a vivid dream scene is apparent, where sets of Cueball and Megan interact in various action packed scenes, involving a mountain range with a blimp overhead, a city with a mushroom cloud and a blimp anchored to a building, a Zeppelin in the foreground which a Cueball clings to as a Megan falls, Megan climbing off of the blimp to a Cueball with a rope, a Megan using a sword to pin down a Cueball over a cup of spilled liquid, another set holding hands, another set looking at a laptop, another set hugging, another set sliding down a separate rope from the blimp and a final set falling. All in all, this seems very much like a wild, apocalyptic adventure and not something one would forget easily. In the inset, Cueball awakens, excited.]
- Cueball: !!!
- [Dream's edges are fading, mountains, city, and Zeppelin less clear. In the inset, Cueball is seen running down stairs.]
- [Zeppelin has faded out of view; city, and mountains are very faded and unclear. The people can still be seen. In the inset, Cueball gets the attention of Megan, who is sitting at a breakfast table.]
- [The dream has almost completely faded; only the outlines of three people can still be seen. In the inset, Cueball looks confused while Megan picks up the cup in front of her.]
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