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Momentum Demonstration

|< < Prev #3312 (October 16, 2026)

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."

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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. An example of this is 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 astrophysical 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 sets the floor on fire and also, presumably via conservation of (linear) momentum, propels Beret Guy upwards. Though the downward-going jet should impart only the same, but opposite, momentum as the upward-going one, the more immediate interaction with and rebound from the the floor may be the prime reason that it propels Beret Guy preferentially upwards against gravity.

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 professors' 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, except perhaps in the metaphorical sense ("My professor is so hot"). 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 his chair is moving upwards, tilting slightly to the left with beams blasting out. Hairbun and Cueball are backing away.]

Beret Guy: Now, to help shed angular momentum, I form an axial jet!

[Beret Guy's chair continues moving upwards and tilting to the left, and fire and smoke are spreading around his jet. Hairbun is covering her eyes and running away, and Cueball is holding his hands out.]

Hairbun: AAAA

Cueball: AAAAA

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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 976: Sail, 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!

Sail

|< < Prev #976 (November 11, 2011) Next > >|

Title text: It only works a few times before you have to capsize the boat in a soap lagoon again.

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Explanation

Welcome to This Week's Explanation for Improvement! Anyone is encouraged to help explain comic 976: Sail; 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:   Could this idea actually work in real life, with the right weather conditions?   In this xkcd universe, how do the sails interact with wind?   Why is Cueball pulling a line? Is he trying to create the bubble or govern the ship?   How would the "soap lagoon" work?

A billowing sail sometimes looks like the first stage of blowing a bubble. Cueball is clearly surprised when a bubble is actually formed by the filled sail.

Readers infer from the title text that the water is also part of this fantastical scenario. If the boat was overturned (capsized) in a soap lagoon, a film would form between the mast and the boom (horizontal bar that adjusts the sail). The film would become like the sail in the comic, filling with air and forming bubbles until the film is used up.

Transcript

[Cueball is sailing a lateen rigged sailboat.]

[The wind picks up, pushing the sail forward.]

[The wind builds strength. Cueball has to stand up to hold on to the main sheet, with curves depicting his tugging.]

[The distorted sail balloons out. Cueball is still holding on to the main sheet. More curves depict a stronger tugging.]

[Part of the sail separates into a bubble. Cueball starts to fall backwards. The sail begins to return to its normal shape.]

[Cueball lands on his rear and scratches his head in confusion as the sail-sized bubble floats away.]

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