explain xkcd - User contributions [en]

2970: Meteor Shower PSA

2024-08-21T21:19:58Z

Jeans: added citation needed

{{comic
| number = 2970
| date = August 9, 2024
| title = Meteor Shower PSA
| image = meteor_shower_psa_2x.png
| imagesize = 561x272px
| noexpand = true
| titletext = If you hold the meteor too long, it may imprint on you and form a contact binary, making reintroduction to space difficult.
}}

==Explanation==
This comic is a {{w|public service announcement}} (PSA) regarding what to do in case you discover a {{w|meteorite}} from the upcoming {{w|Perseid}} {{w|meteor shower}}, which occurred the weekend after this comic appeared. (See here regarding [[1723: Meteorite Identification]]).

In the first panel of the comic, [[Cueball]] finds a {{w|meteorite}} on the ground, likely from the {{w|Perseids}} meteor shower. He then tries to throw it back into space, which is obviously impossible. However, the comic marks this action as wrong (with an X) not because it’s impossible, but for a different reason. The PSA in the next panel (marked as correct with a check mark) suggests that instead of throwing it, Cueball should contact an observatory and deliver the meteorite to them. There, astronomers like [[Ponytail]] can care for it, possibly releasing it back into space during the next launch.

This idea mixes up the concept of meteoroids with baby animals that have been separated from their parents and need rehabilitation—like baby birds that have fallen from their nests (though usually from trees rather than the sky). The advice for finding a wild animal, especially a baby one, is generally to leave it alone because its parents are likely nearby and taking care of it. If the animal is sick, injured, or orphaned, it's best to contact a wildlife rehabilitator who can properly care for it and return it to the wild. However, this advice does not apply to meteors{{Citation needed}}.

If you find a recently fallen meteorite or a {{w|Meteor air burst|fragment}} of one, avoid touching it, as it may be extremely cold. Though the meteor’s surface is heated by the atmosphere during its descent, the interior remains as cold as space, and the surface will rapidly cool if it hasn't already. The coldness depends on the meteorite’s size, fragmentation, and how long it has been on the ground—unless it was {{w|Impact crater#Impact craters on Earth|large enough}} to generate additional heat from the {{w|Lithobraking|impact}}. Experts prefer you not handle meteorites directly, as this can contaminate the specimen, making it less valuable for scientific study. In rare cases, such as with {{tvtropes|GreenRocks|more dangerous examples}}, the meteorite could even contaminate ''you''!

[[1475: Technically|Technically]], [[Randall]] uses the term "meteor" incorrectly. A "meteor" refers to the streak of light seen when a meteoroid enters the atmosphere. If a piece survives and lands on the ground, it's called a "meteorite" (although some refer to it as [[1405: Meteor|magma]]). This might be intentional to fit the "lost baby bird" analogy, as, like baby birds (except for those from {{w|Bird nest#Type|ground-nesting}} species), a true meteor should never be found on the ground.

The title text humorously suggests that handling the meteor for too long might result in forming a {{w|contact binary (small Solar System body)|contact binary}}, which is when two space rocks stick together. While this term is also used for binary star systems, it’s unlikely that a person and a rock would form one. In rehabilitating young animals, preventing {{w|Imprinting_(psychology)|imprinting}}—where the animal forms a psychological attachment to its caregiver—is important because the animal needs to be able to survive in the wild without human help.

==Transcript==
:[A two panel comic with the panels next to each other.]

:[In the left side of the first panel, Cueball spots a meteorite on the ground. It lies a bit buried in the earth between tufts of grass. On the right side, he's shown throwing the rock into the air, with small lines indicating the flight of the meteorite. There's an "X" above him. Above this there is the following text:]
:This meteor shower weekend, remember: If you find a meteor on the ground, don't try to return it to the sky yourself.

:[In the left side of the second panel, Cueball is holding the meteorite in one hand and talking on his cellphone in his other hand, there's a check mark above him. In the middle of the panel Cueball is holding the meteorite out in both hands handing it to Ponytail who is also holding both hand out to receive it. To the right in the panel a rocket is blasting upwards with fire coming out beneath it and a plume of smoke showing its ascending path. Above this there is the following text:]
:Instead, contact an observatory where astronomical rehabbers will care for it, and hopefully release it back into the wild with the next space launch.

{{comic discussion}}

[[Category:Comics featuring Cueball]]
[[Category:Comics featuring Ponytail]]
[[Category:Astronomy]]
[[Category:Space]]
[[Category:Public service announcement]]

2974: Storage Tanks

2024-08-21T21:13:37Z

Jeans: added citation needed

{{comic
| number = 2974
| date = August 19, 2024
| title = Storage Tanks
| image = storage_tanks_2x.png
| imagesize = 321x251px
| noexpand = true
| titletext = We're considering installing a pressurization system to keep the tanks at constant pressure solely to deter them.
}}

==Explanation==
{{incomplete|Created by SUBSTITUTE CALCULUS TEACHER MR. MUNROE ... WITH THE POWER DRILL ... IN THE CONSERVATORY - Do NOT delete this tag too soon.}}
Two construction workers in hardhats are on a scaffold, overlooking an industrial area with two large tanks. One is instructing the head of security that "your primary task" is to "watch for calculus teachers trying to drill holes in their bases." [[Miss Lenhart]] is seen running away with an electric drill after having drilled a hole near the bottom of the tank furthest from the guards.

A common question in introductory {{w|calculus}} courses asks [https://www.haywardflowcontrol.com/media/contentmanager/content//downloads//VessTime.pdf how long it will take a tank to empty.] Assuming the tank is shaped like a cylinder and is open to the environment (so that air can enter the tank to replace water as it flows out), the amount of fluid left in the tank above the hole will follow a quadratic decay, as the rate of pressure decrease will be proportional to the instantaneous pressure at any given moment. (Advanced variations of this question may consider more complicated tank geometries, in which case the pressure at the base will not be simply proportional to the volume of fluid remaining in the tank.) Specifically, the rate at which water flows out of the hole is governed by {{w|Torricelli's law}}, which states that the speed 𝑣 of the water flowing out is proportional to the square root of the height ℎ(𝑡) of the water above the hole.

Also, it's common practice for STEM teachers to use real-world examples to make abstract concepts more memorable for their students (though practical demonstrations like this wouldn't generally be used to teach calculus more than once per term per class{{Citation needed}}).

This comic combines these ideas, implying that there's a rash of calculus teachers that go around drilling actual holes in storage tanks to demonstrate the underlying math principle, so much so that the primary job of the head of security is to prevent this pedagogically-motivated destruction. Perhaps Miss Lenhart's class is watching from afar. (Adding to the drama is the warning triangle (⚠) on the tank, implying the liquid is toxic.)

The title text jokingly alludes to the fact that by maintaining a constant pressure in the tank, the rate of flow would itself become constant, which would simplify the problem greatly from a calculus exercise to a simpler algebraic one, thus deterring calculus teachers — though it may inadvertently attract similarly adventurous algebra teachers. (If the pressure applied was higher than expected, it could also result in a nasty shock to the teacher drilling into it, as the liquid rushes out surprisingly fast.)

An easier deterrence option — compared to installing a pressurization system — would be to make the problem more complex by simply having the tanks fully closed to the environment, forming a vacuum as liquid drained out, which would represent a more complicated problem suitable to an advanced undergraduate or graduate course in Fluid Dynamics or Advanced Differential Equations.

===Analysis and Calculation===
Observation of the comic suggests the following assumptions:
* Tank height above hole: ~20 feet
* Tank radius: ~6 feet
* Type of drill bit: Normal (not a hole saw)
* Size of drill bit: 1 inch (largest commonly available twist drill bit)
* Goal of Miss Lenhart: To demonstrate quadratic decay to her students

With a tank that is 20 feet high, has a 6-foot radius, and a 1-inch diameter drill hole, it would take approximately 21.5 hours for the tank to empty completely — too long for a suitable class demonstration, and likely to be fixed by nearby workers who notice the leak.

That suggests the exploits are being filmed and shown later in an abridged (time-lapse) form.

Another way to put this is that her drill and drill bit seem to be laughably small for the job at hand.

(If she is using a large "hole saw" drill bit — which can be up to 6 inches in diameter — then that would explain the quantity of gushing liquid, and the tank could be emptied in about 36 minutes, but the drill doesn't seem to have that kind of bit on it.)

The most likely explanation to all this is that Randall didn't think through the drill and drill bit size in relation to the apparent hole size, leaving it only to nitpicky editors of a comic explainer website to even notice and care.

Alternatively, the teacher could have drilled multiples holes.

==Transcript==
:[Two guards with helmets are standing on scaffolding to the left of two large tanks with labels at the top. The tanks are cylindrical with a smaller base than the tank above it. The left tank has a small sign with unreadable text and near the bottom of the right tank there is a triangular warning sign with an exclamation mark inside it and a line of unreadable text below it. The guard on the left is talking to the other guard. Miss Lenhart is seen running away from the right tank with an electric drill in one hand. There is a hole in the base of the right tank which has caused the liquid inside to leak out of the tank splashing on the ground in the direction of Miss Lenhart.]
:Left guard: As head of security, your primary task is to monitor the storage tanks and watch for calculus teachers trying to drill holes in their bases.
:Label: Tank #3
:Label: Tank #4

{{comic discussion}}

[[Category:Characters with hats]]
[[Category:Comics featuring Miss Lenhart]]
[[Category:Math]]
[[Category:Physics]]