2539: Flinch

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Premed: "Does this count for a physics credit? Can we shorten the string so I can get it done faster? And can we do one where it hits me in the face? I gotta do a thing for first aid training right after."
Title text: Premed: "Does this count for a physics credit? Can we shorten the string so I can get it done faster? And can we do one where it hits me in the face? I gotta do a thing for first aid training right after."


Cueball is performing a common physics demonstration in which a heavy ball is hung from a rope or cable. The demonstrator, or a volunteer, pulls the ball back until it's close to their face (possibly even touching it), then releases it, allowing it to swing, and then return. Due to conservation of energy, the ball cannot return any further than its original release point, making it impossible for the person to be struck by it. Because a heavy pendulum will tend to lose little energy on each swing (relative to its overall energy), it will come back very close to its original point, so the experiment creates a conflict between the instinctive desire to escape a heavy object flying at your face, and the theoretical knowledge that it won't harm you.

Megan is a physicist, who understands the principles of the experiment and claims she won't flinch, confident that it can't harm her.

Hairy is a biologist, and implies that he has no intention of avoiding the flinch reflex, as he trusts the automatic reflexes that the human body has evolved more than he trusts the premise of the experiment. In both 755: Interdisciplinary and 1670: Laws of Physics, the same experiment is referenced. In the title text of the latter Randall makes a very similar argument as the biologist does here.

Ponytail, an engineer, replies that she doesn't trust Cueball to have hung the pendulum correctly. Engineers are trained in science, but work with practical applications, and tend to be very aware that practice is rarely as simple as scientific theories might imply. Even if the physical laws are constant, the experiment might not go according to plan. For example, if the cable were to snap or come loose while swinging toward the subject, the ball could strike them in the body, or land on their feet. If the cable is more elastic than anticipated, it could stretch unpredictably, once again striking someone. If the anchor point is not stable, it could shift during the experiment, once again causing harm. Also if the ball is not released but pushed, or if the one releasing it leans forward after release they might get hit in the face.

The punch line basically makes the point that failure to trust the safety of an experiment doesn't necessarily imply a lack of scientific knowledge. If you lack confidence in the design of an experiment, then it's not safe to assume that the laws of physics will protect you.

The comic as a whole demonstrates that members of different disciplines have differing perspectives on the world: Physicists trust physics, biologists trust biology, and engineers do not trust engineering. This creates irony, because the reader might expect that an engineer would trust engineering, but in actuality, engineers distrust things designed by humans, since they so often design things poorly and/or encounter things others have designed poorly.

The title text shows a pre-med student's response. Pre-medical university courses have a reputation for being more intense and demanding than other undergraduate degrees, so the student is portrayed as being very stressed and time-conscious; showing little interest in the experiment itself, only in how it impacts their degree. In addition, medical students are commonly the subject of "interesting" medical experiments which may lead to long-term psychological and physical side-effects.

The student first asks if participating in the demonstration will count for a physics credit, implying that they're not willing to spend time on it unless it contributes to their academic requirements. They then ask if they can shorten the string to make the demonstration go faster. Shortening a pendulum does, indeed, cause it to swing faster, but the time saved would be less than the time necessary to make the modification, so the demonstration would not end sooner. Finally, they ask to do a variant where they deliberately get struck in the face, because they have a "thing for first aid training" immediately after. This would likely injure them, but the student is apparently willing to sacrifice their own safety and well-being in service to their academic career. It's not clear how this would help, although it could potentially help others learn first aid by having them practice on the new injury.

Various alternate takes on this experiment have been previously featured in 755: Interdisciplinary and 1670: Laws of Physics, but this is the first time experiment is performed in a proper manner.


[Cueball holds a bowling ball in both hands. It is attached to a string that goes behind him and up disappearing off panel around double his height. He is talking to Megan, Hairy, and Ponytail who is looking at him. Between Cueball and the other three is a cross in a dotted circle on the floor.]
Cueball: If you stand with the bowling ball in front of your face and let go, will you flinch when it swings back?
[Zoom in on Megan in a slim panel. There is a caption in a frame above her.]
Caption: Physicist
Megan: I won't flinch.
Megan: I trust conservation of energy.
[Zoom in on Hairy, in a wide panel. He has lifted arm holding his hand palm up toward Cueball (who is off-panel). There is a caption in a frame above him.]
Caption: Biologist
Hairy: I trust my flinch reflex, which was honed by millions of years of evolution to protect my delicate face. I'm not messing with it.
[Zoom in on Ponytail in a slim panel. There is a caption in a frame above her.]
Caption: Engineer
Ponytail: I don't trust that you hung that thing up correctly.

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A follow-up joke: "Psychologist: I don't trust you not to give it a push." Who, me? (talk) 02:13, 9 November 2021 (UTC)

A sudden wind guest wouldn't add much momentum to a smooth, small object like a bowling ball in one swing. Even given minimal friction losses (air resistance and the chain's internal friction) I very much doubt it would speed it up enough to cause much of an impact. Also, unless Cueball has very bad luck or precognitive powers, he's unlikely to have set up the experiment perfectly in line with the next unexpected gust of wind, meaning any velocity vector change is likely to make the bowling ball miss the target scientist or engineer, not hit harder. Nitpicking (talk) 04:26, 9 November 2021 (UTC)

I disagree with the explanation's contention that the way Cueball is holding the ball means the experiment is being performed incorrectly. I think it's pretty clear he's not saying it will be released from exactly where he's holding it, since it's obviously not in front of any of their faces, and it's not yet above the mark on the floor. Esogalt (talk) 07:44, 9 November 2021 (UTC)

Ditto. Although I have in mind a way in which a (passive) string support could be arranged so that upon the outward swing it unwraps a ting little bit and returns upon a marginally lower/significantly more face-ward back swing (same K+P energy totals at all points), even starting with a taut string. Or of course an active support that moves on command, but that'd be definite cheating-with-intent as opposed to an 'accident'.
(I also imagine Randall saw the original, if not the Youtube parody, of your US Science-Explaining-Guy doing this for real. The Youtube parody had a cartoony 'face smash' edited in as the result as a (faux-?) bite back at the scientific rationalism. If I could remember the guy's name I'd have looked for video links to potentially insert, but all I'm getting is the likes of Brian Cox doing it (successfully), on a quick and broad search.) 10:05, 9 November 2021 (UTC)
"Not hung up correctly" might also mean that the hook suddenly comes loose on the way back, in which case the ball would fly into your face, wouldn't it? -- 10:58, 9 November 2021 (UTC)
No, if the hook came loose the ball would drop to the ground. If it happened to come loose just at or very near the "closest to the target" point, it might fall on his/her foot, though. Nitpicking (talk) 11:40, 9 November 2021 (UTC)
(Edit-conflicted by Nitpicking, who says the start of this more succinctly. Apols. for 'repeating' that as I repaste it all in again!)
More likely hit your chest/fall into your lower body. It would have to be set up very fine/coincidental to still be head(/chin?)height as it was now ballistic, instead of supported at the 'original' (nose?) level. I was thinking more like a small half-loop of string (an inch or two?) round the back and over of the presumed supporting rod and held under the taut cord leading to the nose-held ball with friction enough to preventing it unwrapping immediately.
On swinging away, the dangling cord angles off of the looped bit, the pendulum-arc lenghthens, and if this doesn't dissipate energy in too many other ways then the extra inch or so of length means that the outward swing of the ball (and, more importantly, the return one) will still get up to roughly nose-height at zero kinetic motion, but that would be several inches (assuming total pendulum arc somewhat less than 45°*2) horizontally outward from the centre.
(*Note: there'd be a moment of fall-and-catch with this setup, that a slightly different string-wrapping method might avoid, but this is the archetype for the principle. A more gradual slippage-event would also prevent possible catastrophic cord-snapping upon the completion of the lengthening, which would just drop the ball away from the 'experimentee' and be more dangerous to others.)
Perhaps the string isn't even anchored to the anchor-point, but looped over to a smaller weight that nonetheless catches or swings round like a bolas-ball, when dragged up, to prevent total unwinding beyond the 'accidental' short distance. It could look and feel like a proper hook-tied pendulum (within limits), but probably not so easy to be inadvertently arranged than an accidental twist of a cable over the support when (in 'good faith') setting up the equipment the first time. Which the engineer seems more concerned about (bridges/etc rarely collapse by design, but due to unaccounted-for technical issues/ocersights usually only blindingly obvious after seeing what went wrong) than "you set it up that way deliberately", as I read it. 12:00, 9 November 2021 (UTC)
Rather than your half-loop, which might be very visible, you could use a material for the cord/chain that stretches slightly under maximum tension (inelastically), say a soft alloy chain. Wait, why am I designing booby-trapped physics demonstrations? Nitpicking (talk) 12:24, 9 November 2021 (UTC)

Ponytail hardly ignores the question: "I don't trust that you hung that thing up correctly." is her answer to the question! 11:44, 9 November 2021 (UTC)

Not being familiar with the US university system, my knowledge of "premed" comes entirely from a brief scan of the Wikipedia article. Nonetheless I've expanded on the title text; hopefully it's not too egregiously wrong. Esogalt (talk) 13:15, 9 November 2021 (UTC)

An unmistakable xkcd leitmotif: in a world suckered by theory, engineers are the crafty realists (think 670 and 898). (ezra) 15:09, 9 November 2021 (UTC)

Or overconfident believers that if they can do engineering, they are experts in everything (think 1570). Nitpicking (talk) 15:21, 9 November 2021 (UTC)
We should have a category for "engineering", similar to how we have categories for Category:Physics and Category:Biology. 17:47, 9 November 2021 (UTC)

Maybe in the explanation a link to someone performing the experiment? e.g. https://www.youtube.com/watch?v=77ZF50ve6rs 08:22, 10 November 2021 (UTC)

oh, a similar video has already been linked. ;) 08:24, 10 November 2021 (UTC)