2539: Flinch

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Flinch

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."

Explanation

This explanation may be incomplete or incorrect: Created by a WOUNDED PREMED STUDENT - Please change this comment when editing this page. Do NOT delete this tag too soon. If you can address this issue, please edit the page! Thanks.

Cueball is demonstrating a bowling ball pendulum to a group of bystanders with different science backgrounds. He asks if they'll flinch after the ball is released and swings toward them on its return arc. Black Hat sets up a bowling ball pendulum in 1670: Laws of Physics, but sets it up improperly on purpose in an attempt to hurt Cueball.

The first bystander, a physicist (Megan) understands the principles of the experiment and claims she won't flinch. She knows that, as long as it's just released (as opposed to being pushed) in still air, then the ball won't return further than its point of release. (If the experiment is conducted outdoors, then it is possible for a sudden wind gust to blow the ball enough for it to return further than its point of release; however, this is unlikely with a bowling ball, which has a large mass compared to its surface area.)

The second bystander, a biologist (Hairy) seems to indicate that he expects to flinch, but he feels this is natural and doesn't intend to fight it. It is natural, and in fact the body comes with many kinds of automatic reflexes. In 1670: Laws of Physics, Randall makes a very similar comment in the title text.

The third bystander, an engineer (Ponytail), knows practice usually differs from theory. She ignores Cueball's question and declares his apparatus obviously suspect. Cueball is holding the ball so the string is visibly curved, not stretched, as when the ball pendulums back. The string and ball thus start with greater gravitational potential energy (height) and elastic potential energy than when the ball swings back to the same position over the floor. The excess energy means a near even chance the ball may move past its starting point on the first swing, and sometimes pass or not on following swings.

In the title text, a premed student tries to get course work done.

Transcript

This transcript is incomplete. Please help editing it! Thanks.

[Cueball holds a bowling ball attached to a string as a pendulum. He is talking to Megan, Hairy, and Ponytail.]

Cueball: If you stand with the bowling ball in front of your face and let go, will you flinch when it swings back?

[Caption: Physicist]

Megan: I won't flinch.

Megan: I trust conservation of energy.

[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.

[Caption: Engineer]

Ponytail: I don't trust that you hung that thing up correctly.

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Discussion

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.) 172.70.91.50 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? --172.68.110.141 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. 141.101.98.145 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! 172.70.178.161 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. 172.69.34.91 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 162.158.111.80 08:22, 10 November 2021 (UTC)

oh, a similar video has already been linked. ;) 141.101.105.73 08:24, 10 November 2021 (UTC)