Flinch
Flinch
[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.

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 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.
