The stated theme of Prof. Jackson's video is that the behavior of a ball on a string, swung in a circle and released, is not what physics teachers say it is, and even physicists "get it wrong." This claim is misleading at best.

Physics is about modeling objects and systems, something Prof. Jackson never mentions. When physics teachers cover circular motion, and the ball/string example in particular, they clarify that a simplified model with a massless, infinitely stiff string is being used. In that model, the force of the string disappears instantly when it is released, the string does not influence the motion of the ball, the ball instantly starts to travel in a straight line, and (b) is the correct answer.

We know that strings are not, in fact, massless and infinitely stiff. The video explores the consequences of the "real world" time lag between when the string is released and when the ball experiences a change in forces upon it, during which the ball’s motion remains circular. That’s it. Indeed, the bulk of the video is not about circular motion at all; it’s about wave propagation: a fascinating topic, but one tangential to circular motion. (I'm here all week. More cringe physics teacher jokes to follow. 🙄)

Good physics teachers always make sure their students understand how objects are being modeled. I used to joke: “We paid extra for the frictionless pulleys and massless strings.”

Another joke I would tell relates to balanced forces on someone sitting in a chair. The chair’s job is to balance the weight of the person on it. I would say, "When a lineman on the football team gets up, and a flyer on the cheer squad sits down, how does the chair know not to put on the same amount of force and shoot the cheerleader up to the ceiling?” Of course, in our simplified model, the chair is not even an object, just the source of an outside force on the person sitting in it. In our model, we are ignoring the chair's behavior—it compresses less under the weight of the cheerleader, and its force decreases.

If our goal is to engineer a device or to study a scenario in great detail, every model in first-year physics would be too simple. If our goal is to learn basic physics though, those simple models are exactly what is needed. We should not be surprised when real world behaviors emerge which cannot possibly be predicted by our simple models.

And if one's goal is to entertain, to be able to say things like, “Even physicists get it wrong,” they will tuck modeling into their back pocket, which obscures the very process of physics we want students to appreciate. Doing so makes physics seem mysterious and tricky, the opposite of what physics actually is.

Thus, what Prof. Jackson alluded to, but should have said explicitly, is that the behavior of the ball approaches answer (b) as the behavior of the object holding the ball approaches that of the idealized string. Further, the ball does begin traveling in a straight line the instant the force from what's holding it disappears, just as Newton's laws predict. The "mind-blowing aspect of circular motion" turns out to be more about semantics than physics.

As always, good physics teaching is model-forward physics teaching.