No Source Is Perfect (Textbook Misconceptions)

While I frequently recommend to people that they spend time on the basics and dive into an old-fashioned textbook instead of dropping hundreds or even thousands of dollars on the next trendy modality, cert, etc., that doesn't mean that I don't find problems with texts that purport to cover fundamentals.  Even educational resources that are great overall can have issues, and kinesiology texts are no exception to this.  As I was leafing through one such book earlier today (since I had to do something while I waited for my car to get worked on at the dealership), I came across a passage that made me raise an eyebrow.

Me clutching the book in question while waiting on my car (I could have posted one from a sale page, but this felt a little more personal)

Excerpt on Newton's Third Law of Motion, taken from Brunnstrom's Clinical Kinesiology, 6th Ed. (F.A. Davis Company, 2012) -- Chapter 2 - Mechanical Principles: Kinetics, pp. 34-35:

"The easiest way to discuss this law is to present an example: if you hold your notebook in your hand, there are two equal forces acting on it--your arm muscles to keep the notebook in the position you desire it to he and gravity pulling it to the ground. The forces acting on the notebook are equal since the book has not fallen to the floor and you have not changed the position of the notebook.

"Another example: If two football linemen push each other and neither moves, they are abiding by the law that states for every action there is an equal and opposite reaction. Player A produces an action force toward player B, and player B provides an equal reaction force toward player A. Until one overtakes the other, they are abiding by Newton's third law of motion."

Within this portion of a paragraph are two fairly large back-to-back misconceptions/miscommunications. Can you spot them? Take a moment to see if you know what I'm referring to here.

ALSO -- let me make it abundantly clear that I am not attempting to disparage this book or the authors thereof. I purchased this copy and have no regrets about doing so. Rather, I want to highlight how even renowned educators and respected experts can sometimes present something in less-than-optimal ways. I frequently encounter this in functional anatomy and clinical/exercise anatomy books, as they tend to have an intro section or two on basic physics before going into the bulk of the text which is focused more on the "good stuff" (read: what the book is mainly about, what the reader probably came for, and what the authors have the most expertise in). Thus, basic mechanics can get short shrift, with a "close enough for government work" approach taken -- inadvertently or otherwise.

This book still has value, and I still support purchasing it (and many others) if you wish to explore these topics further.  There's a lot to like about it, and it can be found at a number of outlets if you want to grab a copy.

Did you take a moment to spot any issues?  Yes?  Great.  Now with that out of the way, let's get on to the nerdy stuff!

So in the first example, it is implied that the muscles of the arm are imparting a force to the notebook. They are not. The notebook doesn't "feel" the muscles (at least directly). If you were to draw a free body diagram of this system, it would be the hand that is applying a force to the notebook, and located at our best guess at an average point of force application (gripping on the edge, held mid-palm, etc.) Trying to analyze this from a "muscle on notebook" perspective would present all sorts of problems and leave many students wondering what they're doing wrong, when the error was that it was never the muscle that was acting on the notebook. The muscle acted on the bones, and eventually the influence of connected body structures led to the hand's ability to apply force to the notebook. If we skip steps, we risk missing something important.

Funnily enough, this is similar to the issue that drives a lot of misunderstandings of exercise. We may think that the weight is the resistance, because the weight of the object we lift is (sometimes) what is initially felt by our bodies. However, that is what is felt at the point where the weight interfaces with the body -- not what is felt by the muscles that actually have to move/control the limb that might be lifting that weight. So it is the notebook scenario, but in reverse: the muscle doesn't feel the weight, but rather the pull of the body segment that itself is experiencing forces that directly or indirectly come from the external implement being lifted. To understand the actual challenge a muscle experiences, we must account for these steps and the associated lever systems that are involved. We cannot simply "jump" from weight to muscle.

***

In the second example, the text claims that the two football players "are abiding by the law" if they push each other and neither moves, when the reality is that they are always abiding by Newton's third law. These laws are, for the purposes of "real world" mechanics scenarios and inasmuch as we can describe human function, immutable. They don't apply only some of the time; they apply ALL of the time.

Further language in that example states that "Until one overtakes the other, [the players] are abiding by Newton's third law of motion." Depending on what is meant by "overtakes," this could also be misleading. If it simply means that one player is "winning" the pushing match or that one's *muscular effort or capability* exceeds that of the other, then the overtaking is irrelevant. Newton's laws don't care.

Newton's Laws don't only apply when pushes/pulls happen a certain way.  They *always* apply.

Thus, if the two players were pushing on each other and *moving*, the third law would still apply. Player A would still feel an equal and opposite push from player B, and vice versa. The text is written in a way that might lead people to think that the "equal and opposite" phenomenon only exists when forces are balanced and the system is in equilibrium, but this is not the case. You can never violate Newton's third law (and if you think you can, please message me so we can both get insanely rich).

To some, this may seem like splitting hairs. However, these errors compound across discipline and across one's academic/professional career if they don't get caught. We get used to playing fast and loose with concepts that we *kind of* understand, telling ourselves "Eh, that's good enough." The result is the seasoned veteran who still gets basic things wrong, often with no obvious warning sign of incompetence beforehand. A common refrain is "Weren't they taught this in school?? How did they ever get qualified?!?"

The answer is that they probably were taught it, but they didn't remain vigilant for errors and practice appropriate diligence with their study over time. They may not have sought true mastery of their fundamentals, seeing fit to move on after passing the exam or getting the job. With nobody around to *make* them keep working on these concepts, things just fell by the wayside.

Don't neglect your fundamentals.  They're important for setting yourself apart from others as an enthusiast or professional, and they may *also* help you to spot inconsistencies that arise in the educational materials that you use, thereby allowing you to get more out of those resources without being misled.

This is a process that never really ends, and that's okay.  Students, professionals, teachers, authors -- we're all constantly learning.  And if we're not, we're doing something wrong.

- G


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