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A Bad Example #49



Mechanics Corner

A Journal of Applied Mechanics & Mathematics, #49

A Bad Example

Most of us are inclined to trust an established textbook. We assume (1) the author knows what he is talking about, (2) the book has been carefully vetted by several editors and reviewers, and finally (3) the fact that it is well established in the market means that thousands of other readers have tacitly endorsed it as well. While these assumptions are usually true, they are no guarantee, and they can and do fail us at times. The topic of this post is one such failure. Beware!!

The format of engineering textbooks is fairly uniform, consisting of alternating sections of Theory and Examples. For a student encountering the material for the first time, the Theory is often simply too much, and blows right over the head of the student. The starting student relies heavily on the Examples to comprehend, even if incompletely, what the material is about. The Theory is most useful when the material is studied a second or third time in some depth. Because many will only study the material a single time while in college, it is critically important that the Examples be correct and in no way misleading. Sadly, both of these problems exist in the book discussed here.

The book in question here is Mechanical Vibrations by S.S. Rao, where both the book and the author are well known. The book was published early by Addison-Wesley; I have a 2nd edition dated 1990. It has been re-published more recently by Pearson in a 6th edition with a 2018 copyright date. The problem shown in the Figure is used in both of these, with only very minor changes. The same errors exist in both versions, indicating that in 28 years, Rao has learned nothing about this simple statics problem.

I encourage all readers to examine carefully the discussion in t he attached paper. It demonstrates clearly that even the experts go badly astray at times.






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117 views, and yet no comments! Does no one have any reaction to this?

Do you think Rao had it right in the first place? Do you think he got it wrong, but that getting it wrong does not matter, its only an example problem?

Or did you read it at all?


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Apparently I must write the dullest stuff found anywhere. As I write this, 290 folks have read this article, and not a single one has made a comment. I am really baffled. I had the (obviously mistaken) impression that this was significant, important information. It talks about a problem that is slightly out of the mainstream, a bit of an unusual challenge. It points to the fallibility of all people, even famous, highly successful authors. But no; not a single comment.


I am baffled.


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The new hard drives may have lost my automatic log in but I am in again.

When I use any source I work through any example provided in step by step detail. If steps were skipped in the example, you may not notice the 2 in r/2 may have been dropped, just to mention one of many easy errors.

I recall a professor who must have done the example in class countless times. He wrote 3x3 and came up with 12. Not that he did not know how to multiple, 12 was the incorrect answer, he just wrote down the wrong input but knew what the answer was. Proof readers can make the same error focusing on the answer and overlooking the input.

If no example provided work out the units. And if possible do a simple estimate of the answer to get some idea if the refined answer is possible.

I have briefly communicated with Dr D by email. In the attempt to demonstrate a basic concept the author may have concocted an impossible to build device. 

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Thanks for the comment, JAG.

You (and I) are a whole lot older than most textbook readers, and will read differently from most. As a textbook author, I think it is an absolute requirement that engineering book authors not mislead readers into believing that things can exist which actually cannot. That is called "science fiction."

In this particular example, I think Rao has misled by proposing to analyze a structure that is unstable. For a reader who has never seen a crane, they might just think this could work, and disaster would be the result, possibly with several dead.

To go a step further, I'm baffled at the poor engineering judgement that would enable the author to think that a purely  vertical motion was possible under a simple gravity load. Isn't this man supposed to know kinematics? Isn't he supposed to know what are reasonable modeling assumptions and what are absurd?

Finally, I am amazed that this fallacy has been allowed to persist for so many years and through so many editions of the book. Did no one ever check this problem? Editors? Reviewers? Even the author in preparing a new edition? Wow!!


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