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[Phys-L] Units, gratuitous complication - and memories



On page 32 of the Second Edition (1949) of Synge and Griffith "Principles of Mechanics" we are introduced to Newton's "Law of Motion":

"Relative to a basic frame of reference a particle of mass m, subject to a force P, moves in accordance with the equation

P = k m f ,

where f is the acceleration of the particle and k is a universal positive constant, the value of which depends only on the choice of units of force, mass, and time."

I have left out indications of italics and bolding, but you can guess where that is done. This was the textbook for my third year mechanics course, Physics 105A, at Cal. (It helps if you think about "Phorce" and "fastering" to interpret this equation.)

This was not an engineering course, it was the mainstream physics majors' course.

The first formula* I ever memorized, back in the forties, was:

0.224 K A
C = ----------- (n-1)
d

I learned it so well that I still retain it, and my memory is not getting better. I lose many more important items every day. Memory is a strange phenomenon. Much useless stuff is retained all too well, like advertizing jingles and wartime slogans.

I do remember being taught F = k m a at some point in my education, probably at Sacramento Junior College. My teacher, Clarence Nash, one of the two best teachers** I ever had for a physics course, immediately provided us with the caveat that this was an engineering convention. Our textbook was Sears & Zemansky, and we worked in slugs and pounds, and Mr. Nash taught us about poundals as well. All of this gave me a distaste for multiple unit systems and a feeling that conversion between systems of units was an important part of physics. You can imagine how I managed the transition to Berkeley after two years. I stayed there for graduate school and took an E & M course from Robert Karplus. He insisted on using rationalized cgs units! My research supervisor, Mike Tinkham, preferred (irrational) cgs, which he called "God's own units". I didn't really recover from this unit confusion until I started teaching at SFU 1966. I gladly converged on something close to SI, and I spared my students the unnecessary burden of doing conversions, with the exception of temperature (that was in the old Fahrenheit days).

Nothing really bad has happened to confuse students since I moved to Canada. I was jolted somewhat when the definition of mass changed and there was no longer such a thing as "relativistic mass". I've decided that was a useful reform, however, and I am OK with it now. Sorta gets us full circle, however, back to

F = k m a

Leigh

* Clue: this formula appears in the Amateur Radio Operator's Handbook.

** The other was Luis Alvarez.