Re: Newton's 3rd law? was Re: inertial forces (definition)

[John Mallinckrodt <ajmallinckro@CSUPOMONA.EDU>]

On Sun, 24 Oct 1999, JACK L. URETSKY wrote:

> "Equals" can mean either identity (mathematically, "of the same class as")
> or numerical equality of non-identical quantities.  Your little political
> joke puts a strained meaning on "is" which, as a form of "be" indicates
> identity - but then perhaps that is what you are recognizing.
>         The confusion of the two meanings of "equals" is very common among
> students, which is why I belabor the point.

Arons speaks about these two as well as other different meanings of the
"equals sign" in his "Guide to Introductory Physics Teaching." The
confusion caused in the case of "Centripetal force = mv^2/r" is, I think,
pretty well acknowledged by most teachers.  Perhaps less so is the case of
"impulse = delta p" and "work = delta K."  In these cases the words
impulse and work are important and useful; they can't and shouldn't be
avoided.  But students don't often grasp the important difference between
writing "W = Fd" or "I = Ft" (which are *merely* definitions) and writing
"W = delta K" or "I = delta p" (which are important expressions of
physical relationships between separately defined quantities).  Students
will be a step or two ahead if they understand that they are not doing
physics when they use the former equations.  And lest anyone misinterpret
me here, I certainly don't mean to imply that the former equations should
not be used!

John Mallinckrodt      mailto:ajm@csupomona.edu
Cal Poly Pomona          http://www.csupomona.edu/~ajm