Re: Moment of Inertia or Rotational Inertia

["Prof. John P. Ertel" <jpe@NADN.NAVY.MIL>]

As I look  back in my notes and some texts over the years, it seems that
the two equivalent names "rotational inertia" and "moment of inertia"
have coexisted, more or less, peacefully for over thirty years.

Most of those with a more mathematical bent will probably like to call
it "moment of inertia" due to its connection to the 1st and 2nd moment
calculations that we all try to teach our students.

The 1st moment is the center of mass (and a close relative to the centroid):
        M(total) Rcm(vector) = Integral[R(vector) * dm]
or in one dimension
        M(total) Rcm = Integral[R * dm]

The 2nd moment is or "rotational analogue" of the rectilinear concept of
inertial mass:
        M(total) R^2 = Integral[R^2 * dm]


For all but the physics major, these distinctions are just more "blah
blah".  But for me, I like both terms at different times.  When I'm
trying to emphasize the "rotational analogue" aspect, I prefer to use
the term "rotational inertia".  When I'm trying to show a thread of
logic in the 1st moment (average or centroid) and the 2nd moment, I
prefer the term "moment of inertia"


Robert Carlson wrote:
> I've been writing up some documentation for rotational motion and have always
> referred to mr^2 as rotational inertia.  When looking through Tipler, Serway,
> Simon, Hibbeler, and Beer & Johnston, mr^2 is referred to as moment of
> inertia.  I've taught from these texts and not given it a second thought
> until now.  Wondering where I got the term rotational inertia, I went back to
> my first physics text, Fundamentals of Physics by Halliday & Resnick.  Sure
> enough, there it was, rotational inertia.  I then looked at the fourth
> edition of Physics by Resnick, Halliday & Krane copyright 1992 and there it
> was again, rotational inertia.  Does anyone know why there is this split in
> terminology?  Is one more dated than another?
>
> Thanks,
>
> Bob Carlson

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