Being careful with dimensions (was Re: i,j,k things)

[John Mallinckrodt <ajmallinckro@CSUPOMONA.EDU>]

On Tue, 21 Sep 1999, Timothy Folkerts wrote:

> My problem with this is that I don't like the expression "F=2i-3j" that has
> been used as an example throughout this thread.  In fact I hate it.  I
> would always write "F = (2N)i - (3N)j".
>
> Am I in the minority???

I hope not; I'm 110% with you on this one.  "2i-3j" is no more a force
than "65" is a speed or "9.8" the magnitude of a gravitational field
strength.  When a lab student tells me that he or she determined the
rotational inertia of a wheel that we use in one of our experiments to be
2.4 x 10^5 and asks me if that seems reasonable my answer is always,
"Nope; '2.4 x 10^5' is a *number*, not a rotational inertia."

Moreover, I think texts set a HORRENDOUSLY bad example when they write
things like, "The position of a ham sandwich is given by x = 3 - 5t + 2t^2,
find its speed at t = 5 s."

O.K., I know this probably qualifies as a pet peeve and that most pet
peeves are at least borderline overreactions, but this one really does
push my buttons.

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