Re: [Phys-L] the sign of g

[stefan jeglinski <jeglin@4pi.com>]

Yeah I do all that, but they keep making the same mistake (well not all 
of them of course). My subtle variation of the discussion gets their 
attention because they haven't heard it before so they become more 
actively engaged (that is, argumentative). But it doesn't go as far as I 
would like, somehow. I suppose that it's one of those things that can 
only be presented in one way, and it's up to them to "get" it.


Stefan Jeglinski




On 5/9/16 11:34 AM, Carl Mungan wrote:
> g is the magnitude of the gravitational field vector and thus it is positive, as is a (the magnitude of the acceleration vector) and v (the speed).
>
> Assuming we indicate vertical with the y-axis, then a_y can be either +g or -g depending on whether you choose upward or downward to be the +y direction. I recommend doing about half of your in-class example problems with each of these choices, so students get used to the idea that both choices are viable and sensible. -Carl
>
> > On May 9, 2016, at 11:28 AM, stefan jeglinski <jeglin@4pi.com> wrote:
> >
> > This slays large populations of students on that first exam, and seems to be a serious mental block for them: whether g = +9.8 or -9.8 (units suppressed, substitute the English version over metric if you like). I teach that the sign can't be determined unless a coordinate system is defined, which is a separate but critical step in setting up any problem, but they like to rush. Many will inadvertently (or with intent) define a coordinate system (e.g., up is positive), which naturally works the signs into the algebra, but then at the end, will say "well g is always -9.8" and introduce a sign error when they get out their calculators.
> >
> > I've taken to teaching that g=+9.8 or g=-9.8 is the incorrect way to think about it. Rather, g has merely a value of 9.8, and the sign is an "artificiality" that has nothing to do with g per se. The pushback I get is that "9.8 is the same as +9.8" and I push back in return on that but to skeptical looks.
> >
> > My question is: is there a good mathematical argument I can cite (aside from a coordinate system) for why +9.8 and 9.8 are not the same thing? Or am I myself wrong?
> >
> >
> > Stefan Jeglinski
> >
> >
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