Re: little gee and its sign

[Tom Ford <tomford@THESCIENCESOURCE.COM>]

At 01:23 PM 9/10/01 -0500, you wrote:
> Hello
> I have a problem with my calc-based physics students.  The problem is=
> over the sign of g, the gravitational acceleration.
>
> I have told them the convention is that it is always downward even if=
> the object is going upward. =20
>
> I can't seem to explain to them why this is.  They tell me the book s=
> ays it is positive.  That is not what the book says, the book says ex=
> actly what I am saying.
>
> How can I make it clearer to them about g and its sign?
>
> Thanks
> Tina

I cannot think of a way of resolving this in students' minds with just a
piece of chalk and discussion. My practice has been to use a wheeled object
on a ramp with a motion probe at the top of the ramp and again with it
(careful with the hands) at the bottom. Looking at the position/time,
velocity/time and acceleration/time graphs forms that basis for discussion.
In my clssroom, only the gross shapes of the curves can be used by teacher
or student to illuminate the discussion. Only after consistent use of the
graphs will equations enter the picture and they will be built up USING the
graphss

I find motion graphs do the best job of instilling and verifying these
concepts. Equations come later and vectors after that --- all built on the
understandings developed from the motion graphs.

We used to do these motion graphs with ticker tape and the like. Students
lost track of the ideas involved because of the calculation burden, even
with calculators. Modern devices probeware) using MBL or CBL cut to the
chase and allow concepts to preceed definitions.

Tom Ford