Re: g

[Rondo JEFFERY <RJEFFERY@WEBER.EDU>]

Tina,
   The value g, which is approximately constant near the earth's surface, is actually the *magnitude* of the gravitational force constant.  It is sometimes written 9.8 N/kg, to emphasize the fact that it is the proportionality constant between gravitational weight and mass.
   I have also observed students getting the sign of g confused with the magnitude.  It is just a number.  But, since it is also associated with the vector weight, and is toward the center of the earth, and since much of the time the direction UP is taken as positive, they frequently see the number g with a minus sign in equations.  You really have to do problems with the plus sign both up and down, such as free fall when an object is dropped, so they can see that the sign depends on the arbitrary choice of the positive direction of motion.  Good luck trying to beat this into them.
Rondo Jeffery
Weber State University
Ogden, UT 84408-2508
rjeffery@weber.edu 




> > > FanettT@QUEST.WITCC.CC.IA.US 09/10/01 12:23PM >>>
> 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
>
> Tina Fanetti
> Physics Instructor
> Western Iowa Technical Community College
> 4647 Stone Ave
> Sioux City IA 51102
> 712-274-8733 ext 1429