Re: "acceleration due to gravity"

["John M. Clement" <clement@HAL-PC.ORG>]

The equation F=mg is calculating the gravitational force on a given mass at
the surface of the earth, while F=ma is Ntn's 2 law.  Unfortunately the
majority of texts state that g is the gravitatonal acceleration, while it is
nothing of the kind.  g = G m_earth/r_earth^2 ignoring the complications of
the Earth's rotation.  The first year physics student especially the HS
student, does not understand g in the standard text because obviously the
object is not accelerating.  Under the more rational treatment g is then
stated as being 9.8N/kg at the surface of the earth, not 9.8m/s^2.  The
gravitational acceleration is still 9.8m/s^2, but that is not the same thing
as g.

The review of HS physics texts in The Physics Teacher mentioned this
problem.  It was one of the few points that they made that had a firm
research based pedagogical basis.  Using the correct formulation reduces
student confusion.  If nothing else standard texts could at least be clear,
as they offer little else that helps students dispel their alternate
conceptions.  The research based Minds on Physics Series by Leonard et al.
uses the rational formulation, and cautions the teacher to refer to g as the
gravitational constant not gravitational acceleration.

 From the point of teaching the first year student I would not discuss any
further complications.

John M. Clement
St. Pius X HS, Houston, TX




> Kindly help me understand the difference between
>         F=ma and F=mg.
>  In the first equation  "a" stands for any
> acceleration regardless of the force or mass
> that may be involved?   In the second equation
> "g" stands for the acceleration due to a
> gravitational force resulting from a position
> of a mass in a gravitational field?