Re: [Phys-l] g...

["Richard Tarara" <rbtarara@sprynet.com>]

> [Original Message]
> From: John Denker <jsd@av8n.com>
>
> On 11/18/2006 06:43 PM, Anthony Lapinski wrote in part:
>
> > Call it whatever you want, but the force of Earth's gravity is the
oldest
> > known force, 
>
> And that's relevant because ... why, exactly?

IF (and that's a big IF) you are introducing students to kinematics early
in the curriculum, then the one big limitation of the kinematics equations
as usually presented is that they only work as algebraic equations IF the
acceleration remains constant.  When one tries then to tie this back to
student experience you have a hard time finding realistic situations where
the acceleration is constant.  Free-fall then becomes a prime choice for
such examples although we all know that air resistance quickly makes the
acceleration non-constant in reality.  However, the old 'ignoring
air-resistance' seems to work pretty well with most students.  OTOH, when I
teach this I don't let students write 'g' for the acceleration.  Rather, as
they are identifying the 5 variable is the kinematics equations, they label
the acceleration as 'a' and give it a value of 9.8 (usually just 10 for the
sake of simplifying computations--especially in class) m/s^2.  {They can
add a negative sign ONLY if they have chosen UP as the positive direction
for the problem.}

Now the problem with mg and students identifying 'g' as GRAVITY (that more
often than the force of gravity in my experience) is real.  But from a 2nd
law point of view, if we drop an object of mass 'm' near the earth's
surface, it will accelerate at that 9.8 m/s^2 which (for better or worse)
has been given the shorthand identifier 'g_earth' or more commonly 'g'. 
But we learn that a = netF/m  or net F = ma  and in free fall there is only
one force, that of the earth on the object.  We then tend to call that
force due to the earth (the gravitational force of the earth) WEIGHT. 
OK--we disagree (as per numerous threads) whether weight is the net force
towards the center of the earth or just the gravitational component--but
the general idea is the same.  I really don't like any of the alternatives
that have been offered.  I would go with just beating to death that 'g' is
the symbol for the acceleration due to gravity, and mg is a convenient way
to calculate GM_earthM_object/R^2.  {One strong objection to an earlier
post that suggested gravity was not a force because there wasn't a two-body
interaction--there is, Earth pulls down on object, object pulls up on
earth..but then maybe I misunderstood that post.}

Rick (who has tried this year to have my gen-ed students draw time/motion
free-body diagrams of very simple motions--but after 6 straight quizzes
with only the top students 'getting it' I may punt!)