Re: [Phys-l] A question about the Earth's gravity

["John Clement" <clement@hal-pc.org>]

But in an introductory course we must take the point of view of Newtonian
mechanics until student have mastered it.  Mixing in general relativity will
only confuse the students.

There is research that shows that when you calculate the "gravitational"
force by using g quoted as an acceleration, that students are more confused.
Even if you tell them at 1N/kg = 1m/s^2 they still confused by using
Newton's second law to calculate the force on something that is not
accelerating.  As has been pointed out the factor for calculating the
gravitational force actually comes from the general gravitation law.

In AP courses students are also suffering from confusion over these topics,
as has been demonstrated by testing using the FCI/FMCE type of evaluations.
Actually such testing has also revealed that teachers often suffer from
similar confusion.

I realize that many do not agree, but the research shows that using the
equation F_g = m g where g is 9.8m/s^2 is very confusing.  But that using
g=9.8 N/kg is less confusing.  Also the latter usage when modeled by a lab
where the students derive the equation helps the students acquire
proportional reasoning.  This is very important to later progress.  Remember
that even 5 years of rigorous physics education in China produced students
with exactly the same reasoning ability as US students with on the average
only 1 year of physics.

So while it may be reasonable for a PhD physicist to consider the measured g
to be a measurement of gravitational force, an AP course must separate out
the effect of the rotation from the classical Newtonian gravitational force.
Actually the conventional non AP course should not usually consider this
effect in detail unless it is brought up by a student in class.

Modeling solves the problem of classical vs modern interpretation by always
carefully promoting the idea that the current model can and will usually be
modified as more evidence is accumulated.  So Newtonian mechanics is just
the model that is being studied in class at that point of time.  So you can
not mix in features of other conflicting models.  Other models can be
mentioned, but not used in the Newtonian context.  Students need to
initially learn to work within one model at a time.

John M. Clement
Houston, TX

> > But the question is about the Earth's gravity.  Properly that is
> > the force
> > that the Earth exerts on the body.
>
> That's "proper" in Newtonian physics, but we know better now and have
> for nearly a hundred years.
>
> I'm not saying that we shouldn't teach the antiquated, but still
> useful concept that bodies attract each other by virtue of their mass
> and separation, but we shouldn't actively work against eventual
> better understanding by pretending that centrifugal effects are
> merely unfortunate "corrections" to "real" gravitational forces.
> Indeed, the modern perspective is simple, practical, and easily
> explained: The "gravitational force" in *any* reference frame is m
> times the local value of g and the way you find the local value of g
> is by dropping an object and measuring its acceleration.
>
> John Mallinckrodt
> Cal Poly Pomona
>
> _______________________________________________
> Forum for Physics Educators
> Phys-l@carnot.physics.buffalo.edu
> https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l