Re: [Phys-l] Weight?

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

The big resource to their thinking is MOP Minds on Physics as it embodies
how they do things.  Actually the use g=10 N/kg in the early MOP pages,
later on 9.81 enters.  I do not recall them explicitly equating them
numerically, but I think they do have some questions which bring out this
relationship.

The use of 10 is actually very clever because students have been taught F=mg
where g=9.8 m/s^2 in previous courses, usually IPC.  Using 10 N/kg
essentially decouples this memorized information from the "new" way of
thinking.  It also makes mental calculations possible, and I always
encourage students to do mental work rather than always grab a calculator.

I do not know where the g thing appears in their writings, but it does
appear emphatically in their teacher's manual where they strongly say DO NOT
use g=9.8m/s^2 because it confuses the students.  If I have time I have them
do a simple experiment using a spring scale and some masses to find that the
gravitational force = mg.  And they usually come up with g=10.

Typical of many PER groups they have a large body of data and observations
that have not been published.  This may be one of them.

Thank you for looking at their website.  If we all looked at the evidence,
perhaps we might all come to better agreement.

John M. Clement
Houston, TX

> Isn't UMPERG's comments related more to the units to use?
>
> I couldn't quickly find the spot on their web pages and we have you as a
> resource;  does UMPERG equate numerically g and a_g?
>
> _________________________
>
> Joel Rauber, Ph.D
> Professor and Head of Physics
> Department of Physics
> South Dakota State University
> Brookings, SD 57007
> Joel.Rauber@sdstate.edu
> 605.688.5428 (w)
> 605.688.5878 (fax)
>
> > -----Original Message-----
> > From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-
> > bounces@carnot.physics.buffalo.edu] On Behalf Of John Clement
> > Sent: Wednesday, November 17, 2010 1:02 PM
> > To: 'Forum for Physics Educators'
> > Subject: Re: [Phys-l] Weight?
> >
> > According to the UMPERG group g should be defined as 9.81 N/kg on the
> > surface of the Earth if it is used in the equation F_g=mg.  This is
> > because
> > they have found that g=9.81 m/s^2 is extremely confusing to students.  I
> > can
> > remember thinking as a student it is stupid to calculate F_g = m x 9.81
> > m/s^2 for an object which is at rest because it is not accelerating.
> > Also
> > unit conversions are often just incantations.  So the proposed
> > formulation
> > makes more sense to students.  The idea that N=kg m/s^2 is a very
> > foreign
> > incantation to them.
> >
> > I would use a_g to notate the free fall acceleration.  Again this is
> > inline
> > with the idea that all accelerations should be notated a, all forces F,
> > all
> > energies E, with appropriate subscripts.  The separate symbols for the
> > same
> > type of quantity make students think they are different things, and
> > serve as
> > a barrier to integrated thinking.
> >
> > I am not claiming it is "truth" or "correct" merely that research points
> > to
> > it being pedagogically better.  You are free to disagree, but if you
> > want
> > evidence go to the UMPERG web page and their various papers.
> >
> > The big problem is that the vast majority of intro physics students in
> > an
> > algebra based course do not have proportional thinking.  So many things
> > become incantations rather than understood concepts.  Many of them
> > exhibit
> > the characteristics of concrete operational thinking so multiple steps
> > are
> > extremely hard.  Three variable equations such as a=F_net/m are opaque
> > and
> > have no meaning to them.  Shayer & Adey's "Really Raising Standards" has
> > good information on this problem.
> >
> > All too often the debate is really about what "is is".
> >
> > John M. Clement
> > Houston, TX
> >
> >
> > > Quick question:  You say that mg is the pull of gravity, I'm gathering
> > > from what you said below you mean the integrated contribution of all
> > mass
> > > elements constituting the earth.  Does that mean that g is not the
> > free-
> > > fall acceleration of an object?
> >
> >
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