Re: [Phys-l] definitions ... purely operational, or not

[William Robertson <wrobert9@ix.netcom.com>]

I believe that your students understood the concept as you presented  
it in your course. I'm seeing this from a different perspective, which  
is working with teachers who have taken physics in HS and college, and  
are now trying to teach their students. The teachers I work with  
exhibit a great deal of confusion on the issue, and are therefore  
transmitting their confusion to their students. They are faced with a  
curriculum that gives the moon-earth difference to explain mass and  
weight, yet many have learned something conflicting in their physics  
courses. I am not at all worried about students who will continue to  
study physics or whose goal is to "get it" in the one physics course  
they take. The ones who study more physics can adjust to different  
reference frames and different definitions by different authors, and  
it doesn't matter for students who will never deal with the issue  
beyond the course they are taking.

Bill


William C. Robertson, Ph.D.


On Nov 8, 2010, at 8:48 PM, Hugh Haskell wrote:

> At 21:00 -0700 11/08/2010, William Robertson wrote:
>
> > Like it or not, most elementary and middle school students
> > learn the difference between mass and weight as the fact that mass
> > does not change when we go to the moon, but weight does. That only
> > works if weight is defined as the force of gravity acting on an
> > object. If weight changes all the time depending on the viewer's  
> > frame
> > of reference, then all we have is confusion for the students.
>
> That has not been my experience. I taught weight as the reading on
> the bathroom scale for several years and students seemed to get that
> without too much trouble. What they see fairly quickly is that "most
> of the time" what the bathroom scale reads is equal to, or very close
> to mg, but definitely not always, and in fact its common for that not
> to be so. When we would do the accelerating elevator experiment they
> understood quite well that their weight changed when the elevator
> accelerated, and that in that condition, although mg was still a
> force on them, there were other forces that made the scale reading
> different from mg.
>
> They also understood that g was not a constant (although most of the
> time is was pretty close to constant), and so when they go to the
> moon, their weight is different because, even if they are just
> standing on the NASA version of the bathroom scale, g is noticeably
> different from what it is near the earth.
>
> So they know that the gravitational field strength is g (with
> appropriate corrections for things like the earth's rotation), and
> the force of gravity is mg, and that is always true anywhere. g is
> not a constant but depends on what masses are present and how they
> are arranged, just as E is not a constant but is dependent on what
> charges are present and how they are arranged. And the force of
> gravity is just the gravitational analog of the force of electricity.
>
> This can be explained once and for all in just one page of text. I
> found that teaching g as an acceleration led some students to get the
> idea that all accelerations were g, and confusion reigned. When they
> learned that g was not an acceleration and it was not constant,
> things got easier.
>
> Hugh
> --
>
> Hugh Haskell
> mailto:hugh@ieer.org
> mailto:haskellh@verizon.net
>
> It isn't easy being green.
>
> 		--Kermit Lagrenouille
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