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

[Hugh Haskell <haskellh@verizon.net>]

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