Re: summary of summary of weight

[Richard Tarara <rtarara@SAINTMARYS.EDU>]

One more stab at drawing this together.

Fundamentally (as has been said) because weight is NOT an internationally
defined quantity, we are reasonably free to choose our favorite definition.
There is, however, a huge difference between the W = mg (regardless of
whether g = dv/dt or the net gravitational field) and what the scale reads.
With students moving from one course to another, it will be immensely
confusing for them to move out of a course where the instructor uses the
scale reading into one where the instructor uses mg.  I would say that the
decision has been made (for better or worse) by the textbooks, that the
'standard' definition is W = mg with some disparity about what g really is
in this context, but that disparity is within the range with which one might
expect students to able to deal.

The gulf between the two major camps is easily illustrated with the
elevator.  The scale camp says that the weight is actually changing in the
elevator.  This would encourage many people I know to want to live their
lives in downwards accelerating elevators!  For the mg folks, the weight
remains constant, but the scale pushes with more/less/the same  force as the
weight according to the acceleration of the object on the scale.  If that
object is a person the person feels heavier/lighter/usual because their
SENSATION of weight comes from the upward push of the scale--not the
gravitational attraction of the earth.  IMO, the mg definition is superior
for intro classes because it does force one to look at ALL the forces acting
in a given situation and allows us to easily label one of those forces as
weight, and has a clearly defined source of the force called weight. OTOH,
the scale camp has an advantage in simplicity.

The bottom line, for me, is that:

1)  We need to deal with weight, it is part of our student's vocabulary and
IS a quantity with which they have experience and concern.
2)  The various definitions are all 'correct' and each has advantages and
disadvantages.
3)  The textbooks have essentially 'standardized' the mg definition although
whether g is dv/dt or the net gravitational field does vary between texts.
4)  If one chooses the 'scale reading' definition, then you should be
prepared to deal with the facts that the textbooks don't and that previous
and/or subsequent courses might not.

Rick

----- Original Message -----
From: Joel Rauber <Joel_Rauber@SDSTATE.EDU>
To: <PHYS-L@lists.nau.edu>
Sent: Monday, October 18, 1999 2:02 PM
Subject: summary of summary of weight


> Thanks!!! John and John; i.e. Denker and Mallinckrodt.  Most excellent
> summaries!
>
> Real progress has been made, important clarifications and sharpening of
> arguements.
>
> I hope John Denker, perhaps in collusion with Mallinckrodt, will rewrite
the
> summary in light of the discussions of the summary.
>
> I'm tempted to say there is a 4th camp, the one I resided  in at the
> beginning of the discussion and have now left.
>
> This is the strict or strong, weight is "what the scale" says camp.  In
this
> camp the weight of the object changes when you immerse it in water.  I
have
> left this camp to join Mallinckrodt's camp 2 (Denker's definition 2).
>
> I think there are three sub-camps in camp 2.
>
> sub-camp (a): the strict version of what Denker's #2; i.e. exactly what he
> said, weight depends on reference frame chosen to analyze the problem;
> notice this does *not* necessarily mean the frame of a scale used to weigh
> an object.
>
> sub-camp (b) which Mallinkrodt called camp 3 and as has been noted is
really
> a subset of camp 2; He referred to this as the "what the scale reads"
camp.
> I think this is misnomer; because it is not what the scale reads while
> immersed in water nor is it what the scale reads on the accelerating
> elevator.
>
> sub-camp (c): I think this is where I currently reside. I'd call it the
weak
> form of "what the scale reads".  This is Denker's definition 2, with the
> added proviso that we will preferenctially chose the frame of reference to
> be the frame of reference of the scale that we choose to actually measure
> the "weight" of an object, even if only in a gedanken experiment.  I'm not
> sure if I'm splitting hairs here. But this does make the scale reading on
> the accelerating elevator synonomous with the definition of weight, as
well
> as for the orbiting astronaught; and matches the ordinary meaning of "an
> astronaught is weightless".
>
> Joel Rauber
> Joel_Rauber@sdstate.edu