Re: definition of weight (again)
Well, Rick, I found that it was fine even
after my students opened their books and read some other definitions of
weight. They easily understood the difference between weight and Earth's
gravitational force for an object in free fall and for an object sitting on a
table.. They've never had a problem with my definition of weight,
regardless of what they read in books or on the internet.
Paul O. Johnson
----- Original Message -----
Sent: Thursday, February 07, 2002 3:28
PM
Subject: Re: definition of weight
(again)
> Which is fine until they open a book that deals with free fall and
labels
> the downward force of the earth on the free fall object as
WEIGHT, or draws
> a free body diagram of the book on the table and labels
the downwards force
> as WEIGHT. Since what the scale _actually_
reads is the upwards force of
> the scale on the object, and this is most
often labeled as F-normal and not
> WEIGHT, we have another point of
conflict. So, unless the text is very
> scrupulous about the names
and labels of forces, there is as much, if not
> more, potential confusion
with this definition as with 'the net
> gravitational force' type of
definition--IMO.
>
> Rick
>
> ----- Original Message
-----
> From: "Paul O. Johnson" <pojhome@SWBELL.NET>
> To:
<PHYS-L@lists.nau.edu>
> Sent:
Thursday, February 07, 2002 4:14 PM
> Subject: Re: definition of weight
(again)
>
>
> > I've always taught my students that your
weight is what the scale reads
> that
> > you're standing on.
This works whether you're accelerating in an elevator,
> > in orbit
around the earth, standing still on the earth's surface, or
> >
anyplace else I can think of.
> >
> > Paul O. Johnson
>
> .
> > ----- Original Message -----
> > From: "Larry
Smith" <larry.smith@SNOW.EDU>
> >
To: <PHYS-L@lists.nau.edu>
> >
Sent: Thursday, February 07, 2002 12:14 PM
> > Subject: definition of
weight (again)
> >
> >
> > > This debate seems to
resurface every year (or is it every semester?),
> but
> > >
it would be nice if we could all agree on the definition of weight.
> >
>
> > > Here's what Hewitt says on page 159 of Conceptual Physics
(9e): "In
> > > Chapters 2 and 4 we defined weight as the force due
to gravity on a
> body,
> > > mg. Your weight does have
the value mg if you're not accelerating. To
> be
> > >
more general we now refine this definition and say that the weight of
>
> > something is the force it exerts against a supporting floor or
a
> weighing
> > > scale. According to this definition
you are as heavy as you feel."
> > >
> > > Kirkpatric
and Wheeler (4e) generally agree with Hewitt's _revised_
> > >
definition, with the caveat that they say weight is the support force
>
> > itself (the "reaction" force of Hewitt's weight, equal, of course,
in
> > > magnitude, but opposite in direction).
> >
>
> > > But Serway and Beicher (5e) say on page 119: "the weight
of an object,
> > > being defined as the magnitude of F_g, is
mg." Other than being a
> scalar,
> > > Serway's
definition agrees with Hewitt's original "unrefined"
>
definition.
> > >
> > > Hobson, in Physics: Concepts and
Connections, p. 99, seems to agree with
> > > Serway but keeps the
vector: "The weight of an object refers to the net
> > >
gravitational force exerted on the object by all other objects."
> >
Griffith
> > > (The Physics of Everyday Phenomena p. 64) agrees that
it is a vector:
> "The
> > > force of gravity acting on an
object is what physicists commonly refer
> to
> > > as the
weight of the object."
> > >
> > > This issue is
important enough by itself, but it also affects how we
> talk
> >
> about weightlessness and the definition of g.
> > >
>
> > I guess I'm looking for a little closure on this, folks. Is
there any
> to
> > > be had? Consensus, please.
>
> >
> > > Thanks,
> > > Larry