Re: Weight

[igal@vms.huji.ac.il (Igal Galili)]

Dear colleagues,
        Just a comment.  For years I have been advocating the change of the
gravitational definition of weight (gravitational force) in favor of its
operational definition: the result of standard weighing - the force applied
on the support.  A number of papers were published and presentations given.
Not much changed, but I am optimistic.  I am convinced that it is a matter
of time (may be even a generation of physics teachers) that physicists,
after everybody else, will accept that in a satellite, there is a state of
weightlessness a true one and not "AS IF weightlessness".  Can we teach it,
say in a hundred years?
        May be in a small country, like mine, the transition can be much
less painful.  I am working on this and there are some changes observed by
now in minds of physics educators here.  I know few US college-university
physics textbooks which define weight non-gravitationally (e.g. A. French
in MIT series).  The rest of them, the great majority, remain with Newton,
meanwhile.  As far as I know gravitational weight is the only way to teach
in UK , Australia, South Africa, Western Europe (?)  I think,  Russia is
practicing an operational definition.  I am writing this message as I was
positively surprised to discover views, similar to mine, on this subject of
weight in some contributors to this important forum.  Since 1992 I am
constantly advocating operational weight definition in my talks at AAPT
summer meetings.  Some support in the beginning from Mario Iona was
important.  We see that the story is not finished.

Igal.


> David wrote and Marlow cheered
> > BTW, since gravitational forces (locally) are not "real" but are artifacts of
> > the use of a noninertial frame used to describe the physics, I prefer *not*
> > to define the concept of "weight" as (the magnitude of) the gravitational
> > force on an object.  Such a definition makes an object's weight depend on
> the  coordinate system of the observer.  I prefer to define weight as what
> others define as "apparent weight" which is simply the magnitude of the
> > *non*gravitational force of support on a body which prevents a free fall
> > state (or equivalently, which deflects the motion from a free fall state).
> > This definition of weight is independent of the frame of the observer and
> > agrees with our usual sensations of weight.
>
> I too define weight in this fashion, although for simpler reasons than
> talking about non-inertial frames, and free-fall frames etc.
>
> Simply put, if you define weight to be the gravitational force acting on an
> object; how do you explain why we say astronoughts are weigtless while
> orbiting the earth, since they still have a gravitational force acting on
> them.
>
> Therefore defining weight as the magnitude of the non-gravitational force
> present balancing the gravitational force on the scale used, relieves one
> from the above conundrum and allows one to consistantly understand the term
> weightless up on the space-shuttle
>
> Joel