Re: Weightlessness

[Hugh Haskell <hhaskell@MINDSPRING.COM>]

At 8:55 -0400 5/20/01, Eugene Mosca wrote:

> The letter and answer below came from Marilyn vos Savant's column in today's
> Parade Magazine. I thought some of you might find Marilyn's answer as
> edifying as I do.
>
> > Earth's gravity holds spaceships in orbit, but the things inside them are
> > weightless and float around. Why doesn't gravity have an effect inside?
> > --Jerry Mapes, city unknown
> >
> > Actually, it does--the same way it affects the spacecraft itself. When a
> > space shuttle is orbiting the Earth, the sum of the "downward"
> > (gravitational) force and the "forward'' (inertial) force of the moving ship
> > and its contents nearly equals zero. So both the ship and its contents are
> > in freefall, which makes everything weightless. They stay in orbit while
> > "falling'' (being pulled toward the Earth) because the inertial force
> > (centrifugal force, in this case) of the moving vehicle is radial--away from
> > the Earth. A similar principle applies to the planets: All are in glorious
> > free-fall while orbiting within the gravitational field of the Sun.

Gene, I trust your comment above is meant facetiously. As I suspect
you did, I found her answer to be so muddled, that I'm sure the
original questioner is still scratching his head in confusion.,  As I
read the     her inscrutable answer breakfast coffee this  morning, I
couldn't help but comment to my wife that  the e-mail and web site
that involve Marilyn's wisdom on all topics scientific would be
lighting up by now. I appears I wasn't wrong.

Aside from the fact that she confuses inertia with a force and then
imputes two different directions to it simultaneously, the confusion
apparent in her answer points up that we still haven't dealt with the
issue of "weight" very well in the body of physics. We've visited
this issue before on this list, but I don't think we have come to any
definitive conclusion and I'd like to toss out the definition I have
used for several years, which seems to me to be as unambiguous as it
is likely to get.

Ever since NASA replaced "weightless" with "microgravity," and before
as well, but certainly since, students have been convinced that there
is no gravity in outer space. "Weightless" implies it and
"microgravity" practically shouts it from the minarets. And in trying
to explain how gravity acting on everything can still lead to
"weightlessness" clearly got Marilyn hopelessly confused. So my
definition of "weight" is simply the force being applied to an object
to keep it from falling, in other words, just what a bathroom scale
would read if the object was placed in contact with the object and
between it and the direction it would go if there was no other
restraint. That allows you to be heavier or lighter depending on your
particular circumstances, riding in an elevator, jumping off a diving
board, riding in an acrobatic aircraft or an amusement park ride, or
anywhere else where you might be, relative to your immediate
surroundings. So the astronauts are "weightless" because their
surroundings are being subjected to the same acceleration that they
are, just as anyone would be in a freely falling elevator closer to
the earth. In other words, put the bathroom scale between the object
and the center of the local gravitational force and whatever it reads
is the object's "weight."

Of course for decades or longer, physics texts have defined the
quantity "mg" as the weight of an object. I propose to abandon this
misleading definition, and rename mg to be just the gravitational
force on the object, just as we define qE to be the electrical force
on a charged object. For beginning students, mg is relatively new, so
this shouldn't create onerous problems for them, but the idea of
weight being defined as what the bathroom scale reads is what they
have grown up with, so it shouldn't be too hard a concept to grasp.
Of course there will be confusion, since most of the time mg and
weight as I have defined it here will be the same thing, but it seems
to me that this is a confusion that is relatively easily dealt with
by putting the students on an elevator with a bathroom scale.

As I see it, the important thing here is that students will not be
confused, as the questioner in Marilyn's column, and Marilyn too,
were, about how someone can be under the influence of gravity and
still be weightless.

Comments?

Hugh
--

Hugh Haskell
<mailto://haskell@ncssm.edu>
<mailto://hhaskell@mindspring.com>

(919) 467-7610

Let's face it. People use a Mac because they want to, Windows because they
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