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Re: Weightlessness

Unfortunately this is typical of many "popular" explanations of physical
concepts. I have heard astronauts at NASA giving similar explanations so
this is not at all unusual. A better explanation is often difficult to make
within a few words, without causing more misconceptions in the reader. I
will attempt one:

Actually the earth does pull on the things inside a spaceship in the same
way that it pulls on the spacecraft. The people inside the craft do not
notice the pull because they are accelerating toward earth at the same rate
as the spacecraft. Essentialy the spacecraft and people are in free fall.
Since this is the only significant force on the people and spacecraft, why
does it stay in orbit? The spacecraft is moving forward around the earth,
so by the time it has moved "downward" enough to enter the atmosphere, it
has also moved to the side around the earth's curve. A similar priciple
governs the motion of all planets: All are in glorious free-fall while
orbiting within the gravitational field of the Sun.

I think that some of the expressions she used are quite good, and could be
retained. I suspect that she looked up an article in an encyclopedia and
paraphrased what she thought was the explanation. I would like to look at
her source. My explanation follows a rule that I use in class. NEVER use
the word centrifugal, or centripetal for that matter. When students hear it
they key on the idea that there is a centrifugal force. I ask them where is
the centrifugal that produces this force? They must identify all forces as
interaction where one object produces a force on the other. While the
centrifugal pseudoforce is a valid construction for accelerating reference
frames, this is a distinction which totally excapes most beginning physics
students. If pressed I explain that centrifugal is just a word that
identifies the direction of the force. For example when you swing a ball on
a string the outward tension exerted on the hand by the string is
centrifugal, but the inward tension exerted on the ball by the string is
centripetal. I strongly feel that the word centrifugal should never be used
outside more advanced physics courses, and that we should challenge most
popular uses of this term. Some people dislike "heat", but I dislike
"centrifugal".

Unfortuantely the explation published in the column is typical of many
supposedly scientific writing. I have seen question in texts that imply
that inertia is a force. When children ask why things are in orbit, I
suspect that most teachers tell then about the centrifugal force. I saw a
vocabulary lesson in an English workbook that used the word centrifugal
incorrectly in a physics sense. I just looked up the definition of
centrifugal force in the "World Book" encyclopedia. It is a technically
very correct explanation, but it is pedagocially very inappropriate. Most
readers will not be able to make the distinction between a rotating and
non-rotating reference frame. As a result they will key into the "you could
say that cenrifugal force pulls you away from the center" as a good
explanation. A better explanation would be to say that "you feel as if a
centrifugal...". I would also take out the "flying into space" and replace
it by "continuing along a straight line". One way of helping readers
overcome misconceptions is to always use precise terms which do not conjure
up the wrong images. Eminent physicists can write wonderful explanations,
but their phrasing can often conjure up misconceptions in the average
reader. There is essentially an impedance mismatch between the writer and
the reader since each is coming in with a different set of assumptions.

Perhaps a well known physicist such as John Hubisz should draft a letter
giving the correct explanation on AAPT letterhead stationery. I suspect the
author has already had a number of letters and E-mails by this point in
time.

John M. Clement
Houston, TX



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 ³downward"
(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.


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