Re: What is weight? (was Re: Internal or external?)

[Richard Tarara <rtarara@SAINTMARYS.EDU>]

----- Original Message -----
From: John Mallinckrodt <ajmallinckro@CSUPOMONA.EDU>
> I really don't think this is such a novel idea.  It is, after all, why we
> say the astronauts are weightless; it is why we feel lighter or heavier
> at certain points on a roller coaster; it is why objects have weight in
> a rotating space station, etc.
I'll wade in _again_ (for this round of 'what is weight'?) at this point.
The proponents (myself included) of weight being mg...the gravitational
attraction of the most dominant nearby massive object--usually the earth or
moon in intro courses...would say that the astronauts _are not weightless.
In fact, at their average altitude, they have about 90% of their surface
weight.  There must be a force causing their centripetal acceleration, and
that force IS the gravitational attraction of the earth, which we (at least
my camp) call weight.

The 'apparent' weightlessness of the astronauts, the 'feeling' of weight in
a rotating space station or the elevator accelerating in outer space, are
all explained by the way we EXPERIENCE forces.  For the most part, we
experience them BACKWARDS.  Our sensation of weight is due to the floor or
seat pushing up on us.  When we take off in a jet we FEEL pushed BACK into
the seat, in rotating systems we feel pushed out from the center.  We are so
accustomed to experiencing these forces backwards that when ONLY the force
that is the source of our sensation is present, we will still FEEL 'normal'.
Thus in the space station or accelerating rocket (elevator) with only the
push of the floor or wall present, as long as that push is of the same
magnitude that we would experience at rest, on earth, we feel as though we
have our normal weight.  In the absence of the 'reverse' force, we feel
weightless (free fall, shuttle, etc.).

I understand the other viewpoint, and perhaps it IS superior for Physics
majors, but mostly I'm dealing with our 'liberal arts' class {students who
can't reliably calculate the frictional force for a moving 1000 N box on a
flat surface where mu_sub_k is 0.25--or else must write out the whole 3rd
grade multiplication process to get the answer.)  Using the viewpoint above,
we can analyze THEIR experiences in cars, planes, elevators, and especially
amusement park rides and arrive at some useful and 'simple' conclusions.  I
like to use the Ferris Wheel (one without safety bars) to explore both the
actual sources of the centripetal acceleration and the causes for their
sensations on the ride--lighter near the top (coming down) because their
weight is greater than the upwards push of the chair (and they experience
their weight backwards), heavier near the bottom because the chair bottom
pushes up with more force than their weight, firmly pushed into the chair at
one side since the back of the chair is providing the needed centripetal
force, and ready to slip out of their seat on the other since only friction
can supply the needed force there (and the normal force--the seat bottom--is
less than at rest).

Rick

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Richard W. Tarara
Department of Chemistry & Physics
Notre Dame, IN 46556
219-284-4664
rtarara@saintmarys.edu

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