At 12:00 AM 5/2/01 +0000, Alberto Monteiro wrote:
>Charlie Bell wrote:
> >
> >> Well, it's not _free fall_, because the resistance of the air
> >> slows him down.
> >
> >Still free fall. If you're falling without an aid designed to arrest that
> >fall, it's free fall. Falling at terminal velocity in air is still free
> >fall.
Not exactly. Assume you are in an elevator in a v-e-r-y tall building,
standing on a set of bathroom scales. The cable holding the elevator car
breaks. If all the air were evacuated from the elevator shaft, you would
indeed fall freely, constantly accelerating until you hit the bottom, and
the scale would register your weight as zero all the way down. However, if
there is the normal amount of air in the shaft (and assuming enough room
between the walls of the car and the sides of the shaft that air can flow
freely by the car), air resistance would slow your acceleration to
something less than g, so the scales below you would register _some_ weight.
> >
>I thought free fall was a fall that could be computed by just
>gravitational forces...
>
> >Free fall in orbit is only different because you're missing the
> >ground.... (Very Douglas Adams thought there...).
> >
>:-)))))))
I've tried using that in class, but few of today's students have read HHG,
so all I get are blank looks. I do a better job of getting across the idea
of free fall by jumping off a lab table holding a clear plastic cup whose
contents "float" during the approximately .4 second it takes me to fall to
the floor. Of course, it's a little hard on the knees, since I try to land
without rolling and spilling the cup . . .
A more useful illustration (i.e., one that many of the students actually
get) is found in the movie _Goldeneye_: the giant antenna of the "Soviet
radar site in Cuba" is a duplicate of the Arecibo radio
telescope. Although I do point out that the real one is _not_ filled up
with water so it looks like a lake to reconnaissance satellites when not in
use . . .
-- Ronn! :)
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-- Ronn Blankenship
Instructor of Astronomy/Planetary Science
University of Montevallo
Montevallo, AL
Standard Disclaimer: Unless specifically stated
otherwise, any opinions stated herein are the personal
opinions of the author and do not represent the
official position of the University of Montevallo.
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