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Re: atmosphere and normal force



Why doesn't this effect show up in scales where the load is supported
ON a spring instead of FROM a spring. A standard bathroom scale
shouldn't function properly if the effect you're describing is really
happening.

I'm not sure if I agree with Jeff Marx's explanation that enough air
gets nder your shoes to eliminate the effect. Very smooth soled
shoes, especially a really worn down gum rubber for instance, on a
smooth floor would seem to make as good a seal as some suction cups
I've seen. These might not be ideal suction cups, but they certainly
support enough pressure differential to create a noticeable effect.

Is there some explanation hiding in the fact that our bodies are
pressurized to one atmosphere (or some fraction thereof depending on
out location) and exert a constant outward force of 1 hPa? I don't
have the answer.

Best,
Zach Wolff
University of Arizona


---James Mclean <jmclean@CHEM.UCSD.EDU> wrote:

Dear All,

I just noticed something that I can't believe I've never noticed
before -
or that it didn't come up in all my physics learning and teaching.
Am I
missing something here?

The atmosphere exerts a pressure on me of 1 hPa. If I were
suspended in
the air, that pressure would come from all directions, and the net
force
would be zero. But I'm not usually suspended in air; normally I'm
standing on my feet, with an area of about 0.05 meters. That means
the air
is exerting a net downward force of 5000 N, or about 6 times my
weight! In
other words, the Normal force of the ground on me is dominated by
atmospheric effects, not my mass.

Is this right? I'm trying to think about the idea that air is
getting in
between the floor and my shoes, but I'm not sure that I'm convinced of
that.
Everybody likes to use suction cups to demonstrate how
strong atmospheric pressure is, but that may be overkill.
Are all those diagrams we like to draw with N=mg bogus?
What does this do to the whole 'friction is independent of area'
thing?

Note: As far as I can think, this is not inconsistant with using a
scale to
measure weight, since scales pretty much have to allow air to flow in
underneath you (you are either hanging, or supported by a spring
with a
relatively small contact area).

--
--James McLean
jmclean@chem.ucsd.edu
post doc
UC San Diego, Chemistry


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