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before -
Dear All,
I just noticed something that I can't believe I've never noticed
or that it didn't come up in all my physics learning and teaching.Am I
missing something here?suspended in
The atmosphere exerts a pressure on me of 1 hPa. If I were
the air, that pressure would come from all directions, and the netforce
would be zero. But I'm not usually suspended in air; normally I'mthe air
standing on my feet, with an area of about 0.05 meters. That means
is exerting a net downward force of 5000 N, or about 6 times myweight! In
other words, the Normal force of the ground on me is dominated bygetting in
atmospheric effects, not my mass.
Is this right? I'm trying to think about the idea that air is
between the floor and my shoes, but I'm not sure that I'm convinced ofthing?
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'
scale to
Note: As far as I can think, this is not inconsistant with using a
measure weight, since scales pretty much have to allow air to flow inwith a
underneath you (you are either hanging, or supported by a spring
relatively small contact area).
--
--James McLean
jmclean@chem.ucsd.edu
post doc
UC San Diego, Chemistry