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