Re: atmosphere and normal force

[jeff marx <marxj2@OREGON.UOREGON.EDU>]

Interesting point. I never though about it like that. I would argue,
however, that plenty of air is getting between my shoes and the floor.
Looking down at the bottom of my shoes I see a heal, treads, imbedded
rocks, et cetera. All of these things contribute to the air's ability to
get between my shoes and the floor. You'd have to have microscopically
smooth (smooth on the order of the size of a molecule of nitrogen) soled
shoes and a similarly smooth floor before you'd begin to see the effect you
describe.
later,
       jeff

> 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