Re: A simple problem that puzzles me

Quentin Grady <quentin@xxxxxxxxxxxxxxx> wrote in message
tl5164t769d27tan0nhdgmpbpkoqoajccc@xxxxxxx
G'day G'day Folks,

I provide support for students who have been out of the classroom
for ages often a decade or so or have Asperger's syndrome, dyslexia
and other learning disabilities.

That sounds like 75% of the contributors of this group :-)

Most questions are pretty routine.

Today though I came across a simple problem that puzzled me.

The first part of the question asked them to calculate the volume,
mass and weight of air in a room. No problems there. The weight came
to 418 Newtons. Then the final part asked for the force exerted by
the atmosphere on the floor of the room.

I was tempted to say, "Oh, that has to be equal to the weight of air
in the room."

However the students were also given the atmospheric pressure as
1.01E5 Pa. When combined with the area of the floor this gave a force
of about 14.8E6 Newtons. The numbers don't matter much. Nothing
like it.

What has me puzzled is which calculation gives the force on floor.

Is it F = P x A using atmospheric pressure?

yes, 100%

That seemed the more logical answer to me on reflection.

I'm assuming atmospheric pressure doesn't change significantly
throughout the room, just enough to allow for the weight of air in the
room. It seems that meant the 1.01E5 Pa was only true at ground level.
The pressure really is less at the ceiling enough to allow for the
weight of air in the room. That seems logical enough.

Or is the answer the weight of the air in the room?
I hope not on reflection.

If the weight of the air in the room determines the force and
the pressure on the floor, the force and pressure on the ceiling
and on the walls should be zero. And you'd have a vacuum
neir the ceiling.
Pressure is caused by the motion (mass and velocity) of the air
molecules. Weight is just proportional to mass in this case.


Or is it some combination?
If so what combination?

I told the student I really wasn't sure but I thought I knew some
people who would. Here's hoping I do.

Best wishes,
--
Quentin Grady ^ ^ /

Don't go away - they need you here, and even more so (90%!) on
sci.physics.relativity :-)

Dirk Vdm
.

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