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Pressure in a gas
The molecules of a gas exert a force on every part of the inner wall
of the container.
The pressure is the magnitude F of the
force acting perpendicular to the surface of
an object immersed in the liquid, divided
by the area A on which the force acts
P
F
A
If a small cube is inserted within the
container, the cube experiences forces
acting perpendicular to
each
of its six
faces.
F
F
F
F
F
F
A
Unit: 1 Newton/m
2
= 1 Pascal
Chapter 15 Fluids
Atmospheric pressure
Pressure in water
Atmospheric pressure at sea level is
1.013
10
5
Newton/m
2
= 1 atmosphere.
This is sufficient to crumple a can if the
inside air is pumped out.
1 Newton/m
2
= 1 Pascal
Pressure in water increases by about
1 atmosphere for every 10m
below the
surface.
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Pressure depends on depth and density
Because the outlined volume of fluid is in
equilibrium, the net force acting on it
must vanish:
P
2
P
1
gh
Pressure increases with depth and depends
only
on depth.
P
1
P
2
P
2
A
P
1
A
mg
0
Ah
m
V
m
F
1
=
F
2
=
0
1
2
mg
F
F
The
mass density
is the mass of a given
volume of fluid divided by the volume:
Then we have
hA
m
The pressure at any point in a fluid depends on the
vertical distance h
of the point beneath the surface.
P
A’
= P
B’
P
A
= P
B
Pa
m
s
m
m
kg
Pa
gh
P
5
2
3
3
5
1
10
55
.
1
)
50
.
5
)(
/
8
.
9
)(
/
10
0
.
1
(
10
01
.
1
P
A
= P
B
matter where the point is located horizontally in the fluid.
P
1
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 Fall '09
 yung
 Physics, Force

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