Let us now look at the properties inherent in a fluid.
A fluid differs from a
solid in that it cannot support a shear stress.
In this sense, both liquids and
gases can be described as fluids (in fact, under certain extreme conditions, even
solids exhibit fluidlike properties).
The
density
of a fluid is defined as its mass per unit volume
ρ
=
m
V
(18.1)
Density is considered to be inherent to the fluid and can be used to characterize
it.
The
specific gravity
of a fluid is the ratio of its density to that of water.
It is a
dimensionless number.
Since the density of water is defined to be 1 gm/cm
3
, the
specific gravity of a fluid is the density without the units, if the density was also
given in gm/cm
3
.
Pressure
When a body is submerged in a fluid, the fluid exerts a force perpendicular
to the surface of the body at each point on the surface.
However, for a fluid, the
force does not necessarily need to be a constant over the whole surface.
This
leads us to a more refined definition of pressure.
We define the pressure p at a
point in a fluid as the ratio of the normal force
dF
on a small area
dA
around that
point, to the area
p
dF
dA
=
(18.2)
From this definition we can immediately see that pressure also has units of
Pascals.
The pressure due to a fluid tends to compress a body.
The ratio of the
pressure to the fractional decrease in volume is called the
bulk modulus
B
p
V
V
= 
∆
The inverse of the bulk modulus is called the
compressibility
,
k
.
Variation of Pressure with Depth
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 Spring '09
 Knott
 Physics, mechanics

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