Some Rules about Hydrostatics
(ME 33 Handout, prepared by Professor J. M. Cimbala)
•
For hydrostatics, pressure can be found from the simple equation,
below
above
P
P
g
z
ρ
=
+
∆
•
There are several “rules” that directly result from the above equation:
1.
If you can draw a continuous line through the
same
fluid from
point
1
to point
2
, then P
1
= P
2
if
z
1
= z
2
.
E.g., consider the oddly shaped container in the sketch. By this rule,
P
1
=
P
2
and
P
4
=
P
5
since these points are at the same elevation in the same fluid.
However,
P
2
does not equal
P
3
even though they are at the same elevation,
because one cannot draw a line connecting these points through the
same
fluid.
In fact,
P
2
is
less than
P
3
since mercury is denser than water.
2.
Any free surface open to the atmosphere has atmospheric pressure, P
atm
.
(This rule holds not only for hydrostatics, by the way, but for
any
free surface exposed to the
atmosphere, whether that surface is moving, stationary, flat, or curved.)
Consider the hydrostatics
example of a container of water. The little upsidedown triangle indicates a free surface, and means
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 Fall '05
 CIMBALA
 Statics, Atmosphere, Hydrostatics, hydrostatics equation

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