Fluid Mechanics in a Closed
System
Pascal’s Law
states in a closed system, an
increase of pressure at any point will
produce an equal increase in the pressure at
every point in the liquid.
Difference between force and pressure:
Pressure is exerted on a unit area.
Consider a glass flask. Fill it to the top and
insert stopper into the neck. If you hit the top
of a 1 sq in stopper with an 18 lb force, what
pressure is exerted on the 25 sq. in bottom?
Pressure is transmitted through a liquid.
If the system is closed with equal piston area,
we will have equal inputs and equal outputs.
If the system is “U” shaped, the direction of
the force can be changed.
One force down = One force up
The shape of the container has no effect on
the pressure in it. We could connect the
equal piston through a “small hose” and
move our equal force anywhere we want it.
Suppose the pistons have different areas.
Then the forces are different and the
distance moved are different.
If we connect two pistons in a closed system,
the distance moved is inversely proportional
to their areas. Volume remains constant.
A
B
C
D
Force
500 #
25 #
Area
20 sq in
30 sq in
60 sq in
Pressure
Length
6 in
Volume
A
B
C
D
E
F
Force
11200#
13440 #
Area
5 sq in
12sq in
10sq in
Pressure
1600
psi
2240 psi
Length
12 in
Volume
51.4 cu
in
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