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Fluid
Flow Table of Contents
The pressure of the liquids in each of the
previously cited cases has been due to the weight of the
liquid. Liquid pressures may also result from application of
external forces on the liquid. Consider
the following examples. Figure 2 represents a container
completely filled with liquid. A,
B, C, D, and E represent pistons of equal cross-sectional
areas fitted into the walls of the vessel.
There will be forces acting on the pistons C, D, and E due to
the pressures caused by the
different depths of the liquid. Assume that the forces on the
pistons due to the pressure caused
by the weight of the liquid are as follows: A = 0 lbf, B = 0
lbf, C = 10 lbf, D = 30 lbf, and
E = 25 lbf. Now let an external force of 50 lbf be applied to
piston A. This external force will
cause the pressure at all points in the container to increase
by the same amount. Since the pistons
all have the same cross-sectional area, the increase in
pressure will result in the forces on
the pistons all increasing by 50 lbf. So if an external force
of 50 lbf is applied to piston A, the
force exerted by the fluid on the other pistons will now be
as follows: B = 50 lbf, C = 60 lbf,
D = 80 lbf, and E = 75 lbf.
This effect of
an external force on a confined fluid was first stated by
Pascal in 1653.
Pressure applied to a confined fluid is
transmitted undiminished throughout the confining vessel of
the system.
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