QUESTIONS
Homework
 Jack load 12.2kN
 Gage pressure: 27.7kPa
SG = 0.8
Examples
 Mercury manometer is connected to
an air duct to measure its insice
pressure. The manometer deflection
is 15mm. Atmospheric pressure is
100kPa. Find the duct’s absolute
pressure. Hg  = 13,600kg/m3.
Manometer
 Liquid flow deflection large;
manometer fluid can be Hg, SG =
13.6
 Gas flow deflection small; manometer
fluid SG <~ 0.9. Use inclined
manometer
Example
 Mott 3.71 Figure 3.36 Gage fluid has
a SG = 0.87 and L = 115mm.  =
15o. Find PA.
Fluid statics
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


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Liquid at rest: F = 0
Horizontal: equal in all directions
Horizontal: dependent on depth
Vertical: pressure prism
P = h
Hydrostatic forces



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•
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Pressure prism
Distributed forces
Force → vector
Direction
Magnitude
Line of action: center of pressure
Fluid statics
 F = hcA magnitude
 hcp = hc + (I/ hcA) center of pressure
Submerged surface location
 Top edge at liquid surface
 Top edge submerged
 Pressurized surface –equivalent depth
of fluid
 Inclined surface
 Curved surface
Example
 A heavy car veers off the road into a
lake. It settles upright on the
bottom. The front car door is 1.2m
high and 1m wide. The top of the
door is 8m below the lake surface.
What is the force holding the door
closed? Can the driver open the
door? What can he do to survive?
Example
 A rectangular gate is installed in a
vertical wall of a reservoir. Find the
magnitude of the resultant force and
the center of pressure. What is the
force of each latch?
Example
 The square gate is eccentrically
pivoted so that it automatically opens
at a certain value of h. What is that
value in terms of l ?
Homework
 A gate for the spillway holds water at
a normal high level mark. The
spillway crest elevation is 2090.0 ft.
The gate span is 30 ft. What is the
force on the two pins holding the gate
in place?
Homework
 The air above the liquid is at 40psig.
The SG of the liquid is 0.8. The
rectangular gate is 1.0m wide; y1 =
1.0m; and y2 = 3m. What force P is
needed to hold gate in place?
Homework
 The inclined wall is 4m wide. Find the
magnitude of the resultant force and
the center of pressure.
Centroid of compound area
 Moment of compound area about any
axis is equal to sum of the moments
of its parts around the same axis
 = (A1 x1 + A2 x2)/ AT
Assignment
 Mott Chapters 3 & 4
References
Images & examples
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•
•
Fluid Mechanics Fundamentals & Applications, 6th Edition,
Cengel & Cimbala, McGraw Hill
Applied Fluid Mechanics, 6th Edition, Mott, Prentice Hall
Engineering Fluid Mechanics, 5th Edition Crowe, &
Roberson, Wiley
Which of the examples
were helpful?
Why?