Module 1, Lecture 6: Hydrostatics
Pressure
Density
Mass density:  
m
V
Weight density:  
mg
V
p
F
A
• ______________ pressure: 101.3 kPa = 14.7 psi
Specific Gravity:
Ratio of object’s density to the density of _________
• ______________ pressure: pressure relative to atmospheric pressure
• ______________ pressure: pressure above a vacuum
Absolute pressure = ?
More descriptive name: ___________________
RD 

Pressure in Fluid at Rest
What makes an
object sink?
• Pressure same in all _______________
• Pressure acts ________________ to any solid surface it touches
Water:
ρ = 1000 kg/m3 = 1 g/cm3 = 1.94 slug/ft3
γ = 9810 N/m3 = 62.4 lb/ft3
EF 152 2016 Lecture 1-6
SI unit:
pascal (Pa) = 1N/m2
Pressure variation with depth
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p  gd  p0
EF 152 2016 Lecture 1-6
Examples
Fluid Pressure Distribution
Hoover Dam holds back a height of 576 ft of water. What
is the gauge pressure at the base?
Consider a slice that has constant hydrostatic
pressure.
2
y
dA
dF  pdA  gydA
How would the pressure at the base change if Lake Mead
were only 1 ft long instead of 110 miles long?
F   dF  g  ydA 
What is the height of mercury in the column?
(ρ = 13.6x103 kg/m3)
•
Four 50 ft diameter diversion tunnels built
during construction of Hoover Dam.
•
Closed after construction by bulkhead gates.
•
Force on the gates if the bottom of the tunnels
are at the elevation of the bottom of the lake?
How would the height change if Hg was
replaced by H20?
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Buoyancy Force
Buoyancy Concepts
Does a 12 lb bowling ball float or sink?
F1
d2
d1
Net force exerted by fluid:
_____________
d2-d1
Fnet  F2  F1
radius = 4.25 in
F2
Archimedes’ principle: the buoyant
force on an object immersed in a fluid
is equal to the weight of the fluid
displaced by that object.
How does a concrete canoe float?
Correct equation to determine acceleration?
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Example: Surfboard
Example: Crown of gold
A surfboard has a volume of 3.0 ft3 and can just support a 180
lb surfer (board is just below the water). What is the weight
density of the board?
A crown has a mass of 14.7 kg. When submerged in
water and held up by a spring scale, the scale reads
131 N. Is the crown made of gold?
EF 152 2016 Lecture 1-6
What does the buoyant force equal?
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