Phy 2053 Announcements
Exam 2 Grade Distribution
Results from Exam 2 are in
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Average: 12.58
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Standard deviation: 3.59
Grades are available on UF e-Learning web site
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Exam 2 solutions have been posted:
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http://www.phys.ufl.edu/courses/phy2053/spring09/webpage/
midterm2/index.html
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Still waiting to check IDs for a few students; please come by
my office during office hours
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You have until next Tuesday to look at your test and bubble
sheets
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HW Assignment 9 due this Wednesday, April 8
Pressure and Density
Elastic Moduli
Young’s
Modulus:
F
ΔL
=Y
A
Lo
Sheer
Modulus:
Bulk
Modulus:
Pressure:
force exerted by a fluid on a
submerged object at any
point if perpendicular to the
surface of the object
ρ ≡
m
V
Density:
Pressure vs depth
ΔV
ΔP = −B
V
Pressure and Depth
equation
Liquid in a U- tube
Patm
Patm
Po is normal
atmospheric
pressure
1.013 x 105 Pa
h1
h2
The pressure does
not depend upon
the shape of the
container
P = Patm + ρgh1
P = Patm + ρgh2
1
Pressure Measurements:
Barometer
Liquid in a U- tube
Patm
Patm
Invented by Torricelli
(1608 – 1647)
A long closed tube is
filled with mercury
and inverted in a
dish of mercury
h1
h1 = h2
h2
P = Patm + ρgh1
P = Patm + ρgh2
Buoyant Force (will it
float?)
The magnitude of the buoyant
force always equals the weight of
the displaced fluid
B = ρ fluidVfluid g = wfluid
The forces
balance
ρobj
ρ fluid
=
Vfluid
Vobj
The buoyant force is the same for
a totally submerged object of any
size, shape, or density
Totally Submerged Object
Floating Object
If the object is less
dense than the fluid,
the object
experiences a net
upward force
If the object is more
dense than the fluid
the net force is
downward
Totally Submerged Object
B = ρfluidVfluid g = wfluid
mg = ρobjectVobjectg
For completely
submerged object:
Vobject = Vfluid
If ρobject < ρfluid, mg < B
If ρobject > ρfluid, mg > B
2
Example – P9.43
43. A 1.00-kg beaker containing 2.00 kg of oil (density = 916
kg/m3) rests on a scale. A 2.00-kg block of iron (density =
7.86 × 103 kg/m3) is suspended from a spring scale and is
completely submerged in the oil (Fig. P9.39). Find the
equilibrium readings of both scales.
Equation of Continuity
• A1v1 = A2v2
• The product of the
cross-sectional area of
a pipe and the fluid
speed is a constant
– Speed is high where the
pipe is narrow and speed
is low where the pipe has
a large diameter
• Av is called the flow
rate
Bernoulli’s Equation
• States that the sum of the
pressure, kinetic energy
per unit volume, and the
potential energy per unit
volume has the same
value at all points along a
streamline
P+
1 2
ρv + ρgy = constant
2
Just due to energy conservation
Applications of Bernoulli’s
Principle: Venturi Tube
• Shows fluid flowing
through a horizontal
constricted pipe
• Speed changes as
diameter changes
• Can be used to
measure the speed of
the fluid flow
• Swiftly moving fluids
exert less pressure than
do slowly moving fluids
P+
1 2
ρv + ρgy = constant
2
Application – Airplane Wing
• The air speed above the
wing is greater than the
speed below
• The air pressure above the
wing is less than the air
pressure below
• There is a net upward
force
– Called lift
• Other factors are also
involved
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