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LIFE IN A FLUID
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http://www.physics.usyd.edu.au/teach_res/jp/fluids09
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Ian Cooper
LecturesTutorialsLabsDuty TutorAssignmentsExamTextbookWorked examples
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Identify Setup Execute Evaluate
IDENTIFYIdentify what the question askingIdentify the known and unknown physical quantities (units) Identify the type of problem
SETUP need a good knowledge base (memory + understanding)Visualise the physical situation Diagrams - reference frames / coordination system / origin / directions Write down key concepts, principles, equations, assumptions that maybe needed to answer the question
EXECUTEAnswer to the question from what you know. Numerical questions - solve before calculations - manipulate equations then substitute numbers add comments.
EVALUATECHECK - answer reasonable, assumptions, units, signs, significantfigures, look at limiting cases
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mV
Definition ( Greek letter rho)
density of object
m mass of objectV volume of object
UnitsSI unit kg.m-3
Other units g.mL-1
1 mL = 1 cm3 = (10-2)3 m3 = 10-6 m3
1 g = 10-3 kg 1 g.mL-1 = 103 kg.m-3
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Density values (kg.m-3)
Interstellar space 10-20
Best laboratory vacuum 10-17
Air (20 °C and 1 atm) 1.21Styrofoam 100
Ice 917
Water (20 °C and 1 atm) 998
SaltWater 1024
Whole blood 1060
Earth (average) ~5500
Iron 7900
Mercury 13 600
Osmium (most dense found on earth)
22 500
Uranium nucleus 1017
Neutron star (core) 1018
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… and all the king's horses …
What force is required to separate the hemispheres? Is this force significant?
?What physical quantities do we need to consider?
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Pressure P
pressure !!!Is this pressure?
What pressure is applied to the ground if a person stood on one heel?
? What physical quantities do we need to consider?
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Pressure P
FpA
F force at right angles to surface A area of contact over which force acts
A
F
Units
pressure p Pa pascal 1 Pa = 1 N.m-2
force F N newton 1 N = 1 kg.m.s-2
area A m2
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How large is atmospheric pressure?
Rough calculation
p = F / A
p = m g / A
p = V g / A
p = A h g / A
p = h g
p = (1)(104)(10) Pa
p = 105 Pa
Pressure at Earth’s surface due to weight of atmosphere
air = 1.21 kg.m-3
g = 9.8 m.s-2
h ~ 10 km
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Atmospheric Pressure patm
1.013x105 Pa
101.3 kPa
1013 hPa (hecto)
1013 mb (millibar)
1.013 b (bar)
760 mmHg
14.7 lb.in2 (14.7 psi)
10.34 mH20
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? What does it mean when we are told to inflate a tyre to 100 kPa?
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What does it mean when we are told to inflate a tyre to 100 kPa?
Answer Pressure gauges measure the pressure above and below atmospheric (or barometric) pressure. Need to distinguish between a gauge pressure and absolute pressure.
patm = 101 kPa
pgauge = 100 kPa pabsolute = (101 + 100) kPa = 201 kPa pabsolute = pgauge + patm
0
100
200
300
4000
100
200
300
400
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? How does a gas or liquid exert a pressure on its container walls?
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How does a gas or liquid exert a pressure on its container walls?
Answer
Impact of a molecule on the wall of the container exerts a force on the wall and the wall exerts a force on the molecule. Many impacts occur each second and the total average force per unit area is called the pressure.
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patm = 1x105 Pa
R = m
A = 2 R2
p = F / A
F = p A
F = (1x105)()( )2 N
F = x10 N
70 kg person HOW MANY PEOPLE? N = F / Fperson ~
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pressure !!!
F = ? N
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m = 70 kg g = 10 m.s-2
FG = 700 N
A = 10 mm x 10 mm
A = (10x10-3)(10x10-3) m2
A = 10-4 m2
p = F / A
p = (9700 / 10-4) Pa
p = 700 x 104 Pa
p = 7 x 106 Pa
p / patm = 7x106 / 1x105 atm = 70 atm
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Why is it better is use a sharp knife?
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FLUID PRESSURE p
The oarfish may be the creature that gave rise to the legends about "sea serpents". This long, skinny fish, with its strange-looking head covered in protrusions and reaching lengths perhaps as long as 15 m,
Life Life in the deep sea must adapt to unique conditions of low or no light, high pressure, low energy- except at hot vents (superheated temperatures). One unexplained but fairly common feature of deep animals is gigantism (such as oarfish, the giant squid, etc.). Less than 1% of the deep-sea has been explored. What might be left undiscovered?
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Provided that an object im mersed in a fluid is not moving, the force on each small part of the contact surface is perpendicular to the surface so the interaction can be described completely in terms of pressure. Fluid pressure always acts perpendicular to any surface irrespective of how the surface is oriented. If there is relative motion between the fluid and solid object, the force has components parallel to the surface and the interaction is not simply described by the pressure. Pressure has no intrinsic direction of its own (pressure is a scalar).
t t
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How long can your snorkel be?
Why do your ears feel different when you dive into deep water?
Why does atmospheric pressure change as you go up a mountain?
How does a barometer work?
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Pressure increases with depth in a fluid
p0 pressure acting on surface
h
Liquid – uniform density
A
Weight of columnof liquid
F
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pressure at depth h = pressure acting on surface + pressure due to height of
liquid ph = p0 + F / A
F = weight of column liquid of cross sectional area A F = m g where m of liquid in column m = V = (A h) F / A = g h ph = p0 + g h (equation only applies for static fluids)
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Snorkelling
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200 mm
ph = patm + g h
p = ph – patm
p = g h
p = (1030)(9.8)(0.2) Pa
p = 2x103 Pa
if h > 200 mm p too large, lungs collapse
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A scuba diver's lungs can ruptureif he doesn't breathe while surfacing?
Why?
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You dive deep down into the water.
Does it matter whether it is in the ocean or a swimming pool?
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You dive deep down into the water. Does it matter whether it is in the ocean or a swimming pool?
! The pressure within a uniform stationary fluid is the same at all points in the same horizontal plane.
hh
The pressure exerted by a static fluid depends only upon the depth of the fluid, the density of the fluid, and the acceleration of gravity
ph = p0 + g h
Static pressure does not depend upon mass or surface area of liquid and the shape of container due to pressure exerted by walls.