View
218
Download
1
Category
Tags:
Preview:
Citation preview
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Conceptual Physics Fundamentals
Chapter 7:
FLUID MECHANICS
1
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
This lecture will help you understand:
Density Pressure Pressure in a Liquid Buoyancy in a Liquid Archimedes’ Principle Pressure in a Gas Atmospheric Pressure Pascal’s Principle Buoyancy in a Gas Bernoulli’s Principle
2
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Fluid Mechanics
“Men never do evil so cheerfully and completely as when they do so from religious conviction.”
—Blaise Pascal
3
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Density
Density importantproperty of materials (solids, liquids,
gases) measure of compactness of how much mass an
object occupies “lightness” or “heaviness” of materials of the
same size
4
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Density
in equation form:
units: grams or kilograms
volume: cm3 or m3
example: density of mercury is 13.6 g/cm3, so mercury has 13.6 times as much mass as an equal volume of water (density 1 g/cm3)
density = massvolume
5
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
The volume of 1000 kg of water is _______________.
A. 1 liter
B. slightly less than 1 liter
C. more than 1 liter
D. None of the above
DensityCHECK YOUR NEIGHBOR
6
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
The volume of 1000 kg of water is _______________.
A. 1 liter
B. slightly less than 1 liter
C. more than 1 liter
D. None of the above
DensityCHECK YOUR ANSWER
7
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Density
Weight density in equation form:
example: density of salt water is 64 lb/ft3, more dense
than fresh water (density 62.4 lb/ft3)
weight density = weightvolume
8
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Which of these has the greatest density?
A. 100 kg of lead
B. 100 kg of water
C. Both are the same
D. None of the above
DensityCHECK YOUR NEIGHBOR
9
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Which of these has the greatest density?
A. 100 kg of lead
B. 100 kg of water
C. Both are the same
D. None of the above
Explanation:
They have the same mass and weight, but any amount of lead is more dense than any amount of water.
DensityCHECK YOUR ANSWER
10
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
When water freezes, it expands. This tells us that the density of ice is _______________.
A. less than the density of water
B. equal to the density of water
C. more than the density of water
D. None of the above
DensityCHECK YOUR NEIGHBOR
11
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
When water freezes, it expands. This tells us that the density of ice is _______________
A. less than the density of water
B. equal to the density of water
C. more than the density of water
D. None of the above
DensityCHECK YOUR ANSWER
12
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Compared with the weight of water, the weight of a liter of ice is _______________.
A. more
B. less
C. the same
D. None of the above
DensityCHECK YOUR NEIGHBOR
13
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Compared with the weight of water, the weight of a liter of ice is _______________.
A. more
B. less
C. the same
D. None of the above
Explanation
When a liter of water freezes, its volume increases. To get a liter of ice you’d have to shave part of it off to be the same size. So there is less water in a liter of ice.
DensityCHECK YOUR ANSWER
14
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Pressure
the force per unit area that one object exerts on another
in equation form: depends on area over which force is
distributed
units: N/m2, lb/ft2, or Pa (Pascals)
pressure = forcearea
15
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Pressure
example: The teacher between nails is unharmed because force is applied over many nails. Combined surface area of the nails results in a tolerable pressure that does not puncture the skin.
16
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
When you stand on one foot instead of two, the force you exert on the floor is __________.
A. less
B. the same
C. more
D. None of the above
PressureCHECK YOUR NEIGHBOR
17
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
When you stand on one foot instead of two, the force you exert on the floor is ___________.
A. less
B. the same
C. more
D. None of the above
Explanation:
Distinguish between force and pressure!
PressureCHECK YOUR ANSWER
18
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
When you stand on one foot instead of two, the pressure you exert on the floor is _____________.
A. less
B. the same
C. more
D. None of the above
PressureCHECK YOUR NEIGHBOR
19
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
When you stand on one foot instead of two, the pressure you exert on the floor is _____________.
A. less
B. the same
C. more
D. None of the above
Explanation:
Twice as much, in fact!
PressureCHECK YOUR ANSWER
20
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Pressure in a Liquid force per unit area that a liquid exerts on an
object depth dependent and not volume dependent
example: swim twice as deep, then twice as much weight of water above you produces as much pressure on you
21
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Pressure in a Liquid
acts equally in all directions
example: o your ears feel the same amount of pressure under
water no matter how you tip your heado bottom of a boat is pushed upward by water pressureo pressure acts upward when pushing a beach ball
under water
22
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Pressure in a Liquid
independent of shape of container
whatever the shape of a container, pressure at any particular depth is the same
in equation form:
liquid pressure = weight density depth
23
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Water pressure provided by a water tower is greater if the tower __________.
A. is taller
B. holds more water
C. Both A and B
D. None of the above
Pressure in a LiquidCHECK YOUR NEIGHBOR
24
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Water pressure provided by a water tower is greater if the tower ____________.
A. is taller
B. holds more water
C. Both A and B
D. None of the above
Explanation:
Only depth, not amount of water, contributes to pressure.
Pressure in a LiquidCHECK YOUR ANSWER
25
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Pressure in a Liquid
Effects of water pressure acts perpendicular to surfaces
of a container
liquid spurts at right angles from a hole in the surface curving downward– The greater the depth, the greater
the exiting speed
26
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Buoyancy in a Liquid
Buoyancy apparent loss of weight of a submerged object amount equals the weight of water displaced
27
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Buoyancy in a Liquid
Displacement rule: A completely submerged object always displaces a volume of liquid equal to its own volume.
example: Place a stone in a container that is brim- full of water, and the amount of water overflow equals the volume of the stone
28
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
A cook who measures a specific amount of butter by placing it in a measuring cup with water in it is using the ___________.
A. principle of buoyancy
B. displacement rule
C. concept of density
D. All of the above
Buoyancy in a LiquidCHECK YOUR NEIGHBOR
29
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
A cook who measures a specific amount of butter by placing it in a measuring cup with water in it is using the __________.
A. principle of buoyancy
B. displacement rule
C. concept of density
D. All of the above
Buoyancy in a LiquidCHECK YOUR ANSWER
30
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Buoyancy in a Liquid
Buoyant force net upward force that a fluid
exerts on an immersed object = weight of water displaced
example: the difference in the upward and downward
forces acting on the submerged block is the same at any depth
31
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
How many forces act on a submerged body at rest in a fluid?
A. One—buoyancy
B. Two—buoyancy and the force due to gravity
C. None—in accord with the equilibrium rule, F = 0
D. None of the above
Buoyancy in a LiquidCHECK YOUR NEIGHBOR
32
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
How many forces act on a submerged body at rest in a fluid?
A. One—buoyancy
B. Two—buoyancy and the force due to gravity
C. None—in accord with the equilibrium rule, F = 0
D. None of the above
Buoyancy in a LiquidCHECK YOUR ANSWER
33
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Buoyancy in a Liquid
Sink or float? sink when weight of submerged object is greater
than the buoyant force neither sink nor float when weight of a
submerged object is equal to buoyant force—object will remain at any level
float when weight of submerged object is less than the buoyant force it would have when submerged—when floating, buoyant force = weight of floating object
34
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Archimedes’ Principle
Archimedes’ Principle discovered by Greek scientist Archimedes relates buoyancy to displaced liquid states that an immersed body (completely or
partially) is buoyed up by a force equal to the weight of the fluid it displaces
applies to gases and liquids
35
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Archimedes’ PrincipleApparent weight of a submerged object weight out of water—buoyant force
example: If a 3-kg block submerged in water apparently “weighs” 2 kg, then the buoyant force or weight of water displaced is 1 kg.
36
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
On which of these blocks submerged in water is the buoyant force greatest?
A. 1 kg of lead
B. 1 kg of aluminum
C. 1 kg of uranium
D. All the same
Archimedes’ PrincipleCHECK YOUR NEIGHBOR
37
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
On which of these blocks submerged in water is the buoyant force greatest?
A. 1 kg of lead
B. 1 kg of aluminum
C. 1 kg of uranium
D. all the same
Explanation:
The largest block is the aluminum one. It displaces more water and therefore experiences the greatest buoyant force.
Archimedes’ PrincipleCHECK YOUR ANSWER
38
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
When a fish expands its air bladder, the density of the fish ________________.
A. decreases
B. increases
C. remains the same
D. None of the above
Archimedes’ PrincipleCHECK YOUR NEIGHBOR
39
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
When a fish expands its air bladder, the density of the fish _____________.
A. decreases
B. increases
C. remains the same
D. None of the above
Archimedes’ PrincipleCHECK YOUR ANSWER
40
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
When a fish makes itself less dense, the buoyant force on it ______________.
A. decreases
B. increases
C. remains the same
D. None of the above
Archimedes’ PrincipleCHECK YOUR NEIGHBOR
41
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
When a fish makes itself less dense, the buoyant force on it ______________.
A. decreases
B. increases
C. remains the same
D. None of the above
Archimedes’ PrincipleCHECK YOUR ANSWER
42
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
When a fish decreases the size of its air bladder, the density of the fish ___________.
A. decreases
B. increases
C. remains the same
D. None of the above
Archimedes’ PrincipleCHECK YOUR NEIGHBOR
43
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
When a fish decreases the size of its air bladder, the density of the fish _____________.
A. decreases
B. increases
C. remains the same
D. None of the above
Archimedes’ PrincipleCHECK YOUR ANSWER
44
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
When a submarine takes water into its ballast tanks, its density __________.
A. decreases
B. increases
C. remains the same
D. None of the above
Archimedes’ PrincipleCHECK YOUR NEIGHBOR
45
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
When a submarine takes water into its ballast tanks, its density ______________.
A. decreases
B. increases
C. remains the same
D. None of the above
Archimedes’ PrincipleCHECK YOUR ANSWER
46
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
When a submerged submarine expels water from its ballast tanks, its density __________.
A. decreases
B. increases
C. remains the same
D. None of the above
Archimedes’ PrincipleCHECK YOUR NEIGHBOR
47
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
When a submerged submarine expels water from its ballast tanks, its density __________.
A. decreases
B. increases
C. remains the same
D. None of the above
Explanation:
This is how a submerged submarine is able to surface.
Archimedes’ PrincipleCHECK YOUR ANSWER
48
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Archimedes’ PrincipleFlotation Principle of flotation
o a floating object displaces a weight of fluid equal to its own weightexample: A solid iron 1-ton block may displace 1/8 ton of water
and sink. The same 1 ton of iron in a bowl shape displaces a greater volume of water—the
greater buoyant force allows it to float.
49
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
The reason a person finds it easier to float in salt water, compared with fresh water, is that in salt water __________.
A. the buoyant force is greater
B. a person feels less heavy
C. Neither of these
Archimedes’ PrincipleCHECK YOUR NEIGHBOR
50
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
The reason a person finds it easier to float in salt water, compared with fresh water, is that in salt water __________.
A. the buoyant force is greater
B. a person feels less heavy
C. Neither of these
Explanation:
A floating person has the same buoyant force whatever the density of water. A person floats higher because a smaller volume of the denser salt water is displaced.
Archimedes’ PrincipleCHECK YOUR ANSWER
51
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
On a boat ride, the skipper gives you a life preserver filled with lead pellets. When he sees the skeptical look on your face, he says that you’ll experience a greater buoyant force if you fall overboard than your friends who wear Styrofoam-filled preservers.
A. He apparently doesn’t know his physics.
B. He is correct.
Archimedes’ PrincipleCHECK YOUR NEIGHBOR
52
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
On a boat ride, the skipper gives you a life preserver filled with lead pellets. When he sees the skeptical look on your face, he says that you’ll experience a greater buoyant force if you fall overboard than your friends who wear Styrofoam-filled preservers.
A. He apparently doesn’t know his physics.B. He is correct.
Explanation:He’s correct, but what he doesn’t tell you is you’ll drown! Your life preserver will submerge and displace more water than those of your friends who float at the surface. Although the buoyant force on you will be greater, the net force downward is greater still!
Archimedes’ PrincipleCHECK YOUR ANSWER
53
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Archimedes’ Principle
Denser fluids will exert a greater buoyant force on a body than less dense fluids of the same volume.
example: ship will float higher in salt water (density = 1.03 g/cm3) than in fresh water (density = 1.00 g/cm3)
54
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Archimedes’ Principle
applies in airo The more air an object displaces, the greater the
buoyant force on it.o If an object displaces its weight, it
hovers at a constant altitude,o If an object displaces less air, it
descends,
55
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
As you sit in class, is there a buoyant force acting on you?
A. No, as evidenced by an absence of lift
B. Yes, due to displacement of air
Archimedes’ PrincipleCHECK YOUR NEIGHBOR
56
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
As you sit in class, is there a buoyant force acting on you?
A. No, as evidenced by an absence of lift
B. Yes, due to displacement of air
Explanation:
There is a buoyant force on you due to air displacement, but much less than your weight.
Archimedes’ PrincipleCHECK YOUR ANSWER
57
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Pressure in a Gas
greater distance between molecules in a gas than in a liquid
molecules in a gas are free from the cohesive forces
Molecular motion of gas exerts pressure against a container.
For large volumes of gas, gravitational forces limit size and determine shape.
58
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Pressure in a GasRelationship between pressure and density gas pressure is proportional to density
example: o Air pressure and air density inside
an inflated tire are greater than the atmospheric pressure and density outside.
o twice as many molecules in the same volume air density doubled
o for molecules moving at the same speed (same temperature), collisions are doubled pressure doubled
59
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Pressure in a Gas
Double density of air by doubling the amount of air decreasing the volume to half
60
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Pressure in a Gas
Boyle’s Law relationship between pressure and volume for ideal
gases an ideal gas is one in which intermolecular forces play
no role states that pressure volume is a constant for a given
mass of confined gas regardless of changes in pressure or volume (with temperature remaining unchanged)
pressure volume = constant means that
P1V1 = P2V2
61
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
When you squeeze a party balloon to 0.8 its volume, the pressure in the balloon __________.
A. is 0.8 its former pressure
B. remains the same if you squeeze it slowly
C. is 1.25 times greater
D. is 8 times greater
Pressure in a GasCHECK YOUR NEIGHBOR
62
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
When you squeeze a party balloon to 0.8 its volume, the pressure in the balloon __________.
A. is 0.8 its former pressure
B. remains the same if you squeeze it slowly
C. is 1.25 times greater
D. is 8 times greater
Explanation:
Boyle’s law, sweet and simple: P(1.0 V) = 1.25 P(0.8 V).
Pressure in a GasCHECK YOUR ANSWER
63
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Earth’s AtmosphereAtmosphere ocean of air exerts pressure
The Magdeburg-hemispheres demonstration in 1654 by Otto von Guericke showed the large magnitude of atmosphere’s pressure.
64
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Atmospheric Pressure
Atmospheric pressure caused by weight of air varies from one locality to
another not uniform measurements are used to
predict weather conditions
65
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Atmospheric Pressure
Pressure exerted against bodies immersed in the atmosphere result from the weight of air pressing from above.
at sea level is 101 kilopascals(101 kPa)
weight of air pressing down on 1 m2 at sea level ~ 100,000 N, so atmospheric pressure is ~ 105 N/m2
66
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Atmospheric Pressure Pressure at the bottom of a column of air reaching to the top of
the atmosphere is the same as the pressure at the bottom of a column of water 10.3 m high.
Consequence: The highest the atmosphere can push water up into a vacuum pump is 10.3 m.
Mechanical pumps that don’t depend on atmospheric pressure don’t have the 10.3-m limit.
67
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Atmospheric Pressure
Barometer device to measure atmospheric pressure also determines elevation
Aneroid barometer small portable instrument that measures atmospheric pressure calibrated for altitude, then an altimeter
68
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
The maximum height to which water can be drunk through a straw __________.
A. is 10.3 mB. is about 76 cm C. has no limitD. None of the above
Atmospheric PressureCHECK YOUR NEIGHBOR
69
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
The maximum height to which water can be drunk through a straw __________.
A. is 10.3 m
B. is about 76 cm
C. has no limit
D. None of the above
Explanation:
However strong your lungs may be, or whatever device you use to make a vacuum in the straw, at sea level, the water could not be pushed up by the atmosphere higher than 10.3 m.
Atmospheric PressureCHECK YOUR ANSWER
70
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Pascal’s Principle
Pascal’s principle discovered by Blaise Pascal, a scientist and theologian in
the 17th century states that a change in pressure at any point in an enclosed
fluid at rest is transmitted undiminished to all points in the fluid
applies to all fluids—gasesand liquids
71
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Pascal’s Principle application in hydraulic press
example: o pressure applied to the left piston is transmitted to the
right pistono a 10-kg load on small piston (left) lifts a load of 500 kg
on large piston (right)
72
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Pascal’s Principle application for gases and liquids
o seen in everyday hydraulic devices used in construction
o in auto lifts in service stations• Increased air pressure produced by an air
compressor is transmitted through the air to the surface of oil in an
underground reservoir.The oil transmits thepressure to the piston,which lifts the auto.
73
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
In a hydraulic device, it is impossible for the __________.
A. output piston to move farther than the input piston
B. force output to exceed the force input
C. output piston’s speed to exceed the input piston’s speed
D. energy output to exceed energy input
Pascal’s PrincipleCHECK YOUR NEIGHBOR
74
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
In a hydraulic device, it is impossible for the __________.
A. output piston to move farther than the input piston
B. force output to exceed the force input
C. output piston’s speed to exceed the input piston’s speed
D. energy output to exceed energy input
Pascal’s PrincipleCHECK YOUR ANSWER
75
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Buoyancy in a Gas
Archimedes’ principle applies to air as well as liquids.
states that an object surrounded by air is buoyed up by a force equal to the weight of the air displaced
76
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
A dirigible that hovers in air __________.
A. displaces its volume of air
B. displaces its weight of air
C. Both A and B
D. Neither A nor B
Buoyancy in GasCHECK YOUR NEIGHBOR
77
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
A dirigible that hovers in air __________.
A. displaces its volume of air
B. displaces its weight of air
C. Both A and B
D. Neither A nor B
Explanation:
Like a fish in water, both volume and weight of a fluid are displaced.
Buoyancy in GasCHECK YOUR ANSWER
78
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Buoyancy in Gas
Rule for “lighter-than-air” objects: When the weight of air displaced by an object is
greater than the weight of the object, it rises. When the weight of air displaced by an object
equals the weight of the object, it hovers in air. When the weight of air displaced by an object is
less than the weight of the object, it is not supported by air.
79
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Buoyancy in Gas
Gas-filled balloons gas prevents atmosphere from collapsing them best buoyancy with hydrogen, the lightest gas
(flammable, so seldom used) next-best buoyancy with helium heated air used in sports balloons
As balloons rise, atmosphere becomes less dense with altitude.
80
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Buoyancy in Gas
Gas-filled balloon will continue to rise until the weight of displaced air equals the total
weight of the balloon the buoyant force on the balloon
equals its weight (which says the same thing)
81
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
As a balloon rises higher and higher into the atmosphere, its __________.
A. volume decreases
B. density increases
C. weight increases
D. None of the above
Buoyancy in GasCHECK YOUR NEIGHBOR
82
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
As a balloon rises higher and higher into the atmosphere, its __________.
A. volume decreases
B. density increases
C. weight increases
D. None of the above
Buoyancy in GasCHECK YOUR ANSWER
83
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Fluid Flow
continuous flow Volume of fluid that flows past any cross-section
of a pipe in a given time is the same as that flowing past any other section of the pipe even if the pipe widens or narrows.
Fluid speeds up when it flows from a wide to narrow pipe.
Motion of fluid follows imaginary streamlines.
84
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Bernoulli’s Principle
Bernoulli’s Principle discovered by Daniel Bernoulli, a 15th century
Swiss scientist states that where the speed of a fluid increases,
internal pressure in the fluid decreases applies to a smooth, steady flow
85
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Bernoulli’s Principle
Streamlines thin lines representing fluid motion closer together, flow speed is greater and
pressure within the fluid is less wider, flow speed is less and pressure within the
fluid is greater
86
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Bernoulli’s Principle
Laminar flow smooth steady flow of constant density fluid
Turbulent flow flow speed above a critical point becomes
chaotic
87
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
The pressure in a stream of water is reduced as the stream speeds up. Can a stream of water from a fire hose actually knock a person off his or her feet?
A. It can’t, as Bernoulli’s principle illustrates.
B. It can, and is consistent with Bernoulli’s principle.
C. Bernoulli’s principle works only for laminar flow, which the stream is not.
D. None of the above
Bernoulli’s PrincipleCHECK YOUR NEIGHBOR
88
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
The pressure in a stream of water is reduced as the stream speeds up. Can a stream of water from a fire hose actually knock a person off his or her feet?
A. It can’t, as Bernoulli’s principle illustrates.
B. It can, and is consistent with Bernoulli’s principle.
C. Bernoulli’s principle works only for laminar flow, which the stream is not.
D. None of the above
Explanation:
There’s a difference in the pressure within flowing water and the pressure it can exert when its momentum is changed. The pressure that knocks one off his or her feet is due to the change in the water’s momentum, not the pressure within the water.
Bernoulli’s PrincipleCHECK YOUR ANSWER
89
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Bernoulli’s Principle
Applications of Bernoulli’s Principle Blow on the top surface of a paper and the
paper rises. Reason: The pressure of the moving air is less than the atmospheric pressure beneath it.
Convertible car roof puffs upward as air moves over its top surface. Reason: The pressure of air moving on the top surface is less than the atmospheric pressure inside the car.
90
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Bernoulli’s Principle
Wind blowing across a peaked roof can lift the roof off the house. Reason: Pressure is reduced as wind gains speed as it flows over the roof. The greater pressure inside the house lifts the roof up.
Perfume atomizer
Reason: Air rushes across the open end of the tube when you squeeze the bulb, reducing pressure in the tube. Atmospheric pressure on the liquid below pushes it up into the tube.
91
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Bernoulli’s Principle
Trucks passing closely on a highway are drawn to each other.
Reason: Air pressure between the trucks is less than pressure on their outer sides pushing inward.
Risk of passing ships colliding sideways
Reason: Water pressure between the ships is less than pressure on their outer sides pushing inward.
92
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
On a windy day, waves in a lake or the ocean are higher than their average height. What factor in Bernoulli’s principle contributes to the increased height?
A. Reduced pressure pulls water upward.
B. Pressure builds up over the crests.
C. Air travels faster over the crests than the troughs.
D. None of the above
Bernoulli’s PrincipleCHECK YOUR NEIGHBOR
93
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
On a windy day, waves in a lake or the ocean are higher than their average height. What factor in Bernoulli’s principle contributes to the increased height?
A. Reduced pressure pulls water upward.
B. Pressure builds up over the crests.
C. Air travels faster over the crests than the troughs.
D. None of the above
Explanation:
The troughs of the waves are partially shielded from the wind, so air travels faster over the crests. Pressure there is thus lower than down below in the troughs. The greater pressure in the troughs pushes water into the even higher crests.
Bernoulli’s PrincipleCHECK YOUR ANSWER
94
Recommended