Chapter 11 Forces in Fluids
Chapter Preview Questions
1. Which of the following is an example of a force?
a. water
b. other fluids
c. gravity
d. mass
Chapter 11 Forces in Fluids
Chapter Preview Questions
1. Which of the following is an example of a force?
a. water
b. other fluids
c. gravity
d. mass
Chapter 11 Forces in Fluids
Chapter Preview Questions
2. A fluid can be
a. a gas only.
b. a liquid only.
c. a solid or a gas.
d. a liquid or a gas.
Chapter 11 Forces in Fluids
Chapter Preview Questions
2. A fluid can be
a. a gas only.
b. a liquid only.
c. a solid or a gas.
d. a liquid or a gas.
Chapter 11 Forces in Fluids
Chapter Preview Questions
3. The velocity of an object is
a. its standard reference point.
b. the rate of change of its position.
c. the process of speeding it up.
d. its change in direction.
Chapter 11 Forces in Fluids
Chapter Preview Questions
3. The velocity of an object is
a. its standard reference point.
b. the rate of change of its position.
c. the process of speeding it up.
d. its change in direction.
Chapter 11 Forces in Fluids
Chapter Preview Questions
4. Earth’s gravity pulls you down with a force
a. greater than your weight.
b. the size of your feet.
c. equal to your weight.
d. half your weight.
Chapter 11 Forces in Fluids
Chapter Preview Questions
4. Earth’s gravity pulls you down with a force
a. greater than your weight.
b. the size of your feet.
c. equal to your weight.
d. half your weight.
Chapter 11 Forces in Fluids
Section 1:Pressure
Standard 8.8.d Students know how to predict whether
an object will float or sink.
Standard 8.9.f Apply simple mathematical relationships
to determine a missing quantity in a mathematic
expression, given the two remaining terms
Force = pressure x area
Chapter 11 Forces in Fluids
What Is Pressure?
What does pressure
depend on?
The amount of pressure you
exert depends on the area over
which you exert a force.
Chapter 11 Forces in Fluids
Area
The area of a surface is the number of square units that it
covers. To find the area of a rectangle, multiply its length by
its width. The area of the rectangle below is 2 cm X 3 cm, or
6 cm2.
Chapter 11 Forces in Fluids
Area
Practice Problem
Which has a greater area: a rectangle that is 4 cm X 20 cm
or a square that is 10 cm X 10 cm?
The square has the greater area.
4 cm X 20 cm = 80 cm2
10 cm X 10 cm = 100 cm2
Chapter 11 Forces in Fluids
Calculating Pressure
How is pressure
calculated?
Pressure = Force / Area
Pressure is measured in an SI
unit called a pascal (Pa): 1
N/square meter = 1 Pa.
Example:
Calculate the pressure
produced by a force of
800 N acting on an
area of 2.0 square
meters.
P = F/A
P= (800N) / (2.0 square meters)
P = 400 N / square meter = 400
Pa
Chapter 11 Forces in Fluids
Calculating Pressure
The SI unit for pressure,
the pascal, is named for
French mathematician
Blaise Pascal.
The SI unit for force, the newton,
is named for English physicist Sir
Isaac Newton.
Chapter 11 Forces in Fluids
Math Practice
Calculate the pressure produced by a force of 450 N on an
area of 3 square meters.
P = F/A
P = (450 N) / (3 square meters)
P = 150 N / square meter = 150 Pa
What is the pressure on 2.5 by 3 meter area being acted
upon by a force of 6000 N?
P = 6000 N / (2.5)(3) square meters
P = 6000 N / 7.5 square meters
P = 800 Pa
Chapter 11 Forces in Fluids
Calculating Force from Pressure
How is force calculated
from pressure?
Force is expressed in
Newtons (N)
Force = Pressure x Area
Example
The pressure of a gas contained
in a cylinder with a movable
piston is 300 Pa. The area of the
piston is 0.5 square meters.
Calculate the force that is exerted
on the piston.
F=PxA
F = (300 Pa) x 0.5 m2
F = 150 N
Chapter 11 Forces in Fluids
Math Practice
The pressure of a gas contained in a cylinder with a
movable piston is 700 Pa. The area of the piston is 0.2
square meters. What is the force that is exerted on the
piston?
F=PxA
F = (700 Pa) (0.2 square meters)
F = 140 N
Chapter 11 Forces in Fluids
Fluid Pressure
How do fluids exert
pressure?
All of the forces exerted by the
individual particles in a fluid combine
to make up the pressure exerted by
the fluid.
Chapter 11 Forces in Fluids
Fluid Pressure
Air pressure
Balanced pressure
Air is a mixture of gases that make
up Earth’s atmosphere.
These gases press down on
everything on Earth’s surface.
Air exerts pressure because it has
mass.
Balanced pressure explains why the
tremendous air pressure pushing on
you from all sides does not crush
you.
Pressure from fluids inside your
body balances the air pressure
outside your body.
Chapter 11 Forces in Fluids
Variations in Fluid Pressure
How does fluid
pressure change with
elevation and depth?
Atmospheric pressure decreases
as your elevation increases.
You can measure atmospheric
pressure with an instrument
called a barometer.
Water pressure increases as
depth increases.
Chapter 11 Forces in Fluids
Variations in Fluid Pressure
As your elevation
increases, atmospheric
pressure decreases.
Chapter 11 Forces in Fluids
Variations in Fluid Pressure
Water pressure increases as depth increases.
Chapter 11 Forces in Fluids
Section 1 Quick Quiz
Snowshoes enable a person to walk on deep snow because
the snowshoes
A.decrease the person’s weight on the snow.
B.increase the pressure on the snow.
C.increase the buoyancy of the person.
D.increase the area over which the person’s weight is
distributed.
Answer – D - increase the area over which the person’s
weight is distributed.
Chapter 11 Forces in Fluids
Section 1 Quick Quiz
A unit of pressure is called a
A.pound.
B.meter.
C.pascal.
D.bernoulli.
Answer – C - pascal
Chapter 11 Forces in Fluids
Section 1 Quick Quiz
Pressure can be measured in units of
A.newtons per square meter.
B.newtons per cubic centimeter.
C.newtons per centimeter.
D.newtons.
Answer – A – newtons per square meter
Chapter 11 Forces in Fluids
Section 1 Quick Quiz
Air pressure exerted equally on an object from different
directions is
A.gravitational pressure.
B.balanced pressure.
C.constant pressure.
D.fluid pressure.
Answer – B – balanced pressure
Chapter 11 Forces in Fluids
Section 1 Quick Quiz
Given that the air pressure outside your body is so great,
why aren’t you crushed?
A.Earth’s gravity cancels out the air pressure.
B.Inertia changes the pressure before it comes into contact
with you.
C.Human skin is extremely strong.
D.Pressure inside your body balances the air pressure
outside your body.
Answer – D - Pressure inside your body balances the air
pressure outside your body.
Chapter 11 Forces in Fluids
Section 1 Quick Quiz
Air pressure decreases as
A.elevation increases.
B.velocity increases.
C.gravity increases.
D.acceleration decreases.
Answer – A – elevation increases
Chapter 11 Forces in Fluids
Section 1 Quick Quiz
Water pressure increases as
A.acceleration increases.
B.gravity decreases.
C.depth increases.
D.force decreases.
Answer – C – depth increases
Chapter 11 Forces in Fluids
Section 1 Quick Quiz
A barometer is used to measure pressure
A.in the atmosphere.
B.in hydraulic systems.
C.under a snowshoe.
D.under water.
Answer – A – in the atmosphere
Chapter 11 Forces in Fluids
Section 2: Floating and Sinking
Standard 8.8.c Students know the buoyant force on
an object in a fluid is an upward force equal to the
weight of the fluid the object displaced.
Standard 8.8.d Students know how to predict
whether an object will float or sink.
Chapter 11 Forces in Fluids
Calculating Density
How is density
calculated?
The density of a substance is its mass per
unit of volume.
Density = Mass
Volume
Example:
A sample of liquid has a mass of 24 g
and a volume of 16 mL. What is its
density?
Density = Mass = 24 g = 1.5 g/mL
Volume
16 mL
Chapter 11 Forces in Fluids
Calculating Density
Practice Problem
A piece of metal has a mass of 43.5 g and a
volume of 15 cm3. What is its density?
2.9 g/cm3
Chapter 11 Forces in Fluids
Calculating Density
A block has a mass of 320 g and a volume of 80 cubic
centimeters. What is its density?
4 g / cubic centimeter
An orange has a mass of 250 g and a volume of 750 mL.
What is its density?
.33 g / mL
Chapter 11 Forces in Fluids
Section 2:
Floating and Sinking
How is density calculated?
How can you predict whether an object will
float or sink in a fluid?
What is the effect of the buoyant force?
Chapter 11 Forces in Fluids
Density
Changes in density cause a submarine to dive, rise, or float.
Chapter 11 Forces in Fluids
Density
Changes in density cause a submarine to dive, rise, or float.
Chapter 11 Forces in Fluids
Density
Changes in density cause a submarine to dive, rise, or float.
Chapter 11 Forces in Fluids
Buoyancy
The pressure on the bottom of
a submerged object is greater
than the pressure on the top.
The result is a net force in the
upward direction.
Chapter 11 Forces in Fluids
Buoyancy
The buoyant force works opposite the weight of an object.
Chapter 11 Forces in Fluids
Buoyancy
Archimedes’ principle states
that the buoyant force acting
on a submerged object is
equal to the weight of the fluid
the object displaces.
Chapter 11 Forces in Fluids
Buoyancy
A solid block of steel sinks in water. A steel ship with the
same weight floats on the surface.
Chapter 11 Forces in Fluids
Section 2 Quick Quiz
Which of the following is true of the buoyant force?
A.It acts in the downward direction.
B.It makes an object feel heavier.
C.It acts with the force of gravity.
D.It acts in the upward direction.
Answer – D – It acts in the upward direction
Chapter 11 Forces in Fluids
Section 2 Quick Quiz
If an object floats, the volume of displaced water is equal to
the volume of
A.exactly half of the object.
B.the entire object.
C.the portion of the object that is submerged.
D.the portion of the object that is above water.
Answer – C – the portion of the object that is submerged
Chapter 11 Forces in Fluids
Section 2 Quick Quiz
When water fills a submarine’s flotation tanks, the overall
density of the submarine
A.stays the same.
B.decreases.
C.reduces the buoyant force.
D.increases.
Answer – D - increases
Chapter 11 Forces in Fluids
Section 2 Quick Quiz
An object that is more dense than the fluid in which it is
immersed will
A.rise.
B.sink.
C.sink at first, then rise slowly.
D.neither rise nor sink.
Answer – B - sink
Chapter 11 Forces in Fluids
Section 2 Quick Quiz
Which of these substances is the LEAST dense?
A.rubber
B.mercury
C.copper
D.wood
Answer – D - wood
Chapter 11 Forces in Fluids
Section 2 Quick Quiz
What scientific rule states that the buoyant force on an object
is equal to the weight of the fluid displaced by the object?
A.Archimedes’ principle
B.Newton’s third law of motion
C.Bernoulli’s principle
D.Pascal’s principle
Answer – A – Archimedes’ principle
Chapter 11 Forces in Fluids
Section 3: Pascal’s Principle
Standard 8.8.c Students know the buoyant force on
an object in a fluid is an upward force equal to the
weight of the fluid the object has displaced.
Chapter 11 Forces in Fluids
Section 3:
Pascal’s Principle
What does Pascal’s principle say about change
in fluid pressure?
How does a hydraulic system work?
Chapter 11 Forces in Fluids
Transmitting Pressure in a Fluid
When force is applied to a confined fluid, the change in
pressure is transmitted equally to all parts of the fluid.
Chapter 11 Forces in Fluids
Hydraulic Devices
In a hydraulic device, a force
applied to one piston increases
the fluid pressure equally
throughout the fluid.
Chapter 11 Forces in Fluids
Hydraulic Devices
By changing the size of the
pistons, the force can be
multiplied.
Chapter 11 Forces in Fluids
Comparing Hydraulic Lifts
In a hydraulic device, a force
applied to the piston on the left
produces a lifting force in the
piston on the right. The graph
shows the relationship between
the applied force and the lifting
force for two hydraulic lifts.
Chapter 11 Forces in Fluids
Comparing Hydraulic Lifts
Reading Graphs:
Suppose a force of 1,000 N is
applied to both lifts. Use the
graph to determine the lifting
force of each lift.
Lift A: 4,000 N; lift B: 2,000 N
Chapter 11 Forces in Fluids
Comparing Hydraulic Lifts
Reading Graphs:
For Lift A, how much force
must be applied to lift a
12,000-N object?
3,000 N
Chapter 11 Forces in Fluids
Comparing Hydraulic Lifts
Interpreting Data:
By how much is the applied
force multiplied for Lift A? Lift
B?
Lift A: applied force is
multiplied by four; lift B:
applied force is multiplied by
two.
Chapter 11 Forces in Fluids
Comparing Hydraulic Lifts
Interpreting Data:
What does the slope of each
line represent?
The slope gives the ratio of
the lifting force to the applied
force. The greater the slope,
the more the lift multiplies
force.
Chapter 11 Forces in Fluids
Comparing Hydraulic Lifts
Drawing Conclusions:
Which lift would you choose if
you wanted to lift a weight of
4,000 N? Explain.
Lift A, because it multiplies
force more than lift B.
Chapter 11 Forces in Fluids
Hydraulic Brakes
The hydraulic brake system
of a car multiplies the force
exerted on the brake pedal.
Chapter 11 Forces in Fluids
Section 3 Quick Quiz
The braking system on a car is an example of
A.Bernoulli’s principle.
B.Newton’s third law of motion.
C.a hydraulic system.
D.buoyancy.
Answer – C – a hydraulic system.
Chapter 11 Forces in Fluids
Section 3 Quick Quiz
Which type of substance does Pascal’s principle deal with?
A.fluids
B.powders
C.metals
D.solids
Answer – A - fluids
Chapter 11 Forces in Fluids
Section 3 Quick Quiz
One application of Pascal’s principle is
A.the flight of an airplane.
B.a speedboat’s bottom slapping against the waves.
C.a hydraulic car lift.
D.the buoyancy shown by ducks and other waterfowl.
Answer – C – a hydraulic car lift
Chapter 11 Forces in Fluids
Section 3 Quick Quiz
What does a hydraulic system do?
A.multiply force
B.reduce inertia
C.increase velocity
D.decrease pressure
Answer – A – multiply force
Chapter 11 Forces in Fluids
Section 4: Bernoulli’s Principle
Standard 8.2.e Students know that when the forces
on an object are unbalanced, the object will change
its velocity (that is, it will speed up, slow down, or
change direction).
Chapter 11 Forces in Fluids
Section 4:
Bernoulli’s Principle
How is fluid pressure related to the motion of a fluid?
What are some applications of Bernoulli’s principle?
Chapter 11 Forces in Fluids
Bernoulli’s Principle
Bernoulli’s principle states that as the speed of a moving
fluid increases, the pressure exerted by the fluid decreases.
Chapter 11 Forces in Fluids
Applying Bernoulli’s Principle
Bernoulli’s principle helps explain how planes fly.
Chapter 11 Forces in Fluids
Applying Bernoulli’s Principle
An atomizer is an application of Bernoulli’s principle.
Chapter 11 Forces in Fluids
Applying Bernoulli’s Principle
Thanks in part to Bernoulli's
principle, you can enjoy an
evening by a warm fireplace
without the room filling up with
smoke.
Chapter 11 Forces in Fluids
Applying Bernoulli’s Principle
Like an airplane wing, a flying disk uses a curved upper
surface to create lift.
Chapter 11 Forces in Fluids
Section 4 Quick Quiz
What scientific rule states that the pressure exerted by a
moving stream of fluid is less than the pressure of the
surrounding fluid?
A.Newton’s third law of motion
B.Bernoulli’s principle
C.Archimedes’ principle
D.Pascal’s principle
Answer – B – Bernoulli’s principle
Chapter 11 Forces in Fluids
Section 4 Quick Quiz
Smoke rises up a chimney partly because of
A.Bernoulli’s principle
B.Archimedes’ principle
C.Newton’s third law of motion
D.Pascal’s principle
Answer – A – Bernoulli’s principle